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Cowley, S. W. H., A. J. Deason, and E. J. Bunce. "Axi-symmetric models of auroral current systems in Jupiter's magnetosphere with predictions for the Juno mission." Annales Geophysicae 26, no. 12 (December 12, 2008): 4051–74. http://dx.doi.org/10.5194/angeo-26-4051-2008.
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Abstract. We develop two related models of magnetosphere-ionosphere coupling in the jovian system by combining previous models defined at ionospheric heights with magnetospheric magnetic models that allow system parameters to be extended appropriately into the magnetosphere. The key feature of the combined models is thus that they allow direct connection to be made between observations in the magnetosphere, particularly of the azimuthal field produced by the magnetosphere-ionosphere coupling currents and the plasma angular velocity, and the auroral response in the ionosphere. The two models are intended to reflect typical steady-state sub-corotation conditions in the jovian magnetosphere, and transient super-corotation produced by sudden major solar wind-induced compressions, respectively. The key simplification of the models is that of axi-symmetry of the field, flow, and currents about the magnetic axis, limiting their validity to radial distances within ~30 RJ of the planet, though the magnetic axis is appropriately tilted relative to the planetary spin axis and rotates with the planet. The first exploration of the jovian polar magnetosphere is planned to be undertaken in 2016–2017 during the NASA New Frontiers Juno mission, with observations of the polar field, plasma, and UV emissions as a major goal. Evaluation of the models along Juno planning orbits thus produces predictive results that may aid in science mission planning. It is shown in particular that the low-altitude near-periapsis polar passes will generally occur underneath the corresponding auroral acceleration regions, thus allowing brief examination of the auroral primaries over intervals of ~1–3 min for the main oval and ~10 s for narrower polar arc structures, while the "lagging" field deflections produced by the auroral current systems on these passes will be ~0.1°, associated with azimuthal fields above the ionosphere of a few hundred nT.
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Rostoker, G., and F. Pascal. "Dependence of the response of the magnetosphere–ionosphere current systems on the preconditioning of the auroral oval and on the level of the solar–terrestrial interaction." Canadian Journal of Physics 68, no. 1 (January 1, 1990): 74–80. http://dx.doi.org/10.1139/p90-011.
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It is now well accepted that the impulse response time of the magnetosphere to sudden changes in the interplanetary medium is of the order of 2 h with the shape of the impulse response function approximating a Rayleigh function with a peak near 50 min. In a recent study, Bargatze et al. (J. Geophys. Res. 90, 6387 (1985)) examined the response of the magnetosphere for varying activity levels and found that the impulse response function has two well-defined peaks for moderate activity and a single broad peak for low and high activity levels. They explain the two peaks in the response function as the sequential contributions of the directly driven process and the unloading of stored magnetotail energy. In this paper, we ascribe to the magnetosphere–ionosphere system the bulk properties of self-inductance, capacitance, and resistance. We then proceed to construct an equivalent current system for the magnetosphere–ionosphere coupling process and study its response to changes in the cross polar cap potential drop. In particular, we permit the bulk electrical parameters to change in the manner expected as the input of energy from the solar wind modifies the magnetosphere–ionosphere system. We find that the double peak in the impulse response function identified by Bargatze et al. can be understood purely in terms of changes in the directly driven system without the need to introduce the effects of the unloading of stored energy in the magnetotail.
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Cowley, S. W. H., and E. J. Bunce. "Corotation-driven magnetosphere-ionosphere coupling currents in Saturn’s magnetosphere and their relation to the auroras." Annales Geophysicae 21, no. 8 (August 31, 2003): 1691–707. http://dx.doi.org/10.5194/angeo-21-1691-2003.
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Abstract. We calculate the latitude profile of the equatorward-directed ionospheric Pedersen currents that are driven in Saturn’s ionosphere by partial corotation of the magnetospheric plasma. The calculation incorporates the flattened figure of the planet, a model of Saturn’s magnetic field derived from spacecraft flyby data, and angular velocity models derived from Voyager plasma data. We also employ an effective height-integrated ionospheric Pedersen conductivity of 1 mho, suggested by a related analysis of Voyager magnetic field data. The Voyager plasma data suggest that on the largest spatial scales, the plasma angular velocity declines from near-rigid corotation with the planet in the inner magnetosphere, to values of about half of rigid corotation at the outer boundary of the region considered. The latter extends to ~ 15–20 Saturn radii (RS) in the equatorial plane, mapping along magnetic field lines to ~ 15° co-latitude in the ionosphere. We find in this case that the ionospheric Pedersen current peaks near the poleward (outer) boundary of this region, and falls toward zero over ~ 5°–10° equator-ward of the boundary as the plasma approaches rigid corotation. The peak current near the poleward boundary, integrated in azimuth, is ~ 6 MA. The field-aligned current required for continuity is directed out of the ionosphere into the magnetosphere essentially throughout the region, with the current density peaking at ~ 10 nA m-2 at ~ 20° co-latitude. We estimate that such current densities are well below the limit requiring field-aligned acceleration of magnetospheric electrons in Saturn’s environment ( ~ 70 nAm-2), so that no significant auroral features associated with this ring of upward current is anticipated. The observed ultraviolet auroras at Saturn are also found to occur significantly closer to the pole (at ~ 10°–15° co-latitude), and show considerable temporal and local time variability, contrary to expectations for corotation-related currents. We thus conclude that Saturn’s ‘main oval’ auroras are not associated with corotation-enforcing currents as they are at Jupiter, but instead are most probably associated with coupling to the solar wind as at Earth. At the same time, the Voyager flow observations also suggest the presence of radially localized ‘dips’ in the plasma angular velocity associated with the moons Dione and Rhea, which are ~ 1–2 RS in radial extent in the equatorial plane. The presence of such small-scale flow features, assumed to be azimuthally extended, results in localized several-MA enhancements in the ionospheric Pedersen current, and narrow bi-polar signatures in the field-aligned currents which peak at values an order of magnitude larger than those associated with the large-scale currents. Narrow auroral rings (or partial rings) ~ 0.25° co-latitude wide with intensities ~ 1 kiloRayleigh may be formed in the regions of upward field-aligned current under favourable circumstances, located at co-latitudes between ~ 17° and ~ 20° in the north, and ~ 19° and ~22° in the south.Key words. Magnetospheric physics (current systems; magnetosphere-ionosphere interactions; planetary magnetospheres)
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Tsunomura, S. "Numerical analysis of global ionospheric current system including the effect of equatorial enhancement." Annales Geophysicae 17, no. 5 (May 31, 1999): 692–706. http://dx.doi.org/10.1007/s00585-999-0692-2.
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Abstract. A modeling method is proposed to derive a two-dimensional ionospheric layer conductivity, which is appropriate to obtain a realistic solution of the polar-originating ionospheric current system including equatorial enhancement. The model can be obtained by modifying the conventional, thin shell conductivity model. It is shown that the modification for one of the non-diagonal terms (Σθφ) in the conductivity tensor near the equatorial region is very important; the term influences the profile of the ionospheric electric field around the equator drastically. The proposed model can reproduce well the results representing the observed electric and magnetic field signatures of geomagnetic sudden commencement. The new model is applied to two factors concerning polar-originating ionospheric current systems. First, the latitudinal profile of the DP2 amplitude in the daytime is examined, changing the canceling rate for the dawn-to-dusk electric field by the region 2 field-aligned current. It is shown that the equatorial enhancement would not appear when the ratio of the total amount of the region 2 field-aligned current to that of region 1 exceeds 0.5. Second, the north-south asymmetry of the magnetic fields in the summer solstice condition of the ionospheric conductivity is examined by calculating the global ionospheric current system covering both hemispheres simultaneously. It is shown that the positive relationship between the magnitudes of high latitude magnetic fields and the conductivity is clearly seen if a voltage generator is given as the source, while the relationship is vague or even reversed for a current generator. The new model, based on the International Reference Ionosphere (IRI) model, can be applied to further investigations in the quantitative analysis of the magnetosphere-ionosphere coupling problems.Key words. Ionosphere (electric fields and currents; equatorial ionosphere; ionosphere-magnetosphere interactions)
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Tanaka, T. "Generation mechanisms for magnetosphere-ionosphere current systems deduced from a three-dimensional MHD simulation of the solar wind-magnetosphere-ionosphere coupling processes." Journal of Geophysical Research 100, A7 (1995): 12057. http://dx.doi.org/10.1029/95ja00419.
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Nichols, J. D., and S. W. H. Cowley. "Magnetosphere-ionosphere coupling currents in Jupiter’s middle magnetosphere: dependence on the effective ionospheric Pedersen conductivity and iogenic plasma mass outflow rate." Annales Geophysicae 21, no. 7 (July 31, 2003): 1419–41. http://dx.doi.org/10.5194/angeo-21-1419-2003.
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Abstract. The amplitude and spatial distribution of the coupling currents that flow between Jupiter’s ionosphere and middle magnetosphere, which enforce partial corotation on outward-flowing iogenic plasma, depend on the values of the effective Pedersen conductivity of the jovian ionosphere and the mass outflow rate of iogenic plasma. The values of these parameters are, however, very uncertain. Here we determine how the solutions for the plasma angular velocity and current components depend on these parameters over wide ranges. We consider two models of the poloidal magnetospheric magnetic field, namely the planetary dipole alone, and an empirical current sheet field based on Voyager data. Following work by Hill (2001), we obtain a complete normalized analytic solution for the dipole field, which shows in compact form how the plasma angular velocity and current components scale in space and in amplitude with the system parameters in this case. We then obtain an approximate analytic solution in similar form for a current sheet field in which the equatorial field strength varies with radial distance as a power law. A key feature of the model is that the current sheet field lines map to a narrow latitudinal strip in the ionosphere, at ≈ 15° co-latitude. The approximate current sheet solutions are compared with the results of numerical integrations using the full field model, for which a power law applies beyond ≈ 20 RJ, and are found to agree very well within their regime of applicability. A major distinction between the solutions for the dipole field and the current sheet concerns the behaviour of the field-aligned current. In the dipole model the direction of the current reverses at moderate equatorial distances, and the current system wholly closes if the model is extended to infinity in the equatorial plane and to the pole in the ionosphere. In the approximate current sheet model, however, the field-aligned current is unidirectional, flowing consistently from the ionosphere to the current sheet for the sense of the jovian magnetic field. Current closure must then occur at higher latitudes, on field lines outside the region described by the model. The amplitudes of the currents in the two models are found to scale with the system parameters in similar ways, though the scaling is with a somewhat higher power of the conductivity for the current sheet model than for the dipole, and with a somewhat lower power of the plasma mass outflow rate. The absolute values of the currents are also higher for the current sheet model than for the dipole for given parameters, by factors of approx 4 for the field-perpendicular current intensities, ≈ 10 for the total current flowing in the circuit, and ≈ 25 for the field-aligned current densities, factors which do not vary greatly with the system parameters. These results thus confirm that the conclusions drawn previously from a small number of numerical integrations using spot values of the system parameters are generally valid over wide ranges of the parameter values.Key words. Magnetospheric physics (current systems, magnetosphere-ionosphere interactions, planetary magnetospheres)
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Le, G., C. T. Russell, and K. Takahashi. "Morphology of the ring current derived from magnetic field observations." Annales Geophysicae 22, no. 4 (April 2, 2004): 1267–95. http://dx.doi.org/10.5194/angeo-22-1267-2004.
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Abstract. Our examination of the 20 years of magnetospheric magnetic field data from ISEE, AMPTE/CCE and Polar missions has allowed us to quantify how the ring current flows and closes in the magnetosphere at a variety of disturbance levels. Using intercalibrated magnetic field data from the three spacecraft, we are able to construct the statistical magnetic field maps and derive 3-dimensional current density by the simple device of taking the curl of the statistically determined magnetic field. The results show that there are two ring currents, an inner one that flows eastward at ~3 RE and a main westward ring current at ~4–7 RE for all levels of geomagnetic disturbances. In general, the in-situ observations show that the ring current varies as the Dst index decreases, as we would expect it to change. An unexpected result is how asymmetric it is in local time. Some current clearly circles the magnetosphere but much of the energetic plasma stays in the night hemisphere. These energetic particles appear not to be able to readily convect into the dayside magnetosphere. During quiet times, the symmetric and partial ring currents are similar in strength (~0.5MA) and the peak of the westward ring current is close to local midnight. It is the partial ring current that exhibits most drastic intensification as the level of disturbances increases. Under the condition of moderate magnetic storms, the total partial ring current reaches ~3MA, whereas the total symmetric ring current is ~1MA. Thus, the partial ring current contributes dominantly to the decrease in the Dst index. As the ring current strengthens the peak of the partial ring current shifts duskward to the pre-midnight sector. The partial ring current is closed by a meridional current system through the ionosphere, mainly the field-aligned current, which maximizes at local times near the dawn and dusk. The closure currents flow in the sense of region-2 field-aligned currents, downward into the ionosphere near the dusk and upward out of the ionosphere near the dawn. Key words. Magnetospheric physics (current systems; storms and substorms; magnetospheric configuration and dynamics)
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Neudegg, D. A., B. J. Fraser, F. W. Menk, G. B. Burns, R. J. Morris, and M. J. Underwood. "Magnetospheric sources of Pc1-2 ULF waves observed in the polar ionospheric waveguide." Antarctic Science 14, no. 1 (March 2002): 93–103. http://dx.doi.org/10.1017/s0954102002000627.
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Energy from the outer regions of the magnetosphere may be transferred to the polar ionosphere by plasma waves. A magnetometer array operated during the Antarctic winter observed Ultra-Low-Frequency (ULF) plasma waves in the Pc 1–2 (0.1–10.0 Hz) frequency range, propagating parallel to the surface of the Earth in a waveguide or duct centred at ∼300 km altitude in the ionosphere. These compressional fast mode plasma waves most likely originated in the outer magnetosphere as shear mode plasma waves guided along the geomagnetic field. The region of origin in the magnetosphere for the waves is not certain as several widely spaced volumes map along geomagnetic field lines to a relatively close ensemble in the polar ionosphere. This paper compares the direction of propagation for the waves with signatures of magnetospheric regions geomagnetically projecting onto the ionosphere. Regions such as the polar cusp, low latitude boundary layer and mantle were observed by DMSP spacecraft and a SuperDARN high-frequency radar. The most likely region in the polar ionosphere for the fast mode waves to have originated from is equatorwards of the polar cusp, suggesting the field guided waves originated just inside the magnetopause. A case is made for association of the observed Pc1-2 ULF waves with post-noon, field-aligned-current systems driven by reconnection of the solar Interplanetary Magnetic Field (IMF) and the geomagnetic field near the magnetopause.
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Rostoker, G., D. Savoie, and T. D. Phan. "Response of magnetosphere-ionosphere current systems to changes in the interplanetary magnetic field." Journal of Geophysical Research 93, A8 (1988): 8633. http://dx.doi.org/10.1029/ja093ia08p08633.
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Kivelson, Margaret Galland. "The Current Systems of the Jovian Magnetosphere and Ionosphere and Predictions for Saturn." Space Science Reviews 116, no. 1-2 (January 2005): 299–318. http://dx.doi.org/10.1007/s11214-005-1959-x.
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Cowley, S. W. H., C. S. Arridge, E. J. Bunce, J. T. Clarke, A. J. Coates, M. K. Dougherty, J. C. Gérard, D. Grodent, J. D. Nichols, and D. L. Talboys. "Auroral current systems in Saturn's magnetosphere: comparison of theoretical models with Cassini and HST observations." Annales Geophysicae 26, no. 9 (September 8, 2008): 2613–30. http://dx.doi.org/10.5194/angeo-26-2613-2008.
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Abstract. The first simultaneous observations of fields and plasmas in Saturn's high-latitude magnetosphere and UV images of the conjugate auroral oval were obtained by the Cassini spacecraft and the Hubble Space Telescope (HST) in January 2007. These data have shown that the southern auroral oval near noon maps to the dayside cusp boundary between open and closed field lines, associated with a major layer of upward-directed field-aligned current (Bunce et al., 2008). The results thus support earlier theoretical discussion and quantitative modelling of magnetosphere-ionosphere coupling at Saturn (Cowley et al., 2004), that suggests the oval is produced by electron acceleration in the field-aligned current layer required by rotational flow shear between strongly sub-corotating flow on open field lines and near-corotating flow on closed field lines. Here we quantitatively compare these modelling results (the "CBO" model) with the Cassini-HST data set. The comparison shows good qualitative agreement between model and data, the principal difference being that the model currents are too small by factors of about five, as determined from the magnetic perturbations observed by Cassini. This is suggested to be principally indicative of a more highly conducting summer southern ionosphere than was assumed in the CBO model. A revised model is therefore proposed in which the height-integrated ionospheric Pedersen conductivity is increased by a factor of four from 1 to 4 mho, together with more minor adjustments to the co-latitude of the boundary, the flow shear across it, the width of the current layer, and the properties of the source electrons. It is shown that the revised model agrees well with the combined Cassini-HST data, requiring downward acceleration of outer magnetosphere electrons through a ~10 kV potential in the current layer at the open-closed field line boundary to produce an auroral oval of ~1° width with UV emission intensities of a few tens of kR.
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Bunce, E. J., S. W. H. Cowley, and J. A. Wild. "Azimuthal magnetic fields in Saturn’s magnetosphere: effects associated with plasma sub-corotation and the magnetopause-tail current system." Annales Geophysicae 21, no. 8 (August 31, 2003): 1709–22. http://dx.doi.org/10.5194/angeo-21-1709-2003.
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Abstract. We calculate the azimuthal magnetic fields expected to be present in Saturn’s magnetosphere associated with two physical effects, and compare them with the fields observed during the flybys of the two Voyager spacecraft. The first effect is associated with the magnetosphere-ionosphere coupling currents which result from the sub-corotation of the magnetospheric plasma. This is calculated from empirical models of the plasma flow and magnetic field based on Voyager data, with the effective Pedersen conductivity of Saturn’s ionosphere being treated as an essentially free parameter. This mechanism results in a ‘lagging’ field configuration at all local times. The second effect is due to the day-night asymmetric confinement of the magnetosphere by the solar wind (i.e. the magnetopause and tail current system), which we have estimated empirically by scaling a model of the Earth’s magnetosphere to Saturn. This effect produces ‘leading’ fields in the dusk magnetosphere, and ‘lagging’ fields at dawn. Our results show that the azimuthal fields observed in the inner regions can be reasonably well accounted for by plasma sub-corotation, given a value of the effective ionospheric Pedersen conductivity of ~ 1–2 mho. This statement applies to field lines mapping to the equator within ~ 8 RS (1 RS is taken to be 60 330 km) of the planet on the dayside inbound passes, where the plasma distribution is dominated by a thin equatorial heavy-ion plasma sheet, and to field lines mapping to the equator within ~ 15 RS on the dawn side outbound passes. The contributions of the magnetopause-tail currents are estimated to be much smaller than the observed fields in these regions. If, however, we assume that the azimuthal fields observed in these regions are not due to sub-corotation but to some other process, then the above effective conductivities define an upper limit, such that values above ~ 2 mho can definitely be ruled out. Outside of this inner region the spacecraft observed both ‘lagging’ and ‘leading’ fields in the post-noon dayside magnetosphere during the inbound passes, with ‘leading’ fields being observed both adjacent to the magnetopause and in the ring current region, and ‘lagging’ fields being observed between. The observed ‘lagging’ fields are consistent in magnitude with the sub-corotation effect with an effective ionospheric conductivity of ~ 1–2 mho, while the ‘leading’ fields are considerably larger than those estimated for the magnetopause-tail currents, and appear to be indicative of the presence of another dynamical process. No ‘leading’ fields were observed outside the inner region on the dawn side outbound passes, with the azimuthal fields first falling below those expected for sub-corotation, before increasing, to exceed these values at radial distances beyond ~ 15–20 RS , where the effect of the magnetopause-tail currents becomes significant. As a by-product, our investigation also indicates that modification and scaling of terrestrial magnetic field models may represent a useful approach to modelling the three-dimensional magnetic field at Saturn.Key words. Magnetospheric physics (current systems; magnetosphere-ionosphere interactions; solar wind-magnetosphere interactions)
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Luízar, O., M. V. Stepanova, J. M. Bosqued, E. E. Antonova, and R. A. Kovrazhkin. "Experimental study of the formation of inverted-V structures and their stratification using AUREOL-3 observations." Annales Geophysicae 18, no. 11 (November 30, 2000): 1399–411. http://dx.doi.org/10.1007/s00585-000-1399-6.
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Abstract. Multiple inverted-V structures are commonly observed on the same auroral zone crossing by a low-altitude orbiting satellite. Such structures appear grouped and apparently result from an ionospheric and/or magnetospheric mechanism of stratification. More than two years of AUREOL-3 satellite observations were analyzed to study their properties and their formation in the framework of the ionosphere-magnetosphere coupling model proposed by Tverskoy. This model predicts some natural periodicity in the electrostatic potential profile (and subsequently in the field-aligned current profiles) that could account for oscillations experimentally observed in the auroral zone, such as successive inverted-Vs. Experimental results obtained during quiet or moderately active periods demonstrate that the number of structures observed within a given event is well described by a 'scaling' parameter provided by the hot plasma stratification theory and expressed in terms of the field-aligned current density, the total width of the current band, the plasma sheet ion temperature, and the height-integrated Pedersen conductivity of the ionosphere. The latitudinal width, in the order of 100–200 km at ionospheric altitudes, is relatively independent of the current density, and is determined not only by the existence of a potential difference above the inverted-Vs, but also by basic oscillations of the ionosphere-magnetosphere coupling system predicted by Tverskoy. The large number of cases studied by the AUREOL-3 satellite provides reliable statistical trends which permits the validation of the model and the inference that the multiple structures currently observed can be related directly to oscillations of the magnetospheric potential (or the pressure gradients) on a scale of ~1000-2000 km in the near-Earth plasma sheet. These oscillations arise in the Tverskoy model and may naturally result when the initial pressure gradients needed to generate a large-scale field-aligned current have a sufficiently wide equatorial scale, of about 1 RE or more.Key words: Magnetospheric physics (current systems; energetic particles, precipitating; magnetosphere-ionosphere interactions)
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Nichols, J. D., and S. W. H. Cowley. "Magnetosphere-ionosphere coupling currents in Jupiter's middle magnetosphere: effect of precipitation-induced enhancement of the ionospheric Pedersen conductivity." Annales Geophysicae 22, no. 5 (April 8, 2004): 1799–827. http://dx.doi.org/10.5194/angeo-22-1799-2004.
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Abstract. We consider the effect of precipitation-induced enhancement of the Jovian ionospheric Pedersen conductivity on the magnetosphere-ionosphere coupling current system which is associated with the breakdown of the corotation of iogenic plasma in Jupiter's middle magnetosphere. In previous studies the Pedersen conductivity has been taken to be simply a constant, while it is expected to be significantly enhanced in the regions of upward-directed auroral field-aligned current, implying downward precipitating electrons. We develop an empirical model of the modulation of the Pedersen conductivity with field-aligned current density based on the modelling results of Millward et al. and compute the currents flowing in the system with the conductivity self-consistently dependent on the auroral precipitation. In addition, we consider two simplified models of the conductivity which provide an insight into the behaviour of the solutions. We compare the results to those obtained when the conductivity is taken to be constant, and find that the empirical conductivity model helps resolve some outstanding discrepancies between theory and observation of the plasma angular velocity and current system. Specifically, we find that the field-aligned current is concentrated in a peak of magnitude ~0.25µAm-2 in the inner region of the middle magnetosphere at ~20 RJ, rather than being more uniformly distributed as found with constant conductivity models. This peak maps to ~17° in the ionosphere, and is consistent with the position of the main oval auroras. The energy flux associated with the field-aligned current is ~10mWm-2 (corresponding to a UV luminosity of ~100kR), in a region ~0.6° in width, and the Pedersen conductivity is elevated from a background of ~0.05mho to ~0.7mho. Correspondingly, the total equatorial radial current increases greatly in the region of peak field-aligned current, and plateaus with increasing distance thereafter. This form is consistent with the observed profile of the current derived from Galileo magnetic field data. In addition, we find that the solutions using the empirical conductivity model produce an angular velocity profile which maintains the plasma near to rigid corotation out to much further distances than the constant conductivity model would suggest. Again, this is consistent with observations. Our results therefore suggest that, while the constant conductivity solutions provide an important indication that the main oval is indeed a result of the breakdown of the corotation of iogenic plasma, they do not explain the details of the observations. In order to resolve some of these discrepancies, one must take into account the elevation of the Pedersen conductivity as a result of auroral electron precipitation.Key words. Magnetospheric physics (current systems, magnetosphere-ionosphere interactions, planetary magnetospheres)70d
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Alberti, Tommaso, Mirko Piersanti, Antonio Vecchio, Paola De Michelis, Fabio Lepreti, Vincenzo Carbone, and Leonardo Primavera. "Identification of the different magnetic field contributions during a geomagnetic storm in magnetospheric and ground observations." Annales Geophysicae 34, no. 11 (November 22, 2016): 1069–84. http://dx.doi.org/10.5194/angeo-34-1069-2016.
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Abstract. We used the empirical mode decomposition (EMD) to investigate the time variation of the magnetospheric and ground-based observations of the Earth's magnetic field during both quiet and disturbed periods. We found two timescale variations in magnetospheric data which are associated with different magnetospheric current systems and the characteristic diurnal orbital variation, respectively. On the ground we identified three timescale variations related to the solar-wind–magnetosphere high-frequency interactions, the ionospheric processes, and the internal dynamics of the magnetosphere. This approach is able to identify the different physical processes involved in solar-wind–magnetosphere–ionosphere coupling. In addition, the large-timescale contribution can be used as a local index for the identification of the intensity of a geomagnetic storm on the ground.
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Burrell, Angeline G., Gareth Chisham, Stephen E. Milan, Liam Kilcommons, Yun-Ju Chen, Evan G. Thomas, and Brian Anderson. "AMPERE polar cap boundaries." Annales Geophysicae 38, no. 2 (April 8, 2020): 481–90. http://dx.doi.org/10.5194/angeo-38-481-2020.
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Abstract. The high-latitude atmosphere is a dynamic region with processes that respond to forcing from the Sun, magnetosphere, neutral atmosphere, and ionosphere. Historically, the dominance of magnetosphere–ionosphere interactions has motivated upper atmospheric studies to use magnetic coordinates when examining magnetosphere–ionosphere–thermosphere coupling processes. However, there are significant differences between the dominant interactions within the polar cap, auroral oval, and equatorward of the auroral oval. Organising data relative to these boundaries has been shown to improve climatological and statistical studies, but the process of doing so is complicated by the shifting nature of the auroral oval and the difficulty in measuring its poleward and equatorward boundaries. This study presents a new set of open–closed magnetic field line boundaries (OCBs) obtained from Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) magnetic perturbation data. AMPERE observations of field-aligned currents (FACs) are used to determine the location of the boundary between the Region 1 (R1) and Region 2 (R2) FAC systems. This current boundary is thought to typically lie a few degrees equatorward of the OCB, making it a good candidate for obtaining OCB locations. The AMPERE R1–R2 boundaries are compared to the Defense Meteorological Satellite Program Special Sensor J (DMSP SSJ) electron energy flux boundaries to test this hypothesis and determine the best estimate of the systematic offset between the R1–R2 boundary and the OCB as a function of magnetic local time. These calibrated boundaries, as well as OCBs obtained from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) observations, are validated using simultaneous observations of the convection reversal boundary measured by DMSP. The validation shows that the OCBs from IMAGE and AMPERE may be used together in statistical studies, providing the basis of a long-term data set that can be used to separate observations originating inside and outside of the polar cap.
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Consolini, Giuseppe, Paola De Michelis, Igino Coco, Tommaso Alberti, Maria Federica Marcucci, Fabio Giannattasio, and Roberta Tozzi. "Sign-Singularity Analysis of Field-Aligned Currents in the Ionosphere." Atmosphere 12, no. 6 (May 31, 2021): 708. http://dx.doi.org/10.3390/atmos12060708.
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Field-aligned currents (FACs) flowing in the auroral ionosphere are a complex system of upward and downward currents, which play a fundamental role in the magnetosphere–ionosphere coupling and in the ionospheric heating. Here, using data from the ESA-Swarm multi-satellite mission, we studied the complex structure of FACs by investigating sign-singularity scaling features for two different conditions of a high-latitude substorm activity level as monitored by the AE index. The results clearly showed the sign-singular character of FACs supporting the complex and filamentary nature of these currents. Furthermore, we found evidence of the occurrence of a topological change of these current systems, which was accompanied by a change of the scaling features at spatial scales larger than 30 km. This change was interpreted in terms of a sort of symmetry-breaking phenomenon due to a dynamical topological transition of the FAC structure as a consequence of FACs and substorm current wedge intensification during substorms.
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Moretto, T., N. Olsen, P. Ritter, and G. Lu. "Investigating the auroral electrojets with low altitude polar orbiting satellites." Annales Geophysicae 20, no. 7 (July 31, 2002): 1049–61. http://dx.doi.org/10.5194/angeo-20-1049-2002.
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Abstract. Three geomagnetic satellite missions currently provide high precision magnetic field measurements from low altitude polar orbiting spacecraft. We demonstrate how these data can be used to determine the intensity and location of the horizontal currents that flow in the ionosphere, predominantly in the auroral electrojets. First, we examine the results during a recent geomagnetic storm. The currents derived from two satellites at different altitudes are in very good agreement, which verifies good stability of the method. Further, a very high degree of correlation (correlation coefficients of 0.8–0.9) is observed between the amplitudes of the derived currents and the commonly used auroral electrojet indices based on magnetic measurements at ground. This points to the potential of defining an auroral activity index based on the satellite observations, which could be useful for space weather monitoring. A specific advantage of the satellite observations over the ground-based magnetic measurements is their coverage of the Southern Hemisphere, as well as the Northern. We utilize this in an investigation of the ionospheric currents observed in both polar regions during a period of unusually steady interplanetary magnetic field with a large negative Y-component. A pronounced asymmetry is found between the currents in the two hemispheres, which indicates real inter-hemispheric differences beyond the mirror-asymmetry between hemispheres that earlier studies have revealed. The method is also applied to another event for which the combined measurements of the three satellites provide a comprehensive view of the current systems. The analysis hereof reveals some surprising results concerning the connection between solar wind driver and the resulting ionospheric currents. Specifically, preconditioning of the magnetosphere (history of the interplanetary magnetic field) is seen to play an important role, and in the winther hemisphere, it seems to be harder to drive currents on the nightside than on the dayside.Key words. Ionosphere (electric fields and currents) – Magnetospheric physics (current systems; magnetosphere-ionosphere interactions)
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Lopez, Ramon E., and Daniel N. Baker. "Evidence for Particle Acceleration During Magnetospheric Substorms." International Astronomical Union Colloquium 142 (1994): 531–39. http://dx.doi.org/10.1017/s0252921100077770.
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AbstractMagnetospheric substorms represent the episodic dissipation of energy stored in the geomagnetic tail that was previously extracted from the solar wind. This energy release produces activity throughout the entire magnetosphere-ionosphere system, and it results in a wide variety of phenomena such as auroral intensifications and the generation of new current systems. All of these phenomena involve the acceleration of particles, sometimes up to several MeV. In this paper we present a brief overview of substorm phenomenology. We then review some of the evidence for particle acceleration in Earth’s magnetosphere during substorms. Such in situ observations in this most accessible of all cosmic plasma domains may hold important clues to understanding acceleration processes in more distant astrophysical systems.Subject headings: acceleration of particles — Earth — solar wind
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Gjerloev, J. W., S. Ohtani, T. Iijima, B. Anderson, J. Slavin, and G. Le. "Characteristics of the terrestrial field-aligned current system." Annales Geophysicae 29, no. 10 (October 4, 2011): 1713–29. http://dx.doi.org/10.5194/angeo-29-1713-2011.
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Abstract. We present the first ever comprehensive statistical study of the spatiotemporal characteristics of field-aligned currents in the terrestrial magnetosphere-ionosphere system using multi point measurements. We determine how the FAC density, variability and scale size are coupled. The three ST 5 satellites were in a pearls-on-a-string formation making measurements of the magnetic field with variable inter-spacecraft separations ranging from a few seconds to about 10 min. More than 4700 sets of satellite passes are analyzed using a robust correlation analysis aimed at determining the variability of the FAC system as a function of scale size and satellite spacing. We find significant differences between the FAC characteristics on the dayside and on the nightside in terms of dynamics of the current systems. On the dayside the FAC characteristics are found to be independent of IMF Bz and geomagnetic activity while the nightside indicates increased variability during disturbed conditions. The boundary separating highly and poorly correlated FACs can be fitted by a linear line for satellite separations shorter than 60 s (dayside) and 160 s (nightside). We interpret this as the dayside and nightside magnetospheric reconfiguration times respectively. For times exceeding this the FAC characteristics are suggested to be controlled by the solar wind (dayside) and plasma sheet (nightside) dynamics. Finally, the characteristics of FAC system with scale sizes larger than ~200 km (at ionospheric altitude) appear to be stable and repeatable on time scales of the order of a minute (i.e. comparable to the low-altitude orbiting satellite's traverse time across the auroral belt). In this sense, our results effectively validate the Iijima and Potemra (1978) assumption that on average the large-scale currents with scale sizes of the Region1 and Region2 are quasi-persistently significant in the transport of energy and momentum between the magnetosphere and the ionosphere.
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Belehaki, A., I. Tsagouri, and H. Mavromichalaki. "Study of the longitudinal expansion velocity of the substorm current wedge." Annales Geophysicae 16, no. 11 (November 30, 1998): 1423–33. http://dx.doi.org/10.1007/s00585-998-1423-9.
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Abstract. In this work we examine simultaneous observations from the two geosynchronous satellites GOES-5 and GOES-6 located at 282°E and 265°E respectively, and from middle and low latitude ground observatories located within 250°E and 294°E geographic longitude, during isolated substorms of moderate activity. The spatial distribution of our observation points allows us to make a detailed study of the azimuthal expansion of the substorm current wedge. The data analysis shows evidence that the substorm initiation and development mechanism include the cross-tail current diversion/ disruption, the substorm current wedge formation and the azimuthal expansion of the inner plasma sheet. The triggering mechanism is initially confined in a longitudinally narrow sector, estimated to be less than 15° and located very close to local midnight to the east or to the west. The current disruption region expands both eastward and westward in the magnetotail, so that the location of major field-aligned currents flowing into the ionosphere shifts successively eastward, and the location of the currents flowing out of the ionosphere shifts successively westward. Evidence was found that the perturbation travels toward the west with velocities greater than those expanding the wedge eastward. The drastic decrease of the velocity with the azimuthal distance from the location of the disturbance initiation, i.e., the onset sector, indicates that the energy release is a very localized phenomenon. Finally, the transient D perturbation observed by the geosynchronous satellites suggests that the field-aligned currents forming the wedge have a longitudunally limited extent.Key words. Magnetospheric physics (current systems; magnetosphere-ionosphere interactions)
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de Villiers, J. S., and P. J. Cilliers. "Applying inversion techniques to derive source currents and geoelectric fields for geomagnetically induced current calculations." Annales Geophysicae 32, no. 10 (October 16, 2014): 1263–75. http://dx.doi.org/10.5194/angeo-32-1263-2014.
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Abstract. This research focuses on the inversion of geomagnetic variation field measurement to obtain source currents in the ionosphere. During a geomagnetic disturbance, the ionospheric currents create magnetic field variations that induce geoelectric fields, which drive geomagnetically induced currents (GIC) in power systems. These GIC may disturb the operation of power systems and cause damage to grounded power transformers. The geoelectric fields at any location of interest can be determined from the source currents in the ionosphere through a solution of the forward problem. Line currents running east–west along given surface position are postulated to exist at a certain height above the Earth's surface. This physical arrangement results in the fields on the ground having the magnetic north and down components, and the electric east component. Ionospheric currents are modelled by inverting Fourier integrals (over the wavenumber) of elementary geomagnetic fields using the Levenberg–Marquardt technique. The output parameters of the inversion model are the current strength, height and surface position of the ionospheric current system. A ground conductivity structure with five layers from Quebec, Canada, based on the Layered-Earth model is used to obtain the complex skin depth at a given angular frequency. This paper presents preliminary and inversion results based on these structures and simulated geomagnetic fields. The results show some interesting features in the frequency domain. Model parameters obtained through inversion are within 2% of simulated values. This technique has applications for modelling the currents of electrojets at the equator and auroral regions, as well as currents in the magnetosphere.
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Sandholt, P. E., and C. J. Farrugia. "Plasma flows, Birkeland currents and auroral forms in relation to the Svalgaard-Mansurov effect." Annales Geophysicae 30, no. 5 (May 9, 2012): 817–30. http://dx.doi.org/10.5194/angeo-30-817-2012.
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Abstract. The traditional explanation of the polar cap magnetic deflections, referred to as the Svalgaard-Mansurov effect, is in terms of currents associated with ionospheric flow resulting from the release of magnetic tension on newly open magnetic field lines. In this study, we aim at an updated description of the sources of the Svalgaard-Mansurov effect based on recent observations of configurations of plasma flow channels, Birkeland current systems and aurorae in the magnetosphere-ionosphere system. Central to our description is the distinction between two different flow channels (FC 1 and FC 2) corresponding to two consecutive stages in the evolution of open field lines in Dungey cell convection, with FC 1 on newly open, and FC 2 on old open, field lines. Flow channel FC 1 is the result of ionospheric Pedersen current closure of Birkeland currents flowing along newly open field lines. During intervals of nonzero interplanetary magnetic field By component FC 1 is observed on either side of noon and it is accompanied by poleward moving auroral forms (PMAFs/prenoon and PMAFs/postnoon). In such cases the next convection stage, in the form of flow channel FC 2 on the periphery of the polar cap, is particularly important for establishing an IMF By-related convection asymmetry along the dawn-dusk meridian, which is a central element causing the Svalgaard-Mansurov effect. FC 2 flows are excited by the ionospheric Pedersen current closure of the northernmost pair of Birkeland currents in the four-sheet current system, which is coupled to the tail magnetopause and flank low-latitude boundary layer. This study is based on a review of recent statistical and event studies of central parameters relating to the magnetosphere-ionosphere current systems mentioned above. Temporal-spatial structure in the current systems is obtained by ground-satellite conjunction studies. On this point we emphasize the important information derived from the continuous ground monitoring of the dynamical behaviour of aurora and plasma convection during intervals of well-organised solar wind plasma and magnetic field conditions in interplanetary coronal mass ejections (ICMEs) during their Earth passage.
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Apatenkov, S. V., V. A. Sergeev, R. Pirjola, and A. Viljanen. "Evaluation of the geometry of ionospheric current systems related to rapid geomagnetic variations." Annales Geophysicae 22, no. 1 (January 1, 2004): 63–72. http://dx.doi.org/10.5194/angeo-22-63-2004.
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Abstract. To learn about the geometry and sources of the ionospheric current systems which generate strong geomagnetically induced currents, we categorize differential equivalent current systems (DEC) for events with strong dB/dt by decomposing them into the contributions of electrojet-type and vortex-type elementary systems. By solving the inverse problem we obtain amplitudes and locations of these elementary current systems. One-minute differences of the geomagnetic field values at the IMAGE magnetometer network in 1996–2000 are analysed to study the spatial distributions of large dB/dt events. The relative contributions of the two components are evaluated. In particular, we found that the majority of the strongest dB/dt events (100–1000nT/min) appear to be produced by the vortex-type current structures and most of them occur in the morning LT hours, probably caused by the Ps6 pulsation events associated with auroral omega structures. For strong dB/dt events the solar wind parameters are shifted toward strong (tens nT) southward IMF, enhanced velocity and dynamic pressure, in order for the main phase of the magnetic storms to occur. Although these events appear mostly during magnetic storms when the auroral oval greatly expands, the area of large dB/dt stays in the middle part of the auroral zone; therefore, it is connected to the processes taking part in the middle of the magnetosphere rather than in its innermost region populated by the ring current. Key words. Geomagnetism and paleomagnetism (rapid time variations) – Ionosphere (auroral ionosphere; ionospheric disturbances)
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Giannattasio, Fabio, Alessio Pignalberi, Paola De Michelis, Igino Coco, Michael Pezzopane, Roberta Tozzi, and Giuseppe Consolini. "Parallel Electrical Conductivity at Low and Middle Latitudes in the Topside Ionosphere Derived from CSES-01 Measurements." Remote Sensing 14, no. 20 (October 11, 2022): 5079. http://dx.doi.org/10.3390/rs14205079.
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The study of electrical currents in the topside ionosphere is of great importance, as it may allow a better understanding of the processes involved in the Sun–Earth interaction and magnetosphere–ionosphere–thermosphere coupling, two crucial aspects debated by the Space Weather scientific community. In this context, investigating the electrical conductivity parallel to the geomagnetic field in the topside ionosphere is of primary importance because: (1) it provides information on the capability of the ionosphere to conduct currents; (2) it relates current density and electric field through Ohm’s law; (3) it can help to quantify the dissipation of currents; (4) it is generally modeled and not locally measured by in situ missions. In this work, we used in situ measurements of electron density and temperature recorded between 2019 and 2021 by the China Seismo-Electromagnetic Satellite (CSES-01) flying with an orbital inclination of 97.4∘ and at an altitude of about 500 km to compute the parallel electrical conductivity in the topside ionosphere at low and middle latitudes at the two fixed local times (LT) characterizing the CSES-01 mission: around 02 and 14 LT. The results, which are discussed in light of previous literature, highlight the dependence of conductivity on latitude and longitude and are compared with those obtained using values both measured by the Swarm B satellite (flying at a similar altitude) and modeled by the International Reference Ionosphere in the same time period. In particular, we found a diurnal variation in parallel electrical conductivity, with a slight hemispheric asymmetry. Daytime features are compatible with Sq and equatorial electrojet current systems, containing “anomalous” low values of conductivity in correspondence with the South Atlantic region that could be physical in nature.
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Ritter, P., H. Lühr, A. Viljanen, O. Amm, A. Pulkkinen, and I. Sillanpää. "Ionospheric currents estimated simultaneously from CHAMP satelliteand IMAGE ground-based magnetic field measurements: a statisticalstudy at auroral latitudes." Annales Geophysicae 22, no. 2 (January 1, 2004): 417–30. http://dx.doi.org/10.5194/angeo-22-417-2004.
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Abstract. One important contribution to the magnetic field measured at satellite altitude and at ground level comes from the external currents. We used the total field data sampled by the Overhauser Magnetometer on CHAMP and the horizontal magnetic field measurements of the IMAGE ground-based magnetometer network to study the ionospheric Hall current system in the auroral regions. For the CHAMP data a current model consisting of a series of lines and placed at a height of 110km is fitted to the magnetic field signature sampled on the passage across the polar region. The derived current distributions depend, among others, on season and on the local time of the satellite track. At dawn/dusk the auroral electrojets can be detected most clearly in the auroral regions. Their intensity and location are evidently correlated with the A E activity index. For a period of almost two years the results obtained from space and the currents determined from ground-based observations are studied. For the full IMAGE station array a newly-developed method of spherical elementary current systems (SECS) is employed to compute the 2-D equivalent current distribution, which gives a detailed picture of an area covering latitudes 60° – 80° N and 10° – 30° E in the auroral region. Generally, the current estimates from satellite and ground are in good agreement. The results of this survey clearly show the average dependence of the auroral electrojet on season and local time. This is particularly true during periods of increased auroral activity. The correlation coefficient of the results is close to one in the region of sizeable ionospheric current densities. Also the ratio of the current densities, as determined from above and below the ionosphere, is close to unity. It is the first time that the method of Hall current estimate from a satellite has been validated quantitatively by ground-based observations. Among others, this result is of interest for magnetic main field modelling, since it demonstrates that ground-based observations can be used to predict electrojet signatures in satellite magnetic field scalar data. Key words. Ionosphere (auroral Ionosphere; electric fields and currents; ionosphere-magnetosphere interactions)
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Baumgardner, J., J. Wroten, J. Semeter, J. Kozyra, M. Buonsanto, P. Erickson, and M. Mendillo. "A very bright SAR arc: implications for extreme magnetosphere-ionosphere coupling." Annales Geophysicae 25, no. 12 (January 2, 2007): 2593–608. http://dx.doi.org/10.5194/angeo-25-2593-2007.
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Abstract. In contrast to the polar aurora visible during geomagnetic storms, stable auroral red (SAR) arcs offer a sub-visual manifestation of direct magnetosphere-ionosphere (M-I) coupling at midlatitudes. The SAR arc emission at 6300 Å is driven by field-aligned magnetospheric energy transport from ring current/plasmapause locations into the ionosphere-thermosphere system. The first SAR arc was observed at the dawn of the space age (1956), and the typical brightness levels and occurrence patterns obtained from subsequent decades of observations appear to be consistent with the downward heat conduction theory, i.e., heated ambient F-layer electrons excite oxygen atoms to produce a spectrally pure emission. On very rare occasions, a SAR arc has been reported to be at brightness levels visible to the naked eye. Here we report on the first case of a very bright SAR arc (~13 kilo-Rayleighs) observed by four diagnostic systems that sampled various aspects of the sub-auroral domain near Millstone Hill, MA, on the night of 29 October 1991: an imaging spectrograph, an all-sky camera, an incoherent scatter radar (ISR), and a DMSP satellite. Simulations of emission using the ISR and DMSP data with the MSIS neutral atmosphere succeed in reproducing the brightness levels observed. This provides a robust confirmation of M-I coupling theory in its most extreme aeronomic form within the innermost magnetosphere (L~2) during a rare superstorm event. The unusually high brightness value appears to be due to the rare occurrence of the heating of dense ionospheric plasma just equatorward of the trough/plasmapause location, in contrast to the more typical heating of the less dense F-layer within the trough.
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Tsurutani, Bruce T., and Rajkumar Hajra. "Energetics of Shock-triggered Supersubstorms (SML < −2500 nT)." Astrophysical Journal 946, no. 1 (March 1, 2023): 17. http://dx.doi.org/10.3847/1538-4357/acb143.
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Abstract The solar wind energy input and dissipation in the magnetospheric–ionospheric systems of 17 supersubstorms (SSSs: SML < −2500 nT) triggered by interplanetary shocks during solar cycles 23 and 24 are studied in detail. The SSS events had durations ranging from ∼42 minutes to ∼6 hr, and SML intensities ranging from −2522 nT to −4143 nT. Shock compression greatly strengthens the upstream interplanetary magnetic field southward component (B s), and thus, through magnetic reconnection at the Earth’s dayside magnetopause, greatly enhances the solar wind energy input into the magnetosphere and ionosphere during the SSS events studied. The additional solar wind magnetic reconnection energy input supplements the ∼1.5 hr precursor (growth-phase) energy input and both supply the necessary energy for the high-intensity, long-duration SSS events. Some of the solar wind energy is immediately deposited in the magnetosphere/ionosphere system, and some is stored in the magnetosphere/magnetotail system. During the SSS events, the major part of the solar wind input energy is dissipated into Joule heating (∼30%), with substantially less energy dissipation in auroral precipitation (∼3%) and ring current energy (∼2%). The remainder of the solar wind energy input is probably lost down the magnetotail. It is found that during the SSS events, the dayside Joule heating is comparable to that of the nightside Joule heating, giving a picture of the global energy dissipation in the magnetospheric/ionospheric system, not simply a nightside-sector substorm effect. Several cases are shown where an SSS is the only substorm that occurs during a magnetic storm, essentially equating the two phenomena for these cases.
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Kakad, Bharati, Amar Kakad, Durbha Sai Ramesh, and Gurbax S. Lakhina. "Diminishing activity of recent solar cycles (22–24) and their impact on geospace." Journal of Space Weather and Space Climate 9 (2019): A1. http://dx.doi.org/10.1051/swsc/2018048.
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This study examines the variation of different energies linked with the Sun and the Earth’s magnetosphere-ionosphere systems for solar cycles (SCs) 22–24 for which the gradual decrease in the solar activity is noticed. Firstly, we investigated the variation of solar magnetic energy density (SMED) for SCs 21–24 and its relation to the solar activity. We observed distinct double peak structures in SMED for the past four SCs, 21–24. This feature is consistent with noticeable asymmetry in their two peaks. For SCs 22–24 a significant decrease is observed in the integrated SMED of each SC. This reduction is 37% from SCs 22 to 23 and 51% from SCs 23 to 24, which indicates substantial weakening of Sun’s magnetic field for SC 24. Also, the magnetic, kinetic, and thermal energy densities at the Earth’s bow-shock nose are found to be considerably low for the SC 24. We examined the solar wind Alfven speed, magnetosonic Mach number, solar wind-magnetosphere energy coupling parameter (ε), and the Chapman-Ferraro magnetopause distance (LCF) for the SCs 22–24. The estimated maximum stand-off magnetopause distance is larger for SC 24 (LCF ≤ 10.6 RE) as compared to SC 23 (LCF ≤ 10.2 RE) and SC 22 (LCF ≤ 9.8 RE). The solar wind Alfven speeds during SCs 22 and 23 are in the same range and do not exceed ≈73 km/s whereas, it is below 57 km/s for SC 24. A lower bound of solar wind magnetosonic Mach number for SC 24 is larger (M ≥ 6.9) as compared to SC 22 (M ≥ 5.9) and SC 23 (M ≥ 6). We noticed weakening in the energy coupling parameter for SC 24, which resulted in substantial (15%–38%) decrease in average strength of high latitude ionospheric (AE), low latitude magnetospheric (Dst) and equatorial ionospheric (EEJ) current systems in comparison with SC 23. Subsequently, a reduction of ≈30% is manifested in the high latitude Joule heating for SC 24. Overall this study indicates the significant step down in various energies at Sun, Earth’s bow-shock, and near Earth environment for current SC 24, which will have important implication on our Earth’s atmosphere-ionosphere-magnetosphere system.
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Pulkkinen, A., A. Thomson, E. Clarke, and A. McKay. "April 2000 geomagnetic storm: ionospheric drivers of large geomagnetically induced currents." Annales Geophysicae 21, no. 3 (March 31, 2003): 709–17. http://dx.doi.org/10.5194/angeo-21-709-2003.
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Abstract. Geomagnetically induced currents (GIC) flowing in technological systems on the ground are a direct manifestation of space weather. Due to the proximity of very dynamic ionospheric current systems, GIC are of special interest at high latitudes, where they have been known to cause problems, for example, for normal operation of power transmission systems and buried pipelines. The basic physics underlying GIC, i.e. the magnetosphere – ionosphere interaction and electromagnetic induction in the ground, is already quite well known. However, no detailed study of the drivers of GIC has been carried out and little is known about the relative importance of different types of ionospheric current systems in terms of large GIC. In this study, the geomagnetic storm of 6–7 April 2000 is investigated. During this event, large GIC were measured in technological systems, both in Finland and in Great Britain. Therefore, this provides a basis for a detailed GIC study over a relatively large regional scale. By using GIC data and corresponding geomagnetic data from north European magnetometer networks, the ionospheric drivers of large GIC during the event were identified and analysed. Although most of the peak GIC during the storm were clearly related to substorm intensifications, there were no common characteristics discernible in substorm behaviour that could be associated with all the GIC peaks. For example, both very localized ionospheric currents structures, as well as relatively large-scale propagating structures were observed during the peaks in GIC. Only during the storm sudden commencement at the beginning of the event were large-scale GIC evident across northern Europe with coherent behaviour. The typical duration of peaks in GIC was also quite short, varying between 2–15 min.Key words. Geomagnetism and paleo-magnetism (geomagnetic induction) – Ionosphere (ionospheric disturbances) – Magnetospheric physics (storms and substorms)
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Feldstein, Y. I., L. I. Gromova, A. Grafe, C. I. Meng, V. V. Kalegaev, I. I. Alexeev, and Y. P. Sumaruk. "Auroral electrojet dynamics during magnetic storms, connection with plasma precipitation and large-scale structure of the magnetospheric magnetic field." Annales Geophysicae 17, no. 4 (April 30, 1999): 497–507. http://dx.doi.org/10.1007/s00585-999-0497-3.
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Abstract. Effect of the equatorward shift of the eastward and westward electrojets during magnetic storms main phase is analyzed based on the meridional chains of magnetic observatories EISCAT and IMAGE and several Russian observatories (geomagnetic longitude ~110°, corrected geomagnetic latitudes 74°F 51°.) Magnetic storms of various Dst index intensity where the main phase falls on 1000 UT - 2400 UT interval were selected so that one of the observatory chains was located in the afternoon - near midnight sector of MLT. The eastward electrojet center shifts equatorward with Dst intensity increase: when Dst ~ - 50 nT the electrojet center is located at F ~ 62°, when Dst ~ -300 nT it is placed at F ~54°. The westward electrojet center during magnetic storms main phase for intervals between substorms shifts equatorward with Dst increase: at F~ 62° when Dst ~ -100 nT and at F ~ 55° when Dst ~ -300 nT. During substorms within the magnetic storms intervals the westward electrojet widens poleward covering latitudes F~ 64°- 65°. DMSP (F08, F10 and F11) satellite observations of auroral energy plasma precipitations at upper atmosphere altitudes were used to determine precipitation region structure and location of boundaries of various plasma domains during magnetic storms on May 10-11, 1992, February 5-7 and February 21-22, 1994. Interrelationships between center location, poleward and equatorward boundaries of electrojets and characteristic plasma regions are discussed. The electrojet center, poleward and equatorward boundaries along the magnetic observatories meridional chain were mapped to the magnetosphere using the geomagnetic field paraboloid model. The location of auroral energy oxygen ion regions in the night and evening magnetosphere is determined. Considerations are presented on the mechanism causing the appearance in the inner magnetosphere during active intervals of magnetic storms of ions with energy of tens KeV. In the framework of the magnetospheric magnetic field paraboloid model the influence of the ring current and magnetospheric tail plasma sheet currents on large-scale magnetosphere structure is considered.Key words. Ionosphere (particle precipitation) · Magnetospheric physics (current systems; magnetospheric configuration and dynamics).
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Neudegg, D. A., S. W. H. Cowley, K. A. McWilliams, M. Lester, T. K. Yeoman, J. Sigwarth, G. Haerendel, et al. "The UV aurora and ionospheric flows during flux transfer events." Annales Geophysicae 19, no. 2 (February 28, 2001): 179–88. http://dx.doi.org/10.5194/angeo-19-179-2001.
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Abstract. Far Ultra Violet (FUV) signatures in the polar ionosphere during a period of magnetopause reconnection are compared with ionospheric flows measured in the cusp ‘throat’ and dusk cell by the CUTLASS Hankasalmi HF radar. Regions of peak FUV emission in the 130.4 nm and 135.6 nm range, observed by the Polar spacecraft’s VIS Earth Camera, consistently lie at the turning point of the flows from the dusk cell, poleward into the throat, and at the equatorward edge of the region of high and varied radar spectral-width associated with the cusp. The Equator-S spacecraft was near the magnetopause at the time of the ionospheric observations and geomagnetically conjugate with the region of ionosphere observed by the radar. Flux transfer events (FTEs), suggestive of bursty reconnection between the IMF and geomagnetic fields, were observed by Equator-S prior to and during the periods of high FUV emission. Enhanced poleward ionospheric flow velocities in the polar cusp region, previously shown to be associated with bursty reconnection, consistently lie poleward of the enhanced FUV optical feature. The enhanced optical feature is consistent with the expected position of the largest upward region 1 field-aligned current, associated with electron precipitation, on the dusk edge of the merging gap. The optical feature moves duskward and equatorward during the course of the reconnection sequence, consistent with expansion of the merging line and the polar cap with newly added open magnetic flux by the FTEs. The DMSP F14 spacecraft passed through the enhanced FUV region and measured strong, structured electron precipitation far greater than in the adjacent regions.Key words. Magnetospheric physics (current systems; magnetopause, cusp and boundary layers; magnetosphere-ionosphere interactions)
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Vanhamäki, H., O. Amm, and A. Viljanen. "Role of inductive electric fields and currents in dynamical ionospheric situations." Annales Geophysicae 25, no. 2 (March 8, 2007): 437–55. http://dx.doi.org/10.5194/angeo-25-437-2007.
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Abstract. We study the role of ionospheric induction in different commonly observed ionospheric situations. These include an intensifying electrojet, westward travelling surge (WTS) and Ω-band. We use data based, realistic models for these phenomena and calculate the inductive electric fields that are created due to the temporal variations of ionospheric currents. The ionospheric induction problem is solved using a new calculation technique that can handle non-uniform, time-dependent conductances and electric fields of any geometry. We find that in some situations inductive effects are not negligible and the ionospheric electric field is not a pure potential field, but has a significant induced rotational part. In the WTS and Ω-band models the induced electric field is concentrated in a small area, where the time derivatives are largest. In the electrojet model the induced field is significant over a large part of the jet area. In these examples the induced electric field has typical values of few mV/m, which amounts to several tens of percents of the potential electric field present at the same locations. The induced electric field is associated with ionospheric and field aligned currents (FAC), that modify the overall structure of the current systems. Especially the induced FAC are often comparable to the non-inductive FAC, and may thus modify the coupling between the ionosphere and magnetosphere in the most dynamical situations. We also present some examples with very simple ionospheric current systems, where the effect of different ionospheric parameters on the induction process is studied.
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Mishin, V. M., V. V. Mishin, S. B. Lunyushkin, J. Y. Wang, and A. V. Moiseev. "27 August 2001 substorm: Preonset phenomena, two main onsets, field-aligned current systems, and plasma flow channels in the ionosphere and in the magnetosphere." Journal of Geophysical Research: Space Physics 122, no. 5 (May 2017): 4988–5007. http://dx.doi.org/10.1002/2017ja023915.
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Wild, J. A., S. E. Milan, C. J. Owen, J. M. Bosqued, M. Lester, D. M. Wright, H. Frey, C. W. Carlson, A. N. Fazakerley, and H. Rème. "The location of the open-closed magnetic field line boundary in the dawn sector auroral ionosphere." Annales Geophysicae 22, no. 10 (November 3, 2004): 3625–39. http://dx.doi.org/10.5194/angeo-22-3625-2004.
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Abstract. As a measure of the degree of coupling between the solar wind-magnetosphere-ionosphere systems, the rate at which the size of the polar cap (the region corresponding to ionospheric termini of open magnetic flux tubes) varies is of prime importance. However, a reliable technique by which the extent of the polar cap might be routinely monitored has yet to be developed. Current techniques provide particularly ambiguous indications of the polar cap boundary in the dawn sector. We present a case study of space- and ground-based observations of the dawn-sector auroral zone and attempt to determine the location of the polar cap boundary using multi-wavelength observations of the ultraviolet aurora (made by the IMAGE FUV imager), precipitating particle measurements (recorded by the FAST, DMSP, and Cluster 1 and 3 satellites), and SuperDARN HF radar observations of the ionospheric Doppler spectral width boundary. We conclude that in the dawn sector, during the interval presented, neither the poleward edge of the wideband auroral UV emission (140-180nm) nor the Doppler spectral width boundary were trustworthy indicators of the polar cap boundary location, while narrow band UV emissions in the range 130-140nm appear to be much more reliable.
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Amm, O., A. Aikio, J. M. Bosqued, M. Dunlop, A. Fazakerley, P. Janhunen, K. Kauristie, et al. "Mesoscale structure of a morning sector ionospheric shear flow region determined by conjugate Cluster II and MIRACLE ground-based observations." Annales Geophysicae 21, no. 8 (August 31, 2003): 1737–51. http://dx.doi.org/10.5194/angeo-21-1737-2003.
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Abstract. We analyse a conjunction event of the Cluster II spacecraft with the MIRACLE ground-based instrument net-work in northern Fennoscandia on 6 February 2001, between 23:00 and 00:00 UT. Shortly after the spacecraft were located at perigee, the Cluster II satellites’ magnetic footpoints move northwards over Scandinavia and Svalbard, almost perfectly aligned with the central chain of the IMAGE magnetometer network, and cross a morning sector ionospheric shear zone during this passage. In this study we focus on the mesoscale structure of the ionosphere. Ionospheric conductances, true horizontal currents, and field-aligned currents (FAC) are calculated from the ground-based measurements of the IMAGE magnetometers and the STARE coherent scatter radar, using the 1-D method of characteristics. An excellent agreement between these results and the FAC observed by Cluster II is reached after averaging the Cluster measurements to mesoscales, as well as between the location of the convection reversal boundary (CRB), as observed by STARE and by the Cluster II EFW instrument. A sheet of downward FAC is observed in the vicinity of the CRB, which is mainly caused by the positive divergence of the electric field there. This FAC sheet is detached by 0.5°–2° of latitude from a more equatorward downward FAC sheet at the poleward flank of the westward electrojet. This latter FAC sheet, as well as the upward FAC at the equatorward flank of the jet, are mainly caused by meridional gradients in the ionospheric conductances, which reach up to 25 S in the electrojet region, but only ~ 5 S poleward of it, with a minimum at the CRB. Particle measurements show that the major part of the downward FAC is carried by upward flowing electrons, and only a small part by downward flowing ions. The open-closed field line boundary is found to be located 3°–4° poleward of the CRB, implying significant errors if the latter is used as a proxy of the former.Key words. Ionosphere (electric fields and currents) – Magnetosphere physics (current systems; plasma convection)
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Feldstein, Y. I., L. I. Gromova, J. Woch, I. Sandahl, L. Blomberg, G. Marklund, and C. I. Meng. "Structure of the auroral precipitation region in the dawn sector: relationship to convection reversal boundaries and field-aligned currents." Annales Geophysicae 19, no. 5 (May 31, 2001): 495–519. http://dx.doi.org/10.5194/angeo-19-495-2001.
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Abstract. Abstract. Simultaneous DMSP F7 and Viking satellite measurements of the dawnside high-latitude auroral energy electron and ion precipitation show that the region of the low and middle altitude auroral precipitation consists of three characteristic plasma regimes. The recommendation of the IAGA Working Group IIF/III4 at the IAGA Assembly in Boulder, July 1995 to decouple the nomenclature of ionospheric populations from magnetospheric population is used for their notation. The most equatorial regime is the Diffuse Auroral Zone (DAZ) of diffuse spatially unstructured precipitating electrons. It is generated by the plasma injection to the inner magnetosphere in the nightside and the subsequent drift plasma to the dawnside around the Earth. Precipitating particles have a hard spectrum with typical energies of electrons and ions of more than 3 keV. In the DAZ, the ion pitch-angle distribution is anisotropic, with the peak near 90°. The next part is the Auroral Oval (AO), a structured electron regime which closely resembles the poleward portion of the night-side auroral oval. The typical electron energy is several keV, and the ion energy is up to 10 keV. Ion distributions are pre-dominantly isotropic. In some cases, this plasma regime may be absent in the pre-noon sector. Poleward of the Auroral Oval, there is the Soft Small Scale Luminosity (SSSL) regime. It is caused by structured electron and ion precipitation with typical electron energy of about 0.3 keV and ion energy of about 1 keV. The connection of these low-altitude regimes with plasma domains of the distant magnetosphere is discussed. For mapping of the plasma regimes to the equatorial plane of the magnetosphere, the empirical model by Tsyganenko (1995) and the conceptual model by Alexeev et al. (1996) are used. The DAZ is mapped along the magnetic field lines to the Remnant Layer (RL), which is located in the outer radiation belt region; the zone of structured electrons and isotropic ion precipitation (AO) is mapped to the dawn periphery of the Central Plasma Sheet (CPS); the soft small scale structured precipitation (SSSL) is mapped to the outer magnetosphere close to the magnetopause, i.e. the Low Latitude Boundary Layer (LLBL). In the near-noon sector, earthward fluxes of soft electrons, which cause the Diffuse Red Aurora (DRA), are observed. The ion energies decrease with increasing latitude. The plasma spectra of the DRA regime are analogous to the spectra of the Plasma Mantle (PM). In the dawn sector, the large-scale field-aligned currents flow into the ionosphere at the SSSL latitudes (Region 1) and flow out at the AO or DAZ latitudes (Region 2). In the dawn and dusk sectors, the large-scale Region 1 and Region 2 FAC generation occurs in different plasma domains of the distant magnetosphere. The dawn and dusk FAC connection to the traditional Region 1 and Region 2 has only formal character, as FAC generating in various magnetospheric plasma domains integrate in the same region (Region 1 or Region 2). In the SSSL, there is anti-sunward convection; in the DAZ and the AO, there is the sunward convection. At PM latitudes, the convection is controlled by the azimuthal IMF component (By ). It is suggested to extend the notation of the plasma pattern boundaries, as proposed by Newell et al. (1996), for the nightside sector of the auroral oval to the dawn sector.Key words. Magnetospheric physics (current systems; magnetospheric configuration and dynamics; plasma convection)
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Alberti, Tommaso, Davide Faranda, Giuseppe Consolini, Paola De Michelis, Reik V. Donner, and Vincenzo Carbone. "Concurrent Effects between Geomagnetic Storms and Magnetospheric Substorms." Universe 8, no. 4 (April 6, 2022): 226. http://dx.doi.org/10.3390/universe8040226.
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An accurate understanding of dissimilarities in geomagnetic variability between quiet and disturbed periods has the potential to vastly improve space weather diagnosis. In this work, we exploit some recently developed methods of dynamical system theory to provide new insights and conceptual ideas in space weather science. In particular, we study the co-variation and recurrence statistics of two geomagnetic indices, SYM-H and AL, that measure the intensity of the globally symmetric component of the equatorial electrojet and that of the westward auroral electrojet, respectively. We find that the number of active degrees of freedom, required to describe the phase space dynamics of both indices, depends on the geomagnetic activity level. When the magnetospheric substorm activity, as monitored by the AL index, increases, the active number of degrees of freedom increases at high latitudes above the dimension obtained through classical time delay embedding methods. Conversely, a reduced number of degrees of freedom is observed during geomagnetic storms at low latitude by analysing the SYM-H index. By investigating time-dependent relations between both indices we find that a significant amount of information is shared between high and low latitude current systems originating from coupling mechanisms within the magnetosphere–ionosphere system as the result of a complex interplay between processes and phenomena of internal origin activated by the triggering of external source processes. Our observations support the idea that the near-Earth electromagnetic environment is a complex system far from an equilibrium.
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Kellinsalmi, Mirjam, Ari Viljanen, Liisa Juusola, and Sebastian Käki. "The time derivative of the geomagnetic field has a short memory." Annales Geophysicae 40, no. 4 (August 30, 2022): 545–62. http://dx.doi.org/10.5194/angeo-40-545-2022.
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Abstract. Solar eruptions and other types of space weather effects can pose a hazard to the high voltage power grids via geomagnetically induced currents (GICs). In worst cases, they can even cause large-scale power outages. GICs are a complex phenomenon, closely related to the time derivative of the geomagnetic field. However, the behavior of the time derivative is chaotic and has proven to be tricky to predict. In our study, we look at the dynamics of the geomagnetic field during active space weather. We try to characterize the magnetic field behavior, to better understand the drivers behind strong GIC events. We use geomagnetic data from the IMAGE (International Monitor for Auroral Geomagnetic Effect) magnetometer network between 1996 and 2018. The measured geomagnetic field is primarily produced by currents in the ionosphere and magnetosphere, and secondarily by currents in the conducting ground. We use the separated magnetic field in our analysis. The separation of the field means that the measured magnetic field is computationally divided into external and internal parts corresponding to the ionospheric and telluric origin, respectively. We study the yearly directional distributions of the baseline subtracted, separated horizontal geomagnetic field, ΔH, and its time derivative, dΔH/dt. The yearly distributions do not have a clear solar cycle dependency. The internal field distributions are more scattered than the external field. There are also clear, station-specific differences in the distributions related to sharp conductivity contrasts between continental and ocean regions or to inland conductivity anomalies. One of our main findings is that the direction of dΔH/dt has a very short “reset time“, around 2 min, but ΔH does not have this kind of behavior. These results hold true even with less active space weather conditions. We conclude that this result gives insight into the time scale of ionospheric current systems, which are the primary driver behind the time derivative's behavior. It also emphasizes a very short persistence of dΔH/dt compared to ΔH, and highlights the challenges in forecasting dΔH/dt (and GIC).
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Johansson, T., T. Karlsson, G. Marklund, S. Figueiredo, P. A. Lindqvist, and S. Buchert. "A statistical study of intense electric fields at 4−7 R<sub><i>E</i></sub> geocentric distance using Cluster." Annales Geophysicae 23, no. 7 (October 14, 2005): 2579–88. http://dx.doi.org/10.5194/angeo-23-2579-2005.
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Abstract. Intense high-latitude electric fields (>150 mV/m mapped to ionospheric altitude) at 4–7 RE geocentric distance have been investigated in a statistical study, using data from the Cluster satellites. The orbit of the Cluster satellites limits the data collection at these altitudes to high latitudes, including the poleward part of the auroral oval. The occurrence and distribution of the selected events have been used to characterize the intense electric fields and to investigate their dependance on parameters such as MLT, CGLat, altitude, and also Kp. Peaks in the local time distribution are found in the evening to morning sectors but also in the noon sector, corresponding to cusp events. The electric field intensities decrease with increasing latitude in the region investigated (above 60 CGLat). A dependence on geomagnetic activity is indicated since the probability of finding an event increases up to Kp=5–6. The scales sizes are in the range up to 10 km (mapped to ionospheric altitude) with a maximum around 4–5km, consistent with earlier findings at lower altitudes and Cluster event studies. The magnitudes of the electric fields are inversely proportional to the scale sizes. The type of electric field structure (convergent or divergent) is consistent with the FAC direction for a subset of events with electric field intensities in the range 500–1000 mV/m and with clear bipolar signatures. The FAC directions are also consistent with the Region 1 and NBZ current systems, the latter of which prevail only during northward IMF conditions. For scale sizes less than 2 km the majority of the events were divergent electric field structures. Both converging and diverging electric fields were found throughout the investigated altitude range (4–7 RE geocentric distance). Keywords. Magnetospheric physics (Electric fields; Auroral phenomena; Magnetosphere-ionosphere interactions)
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Bannister, N. P., E. J. Bunce, S. W. H. Cowley, R. Fairbend, G. W. Fraser, F. J. Hamilton, J. S. Lapington, et al. "A Wide Field Auroral Imager (WFAI) for low Earth orbit missions." Annales Geophysicae 25, no. 2 (March 8, 2007): 519–32. http://dx.doi.org/10.5194/angeo-25-519-2007.
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Abstract. A comprehensive understanding of the solar wind interaction with Earth's coupled magnetosphere-ionosphere system requires an ability to observe the charged particle environment and auroral activity from the same platform, generating particle and photon image data which are matched in time and location. While unambiguous identification of the particles giving rise to the aurora requires a Low Earth Orbit satellite, obtaining adequate spatial coverage of aurorae with the relatively limited field of view of current space bourne auroral imaging systems requires much higher orbits. A goal for future satellite missions, therefore, is the development of compact, wide field-of-view optics permitting high spatial and temporal resolution ultraviolet imaging of the aurora from small spacecraft in low polar orbit. Microchannel plate optics offer a method of achieving the required performance. We describe a new, compact instrument design which can observe a wide field-of-view with the required spatial resolution. We report the focusing of 121.6 nm radiation using a spherically-slumped, square-pore microchannel plate with a focal length of 32 mm and an F number of 0.7. Measurements are compared with detailed ray-trace simulations of imaging performance. The angular resolution is 2.7±0.2° for the prototype, corresponding to a footprint ~33 km in diameter for an aurora altitude of 110 km and a spacecraft altitude of 800 km. In preliminary analysis, a more recent optic has demonstrated a full width at half maximum of 5.0±0.3 arcminutes, corresponding to a footprint of ~1 km from the same spacecraft altitude. We further report the imaging properties of a convex microchannel plate detector with planar resistive anode readout; this detector, whose active surface has a radius of curvature of only 100 mm, is shown to meet the spatial resolution and sensitivity requirements of the new wide field auroral imager (WFAI).
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Sarafopoulos, D. V. "Distinct solar wind pressure pulses producing convection twin-vortex systems in the ionosphere." Annales Geophysicae 22, no. 6 (June 14, 2004): 2201–11. http://dx.doi.org/10.5194/angeo-22-2201-2004.
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Abstract. We provide conclusive observational evidence demonstrating that a solar wind pressure pulse produces a twin-vortex system of ionospheric currents, while a stepwise pressure increase/decrease creates a single vortex structure, at high-latitude ground magnetograms. Multi-satellite (Wind, Geotail, Interball, IMP 8 and GOES 8) and multi-instrument observations of plasma and magnetic field confirm with in situ measurements that solar wind inherent pressure pulses and stepwise variations strike on the Earth's magnetosphere in a global scale. The ground signatures are scrutinized in detail and the twin- or single-vortex current systems are studied using the IMAGE array stations. One more finding of this work is that a single vortex can produce monopolar and bipolar features, depending on the station position relative to the overhead travelling convection vortex (TCV). All of the above observational evidence confirms Glassmeier's (1992) prediction model that associates solar wind's steep changes of pressure with ionospheric TCVs.
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Zhang, Q. H., M. W. Dunlop, R. Holme, and E. E. Woodfield. "Comparison of eight years magnetic field data from Cluster with Tsyganenko models in the inner magnetosphere." Annales Geophysicae 28, no. 1 (January 29, 2010): 309–26. http://dx.doi.org/10.5194/angeo-28-309-2010.
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Abstract. Eight years of magnetic field data, taken while the four Cluster spacecraft pass through, or adjacent to, the equatorial ring current, have been surveyed to investigate the effects on the Earth's magnetic field of the externally driven current systems connecting the ionosphere, cusp and ring current regions. This study extends previous work to cover a greater range of orbit location and external conditions. We compare the modeled magnetic field from different global field models (Tsyganenko, 1989, 1996, and 2001, hereafter T89, T96 and T01) with data from the four Cluster spacecraft. Comparing with the different models allow us not only to characterize each model's performance, but also provides insight into the physical sources of observed signals. The data generally deviate much less from the expected model field during the years close to the solar minimum, implying that the models perform better during weaker geomagnetic activity. There are particular deviations from the models associated with the ring current (well-defined smooth trends) and region 2 field aligned currents (FACs) or low-altitude cusp FACs (sharp bipolar signatures). During the ring current crossings (through perigee, at 4–5 RE), the T96 model always overestimates the ring current, while the T01 and T89 models sometimes underestimate it. The sharp bipolar signatures are not always sampled, implying a localized extent, but only the T96 and T01 models include forms for the region 2 FACs and T01 appears to model these better. Overall, all deviations from T01 are much smaller than for the other models, indicating that this model achieves the best fit to the data. The 4 Cluster spacecraft observe nearly the same signatures at small separations (during the early years of the mission) but do sample different signatures at the large separations (during the later years). Using the four-spacecraft technique, we infer that the region 2 FACs, with a transverse thickness of ~0.17–0.54 RE, and cusp FACs, with a thickness of ~0.06–0.12 RE, are very stable in size and location.
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Le, Guan, Delores J. Knipp, Lutz Rastätter, Gang Lu, Dogacan Su Ozturk, James A. Slavin, Astrid Maute, et al. "Next generation magnetic field measurements from low-earth orbit satellites enable enhanced space weather operations." Frontiers in Astronomy and Space Sciences 9 (November 18, 2022). http://dx.doi.org/10.3389/fspas.2022.1076892.
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Large-scale current systems in the ionosphere and the magnetosphere are intimately controlled by the solar wind-magnetosphere interaction and the magnetosphere-ionosphere coupling. During space weather events, these currents reconfigure and intensify significantly in response to enhanced solar wind-magnetosphere interaction, facilitating explosive energy input from the magnetosphere into the ionosphere-thermosphere system and inducing electric current surges in electric power systems on the ground. Therefore, measurements of magnetic manifestations associated with the dynamic changes of the current systems are crucial for specifying the energy input into the ionosphere-thermosphere system, understanding energy dissipation mechanisms, and predicting the severity of their space weather impacts. We investigate the potential uses of high-quality magnetic field data for space weather operations and propose real-time data products from next generation constellation missions that enable improved space weather forecasting and mitigation.
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Borovsky, Joseph E. "On the Saturation (or Not) of Geomagnetic Indices." Frontiers in Astronomy and Space Sciences 8 (October 14, 2021). http://dx.doi.org/10.3389/fspas.2021.740811.
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Most geomagnetic indices are associated with processes internal to the magnetosphere-ionosphere system: convection, magnetosphere-ionosphere current systems, particle pressure, ULF wave activity, etc. The saturation (or not) of various geomagnetic indices under various solar-wind driver functions (a.k.a. coupling functions) is explored by examining plots of the various indices as functions of the various driver functions. In comparing an index with a driver function, “saturation” of the index means that the trend of stronger geomagnetic activity with stronger driving weakens in going from mid-range driving to very strong driving. Issues explored are 1) whether the nature of the index matters (i.e., what the index measures and how the index measures it), 2) the relation of index saturation to cross-polar-cap potential saturation, and 3) the role of the choice of the solar-wind driver function. It is found that different geomagnetic indices exhibit different amounts of saturation. For example the SuperMAG auroral-electrojet indices SME, SML, and SMU saturate much less than do the auroral-electrojet indices AE, AL, and AU. Additionally it is found that different driver functions cause an index to show different degrees of saturation. Dividing a solar-wind driver function by the theoretical cross-polar-cap-potential correction (1+Q) often compensates for the saturation of an index, even though that index is associated with internal magnetospheric processes whereas Q is derived for solar-wind processes. There are composite geomagnetic indices E(1) that show no saturation when matched to their composite solar-wind driver functions S(1). When applied to other geomagnetic indices, the composite S(1) driver functions tend to compensate for index saturation at strong driving, but they also tend to introduce a nonlinearity at weak driving.
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Borovsky, Joseph E. "On the Saturation (or Not) of Geomagnetic Indices." Frontiers in Astronomy and Space Sciences 8 (October 14, 2021). http://dx.doi.org/10.3389/fspas.2021.740811.
Full textAbstract:
Most geomagnetic indices are associated with processes internal to the magnetosphere-ionosphere system: convection, magnetosphere-ionosphere current systems, particle pressure, ULF wave activity, etc. The saturation (or not) of various geomagnetic indices under various solar-wind driver functions (a.k.a. coupling functions) is explored by examining plots of the various indices as functions of the various driver functions. In comparing an index with a driver function, “saturation” of the index means that the trend of stronger geomagnetic activity with stronger driving weakens in going from mid-range driving to very strong driving. Issues explored are 1) whether the nature of the index matters (i.e., what the index measures and how the index measures it), 2) the relation of index saturation to cross-polar-cap potential saturation, and 3) the role of the choice of the solar-wind driver function. It is found that different geomagnetic indices exhibit different amounts of saturation. For example the SuperMAG auroral-electrojet indices SME, SML, and SMU saturate much less than do the auroral-electrojet indices AE, AL, and AU. Additionally it is found that different driver functions cause an index to show different degrees of saturation. Dividing a solar-wind driver function by the theoretical cross-polar-cap-potential correction (1+Q) often compensates for the saturation of an index, even though that index is associated with internal magnetospheric processes whereas Q is derived for solar-wind processes. There are composite geomagnetic indices E(1) that show no saturation when matched to their composite solar-wind driver functions S(1). When applied to other geomagnetic indices, the composite S(1) driver functions tend to compensate for index saturation at strong driving, but they also tend to introduce a nonlinearity at weak driving.
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Smith, A. R. A., and M. Pačes. "Python tools for ESA’s Swarm mission: VirES for Swarm and surrounding ecosystem." Frontiers in Astronomy and Space Sciences 9 (October 31, 2022). http://dx.doi.org/10.3389/fspas.2022.1002697.
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ESA’s Swarm mission is a constellation probing both Earth’s interior and geospace, delivering magnetic and plasma measurements which are used to generate many derived data products. From empirical magnetic field models of the core, crust, ionosphere, and magnetosphere, to multi-point estimates of ionospheric currents and in-situ plasma properties, these are challenging to navigate, process, and visualize. The VirES for Swarm platform (https://vires.services) has been built to tackle this problem, providing tools to increase usability of Swarm data products. The VirES (Virtual environments for Earth Scientists) platform provides both a graphical web interface and an API to access and visualise Swarm data and models. This is extended with a cloud-hosted development environment powered by JupyterHub (the “Virtual Research Environment/VRE”). VirES provides two API’s: the full VirES API for which a dedicated Python client is provided, viresclient, and the more interoperable Heliophysics API (HAPI). The VRE is furnished with a bespoke Python environment containing thematic libraries supporting science with Swarm. This service aims to ease the pathway for scientists writing computer code to analyze Swarm data products, increase opportunities for collaboration, and leverage cloud technologies. Beyond simply providing data and model access to Python users, it is extremely helpful to provide higher-level analysis and visualization tools, and ready-to-use code recipes that people can explore and extend. Critically for space physics, this involves crossover with many other datasets and so it is highly valuable to embed such tools within the wider data and software ecosystems. Through Swarm DISC (Data, Innovation, and Science Cluster), we are tackling this through cookbooks and Python libraries. Cookbooks are built and presented using Jupyter technologies, and tested to work within the VRE. A new library we are building is SwarmPAL, which includes tools for time-frequency analysis and inversion of magnetic field measurements for electric current systems, among others, while relying on the VirES server to provide data portability and other utilities. This paper reviews the current state of these tools and services for Swarm, particularly in the context of the Python in Heliophysics Community, and the wider heliophysics and geospace data environment.
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Buzulukova, Natalia, and Bruce Tsurutani. "Space Weather: From solar origins to risks and hazards evolving in time." Frontiers in Astronomy and Space Sciences 9 (December 22, 2022). http://dx.doi.org/10.3389/fspas.2022.1017103.
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Space Weather is the portion of space physics that has a direct effect on humankind. Space Weather is an old branch of space physics that originates back to 1808 with the publication of a paper by the great naturalist Alexander von Humboldt (Von Humboldt, Ann. Phys. 1808, 29, 425–429), first defining a “Magnetische Ungewitter” or magnetic storm from auroral observations from his home in Berlin, Germany. Space Weather is currently experiencing explosive growth, because its effects on human technologies have become more and more diverse. Space Weather is due to the variability of solar processes that cause interplanetary, magnetospheric, ionospheric, atmospheric and ground level effects. Space Weather can at times have strong impacts on technological systems and human health. The threats and risks are not hypothetical, and in the event of extreme Space Weather events the consequences could be quite severe for humankind. The purpose of the review is to give a brief overall view of the full chain of physical processes responsible for Space Weather risks and hazards, tracing them from solar origins to effects and impacts in interplanetary space, in the Earth’s magnetosphere and ionosphere and at the ground. In addition, the paper shows that the risks associated with Space Weather have not been constant over time; they have evolved as our society becomes more and more technologically advanced. The paper begins with a brief introduction to the Carrington event, arguably the greatest geomagnetic storm in recorded history. Next, the descriptions of the strongest known Space Weather processes are reviewed, tracing them from their solar origins. The concepts of geomagnetic storms and substorms are briefly introduced. The main effects/impacts of Space Weather are also considered, including geomagnetically induced currents (GICs) which are thought to cause power outages. The effects of radiation on avionics and human health, ionospheric effects and impacts, and thermosphere effects and satellite drag will also be discussed. Finally, we will discuss the current challenges of Space Weather forecasting and examine some of the worst-case scenarios.