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1
Wright, D. M., T. K. Yeoman i J. A. Davies. "A comparison of EISCAT and HF Doppler observations of a ULF wave". Annales Geophysicae 16, nr 10 (31.10.1998): 1190–99. http://dx.doi.org/10.1007/s00585-998-1190-7.
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Abstract. Since the middle of 1995, an HF Doppler sounder has been running almost continuously in northern Norway, with the receiver at Ramfjordmoen and the transmitter at Seljelvnes. Concurrent operation of the EISCAT UHF radar in common programme (CP-1) mode has made it possible to study the ionospheric signature of a magnetospheric ULF wave. These are the first results of such wave signatures observed simultaneously in both instruments. It has been demonstrated that the observed Doppler signature was mainly due to the vertical bulk motion of the ionosphere caused by the electric field perturbation of the ULF wave and the first direct observational confirmation of a numerical simulation has been achieved. The wave, which was Alfvénic in nature, was detected by the instruments 8° equatorward of the broad resonance region. The implications for the deduced wave modes in the ionosphere and the mechanism producing the HF Doppler variations are discussed.Key words. Magnetosphere-ionosphere interactions · MHD waves and instabilities · Radio science · Ionospheric physics
2
Lockwood, M., i S. K. Morley. "A numerical model of the ionospheric signatures of time-varying magneticreconnection: I. ionospheric convection". Annales Geophysicae 22, nr 1 (1.01.2004): 73–91. http://dx.doi.org/10.5194/angeo-22-73-2004.
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Abstract. This paper presents a numerical model for predicting the evolution of the pattern of ionospheric convection in response to general time-dependent magnetic reconnection at the dayside magnetopause and in the cross-tail current sheet of the geomagnetic tail. The model quantifies the concepts of ionospheric flow excitation by Cowley and Lockwood (1992), assuming a uniform spatial distribution of ionospheric conductivity. The model is demonstrated using an example in which travelling reconnection pulses commence near noon and then move across the dayside magnetopause towards both dawn and dusk. Two such pulses, 8min apart, are used and each causes the reconnection to be active for 1min at every MLT that they pass over. This example demonstrates how the convection response to a given change in the interplanetary magnetic field (via the reconnection rate) depends on the previous reconnection history. The causes of this effect are explained. The inherent assumptions and the potential applications of the model are discussed. Key words. Ionosphere (ionosphere-magnetosphere interactions; plasma convection) – Magnetospheric physics (magnetosphere-ionosphere interactions; solar wind-magnetosphere interactions)
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Wright, D. M., T. K. Yeoman i P. J. Chapman. "High-latitude HF Doppler observations of ULF waves. 1. Waves with large spatial scale sizes". Annales Geophysicae 15, nr 12 (31.12.1997): 1548–56. http://dx.doi.org/10.1007/s00585-997-1548-2.
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Abstract. A quantitative study of observations of the ionospheric signatures of magnetospheric ultra low frequency (ULF) waves by a high-latitude (geographic: 69.6°N 19.2°E) high-frequency Doppler sounder has been undertaken. The signatures, which are clearly correlated with pulsations in ground magnetometer data, exhibit periods in the range 100–400 s and have azimuthal wave numbers in the range 3–8. They are interpreted here as local field line resonances. Phase information provided by O- and X-mode Doppler data support the view that these are associated with field line resonances having large azimuthal scale sizes. The relative phases and amplitudes of the signatures in the Doppler and ground magnetometer data are compared with a model for the generation of Doppler signatures from incident ULF waves. The outcome suggests that the dominant mechanism involved in producing the Doppler signature is the vertical component of an E × B bulk motion of the local plasma caused by the electric field perturbation of the ULF wave.Key words. Auroral ionosphere · Magnetosphere-ionosphere interactions · MHD waves and instabilities HF Doppler · ULF Waves
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Wild, J. A., S. E. Milan, S. W. H. Cowley, M. W. Dunlop, C. J. Owen, J. M. Bosqued, M. G. G. T. Taylor i in. "Coordinated interhemispheric SuperDARN radar observations of the ionospheric response to flux transfer events observed by the Cluster spacecraft at the high-latitude magnetopause". Annales Geophysicae 21, nr 8 (31.08.2003): 1807–26. http://dx.doi.org/10.5194/angeo-21-1807-2003.
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Abstract. At 10:00 UT on 14 February 2001, the quartet of ESA Cluster spacecraft were approaching the Northern Hemisphere high-latitude magnetopause in the post-noon sector on an outbound trajectory. At this time, the interplanetary magnetic field incident upon the dayside magnetopause was oriented southward and duskward (BZ negative, BY positive), having turned from a northward orientation just over 1 hour earlier. As they neared the magnetopause the magnetic field, electron, and ion sensors on board the Cluster spacecraft observed characteristic field and particle signatures of magnetospheric flux transfer events (FTEs). Following the traversal of a boundary layer and the magnetopause, the spacecraft went on to observe further signatures of FTEs in the magnetosheath. During this interval of ongoing pulsed reconnection at the high-latitude post-noon magnetopause, the footprints of the Cluster spacecraft were located in the fields-of-view of the SuperDARN Finland and Syowa East radars located in the Northern and Southern Hemispheres, respectively. This study extends upon the initial survey of Wild et al. (2001) by comparing for the first time in situ magnetic field and plasma signatures of FTEs (here observed by the Cluster 1 spacecraft) with the simultaneous flow modulations in the conjugate ionospheres in the two hemispheres. During the period under scrutiny, the flow disturbances in the conjugate ionospheres are manifest as classic "pulsed ionospheric flows" (PIFs) and "poleward moving radar auroral forms" (PMRAFs). We demonstrate that the ionospheric flows excited in response to FTEs at the magnetopause are not those expected for a spatially limited reconnection region, somewhere in the vicinity of the Cluster 1 spacecraft. By examining the large- and small-scale flows in the high-latitude ionosphere, and the inter-hemispheric correspondence exhibited during this interval, we conclude that the reconnection processes that result in the generation of PIFs/PMRAFs must extend over many (at least 4) hours of magnetic local time on the pre- and post-noon magnetopause.Key words. Ionosphere (plasma convection) – Magnetospheric physics (magnetosphere-ionosphere interactions; magnetospheric configuration and dynamics)
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Pryse, S. E., A. M. Smith, I. K. Walker i L. Kersley. "Multi-instrument study of footprints of magnetopause reconnection in the summer ionosphere". Annales Geophysicae 18, nr 9 (30.09.2000): 1118–27. http://dx.doi.org/10.1007/s00585-000-1118-3.
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Abstract. Results are presented from a multi-instrument investigation of the signatures of equatorial reconnection in the summer, sunlit ionosphere. Well-established ion dispersion signatures measured during three DMSP satellite passes were used to identify footprints in ionospheric observations made by radio tomography, and both the EISCAT ESR and mainland radars. Under the prevalent conditions of southward IMF with the Bz component increasing in magnitude, the reconnection footprint was seen to move equatorward through the ESR field-of-view. The most striking signature was in the electron temperatures of the F2 region measured by the EISCAT mainland radar that revealed significantly enhanced temperatures with a steep equatorward edge, in general agreement with the leading edge of the ion dispersion. It is suggested that this sharp transition in the electron temperature may be an indicator of the boundary, mapping from the reconnection site, between closed geomagnetic field lines and those opened along which magnetosheath ions precipitate.Key words: Ionosphere (ionosphere-magnetosphere interactions; particle precipitation; plasma temperature and density)
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Neudegg, D. A., S. W. H. Cowley, S. E. Milan, T. K. Yeoman, M. Lester, G. Provan, G. Haerendel i in. "A survey of magnetopause FTEs and associated flow bursts in the polar ionosphere". Annales Geophysicae 18, nr 4 (30.04.2000): 416–35. http://dx.doi.org/10.1007/s00585-000-0416-0.
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Abstract. Using the Equator-S spacecraft and SuperDARN HF radars an extensive survey of bursty reconnection at the magnetopause and associated flows in the polar ionosphere has been conducted. Flux transfer event (FTE) signatures were identified in the Equator-S magnetometer data during periods of magnetopause contact in January and February 1998. Assuming the effects of the FTEs propagate to the polar ionosphere as geomagnetic field-aligned-currents and associated Alfvén-waves, appropriate field mappings to the fields-of-view of SuperDARN radars were performed. The radars observed discrete ionospheric flow channel events (FCEs) of the type previously assumed to be related to pulse reconnection. Such FCEs were associated with \\sim80% of the FTEs and the two signatures are shown to be statistically associated with greater than 99% confidence. Exemplary case studies highlight the nature of the ionospheric flows and their relation to the high latitude convection pattern, the association methodology, and the problems caused by instrument limitations.Key words: Ionosphere (polar ionosphere) · Magnetospheric physics (magnetosphere-ionosphere interaction; solar wind-magnetosphere interactions)
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Stocker, A. J., N. F. Arnold i T. B. Jones. "The synthesis of travelling ionospheric disturbance (TID) signatures in HF radar observations using ray tracing". Annales Geophysicae 18, nr 1 (31.01.2000): 56–64. http://dx.doi.org/10.1007/s00585-000-0056-4.
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Abstract. Characteristic signatures are often observed in HF radar range-time-intensity plots when travelling ionospheric disturbances (TIDs) are present. These signatures, in particular the variation of the F-region skip distance, have been synthesised using a ray tracing model. The magnitude of the skip variation is found to be a function of the peak electron density perturbation associated with the TID and radar frequency. Examination of experimental observations leads to an estimate of the peak electron density perturbation amplitude of around 25% for those TIDs observed by the CUTLASS radar system. The advantage of using the skip variation over the radar return amplitude as an indicator of density perturbation is also discussed. An example of a dual radar frequency experiment has been given. The investigation of the effect of radar frequency on the observations will aid the optimisation of future experiments..Key words. Ionosphere (auroral ionosphere; ionosphere -atmosphere interactions; ionospheric disturbances)
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Thorolfsson, A., J. C. Cerisier, M. Lockwood, P. E. Sandholt, C. Senior i M. Lester. "Simultaneous optical and radar signatures of poleward-moving auroral forms". Annales Geophysicae 18, nr 9 (30.09.2000): 1054–66. http://dx.doi.org/10.1007/s00585-000-1054-2.
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Abstract. Dayside poleward moving auroral forms (PMAFs) were detected between 06:30 and 07:00 UT on December 16, 1998, by the meridian scanning photometer and the all-sky camera at Ny Ålesund, Svalbard. Simultaneous SuperDARN HF radar measurements permitted the study of the associated ionospheric velocity pattern. A good general agreement is observed between the location and movement of velocity enhancements (flow channels) and the PMAFs. Clear signatures of equatorward flow were detected in the vicinity of PMAFs. This flow is believed to be the signature of a return flow outside the reconnected flux tube, as predicted by the Southwood model. The simulated signatures of this model reproduce globally the measured signatures, and differences with the experimental data can be explained by the simplifications of the model. Proposed schemes of the flow modification due to the presence of several flow channels and the modification of cusp and region 1 field-aligned currents at the time of sporadic reconnection events are shown to fit well with the observations.Key words: Ionosphere (auroral ionosphere; plasma convection) - Magnetospheric physics (magnetopause; cusp and boundary layers)
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Berry, S. T., L. Kersley, J. Moen i W. F. Denig. "Ionospheric signatures of magnetospheric boundaries in the post-noon sector". Annales Geophysicae 18, nr 1 (31.01.2000): 74–80. http://dx.doi.org/10.1007/s00585-000-0074-2.
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Abstract. Spatial structures in ionospheric electron density revealed in a tomographic image have been identified with auroral forms and related to their sources in precipitating particles observed by DMSP satellites. The observations of plasma enhancements relate to discrete auroral arcs seen in the post-noon sector, identified by both red- and green-line emissions measured by a meridional scanning photometer. The features lie within a very narrow latitudinal band on L-shells where the satellite detectors observed electron precipitation classified as from the boundary plasma sheet (BPS). The harder particles are identified with an E-region structure, while further north the precipitation is softer, resulting in a localised F-layer blob and 630.0 nm emissions. A steep gradient in plasma density represent a signature in the ionosphere of the central plasma sheet (CPS)/BPS boundary. A transition to a less-structured F-layer is found on crossing the convection reversal boundary..Key words. Ionosphere (auroral ionosphere; ionosphere-magnetosphere interactions; polar ionosphere)
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Provan, G., i T. K. Yeoman. "Statistical observations of the MLT, latitude and size of pulsed ionospheric flows with the CUTLASS Finland radar". Annales Geophysicae 17, nr 7 (31.07.1999): 855–67. http://dx.doi.org/10.1007/s00585-999-0855-1.
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Abstract. A study has been performed on the occurrence of pulsed ionospheric flows as detected by the CUTLASS Finland HF radar. These flows have been suggested as being created at the ionospheric footprint of newly-reconnected field lines, during episodes of magnetic flux transfer into the terrestrial magnetosphere (flux transfer events or FTEs). Two years of both high-time resolution and normal scan data from the CUTLASS Finland radar have been analysed in order to perform a statistical study of the extent and location of the pulsed ionospheric flows. We note a great similarity between the statistical pattern of the coherent radar observations of pulsed ionospheric flows and the traditional low-altitude satellite identification of the particle signature associated with the cusp/cleft region. However, the coherent scatter radar observations suggest that the merging gap is far wider than that proposed by the Newell and Meng model. The new model for cusp low-altitude particle signatures, proposed by Lockwood and Onsager and Lockwood provides a unified framework to explain the dayside precipitation regimes observed both by the low-altitude satellites and by coherent scatter radar detection.Key words. Magnetospheric physics (magnetosphere · ionosphere interactions; plasma convection; solar wind-magnetosphere interactions)
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Provan, G., T. K. Yeoman i S. E. Milan. "CUTLASS Finland radar observations of the ionospheric signatures of flux transfer events and the resulting plasma flows". Annales Geophysicae 16, nr 11 (30.11.1998): 1411–22. http://dx.doi.org/10.1007/s00585-998-1411-0.
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Abstract. The CUTLASS Finland radar has been run in a two-beam special scan mode, which offered excellent temporal and spatial information on the flows in the high-latitude ionosphere. A detailed study of one day of this data revealed a convection reversal boundary (CRB) in the CUTLASS field of view (f.o.v) on the dayside, the direction of plasma flow either side of the boundary being typical of a dawn-cell convection pattern. Poleward of the CRB a number of pulsed transients are observed, seemingly moving away from the radar. These transients are identified here as the ionospheric signature of flux transfer events (FTEs). Equatorward of the CRB continuous backscatter was observed, believed to be due to the return flow on closed field lines. The two-beam scan offered a new and innovative opportunity to determine the size and velocity of the ionospheric signatures associated with flux transfer events and the related plasma flow pattern. The transient signature was found to have an azimuthal extent of 1900 ± 900 km and an poleward extent of ∼250 km. The motion of the transient features was in a predominantly westward azimuthal direction, at a velocity of 7.5 ± 3 km.Key words. Magnetospheric physics (auroral phenomena; magnetopause · cusp and boundary layers; magnetosphere - ionosphere interaction)
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Dougal, E. R., K. Nykyri i T. W. Moore. "Mapping of the quasi-periodic oscillations at the flank magnetopause into the ionosphere". Annales Geophysicae 31, nr 11 (18.11.2013): 1993–2011. http://dx.doi.org/10.5194/angeo-31-1993-2013.
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Abstract. We have estimated the ionospheric location, area, and travel time of quasi-periodic oscillations originating from the magnetospheric flanks. This was accomplished by utilizing global and local MHD models and Tsyganenko semi-empirical magnetic field model on multiple published and four new cases believed to be caused by the Kelvin–Helmholtz Instability. Finally, we used auroral, magnetometer, and radar instruments to observe the ionospheric signatures. The ionospheric magnetic latitude determined using global MHD and Tsyganenko models ranged from 58.3–80.2 degrees in the Northern Hemisphere and −59.6 degrees to −83.4 degrees in the Southern Hemisphere. The ionospheric magnetic local time ranged between 5.0–13.8 h in the Northern Hemisphere and 1.3–11.9 h in the Southern Hemisphere. Typical Alfvén wave travel time from spacecraft location to the closest ionosphere ranged between 0.6–3.6 min. The projected ionospheric size calculated at an altitude of 100 km ranged from 47–606 km, the same order of magnitude as previously determined ionospheric signature sizes. Stationary and traveling convection vortices were observed in SuperDARN radar data in both hemispheres. The vortices were between 1000–1800 km in size. Some events were located within the ionospheric footprint ranges. Pc5 magnetic oscillations were observed in SuperMAG magnetometer data in both hemispheres. The oscillations had periods between 4–10 min with amplitudes of 3–25 nT. They were located within the ionospheric footprint ranges. Some ground magnetometer data power spectral density peaked at frequencies within one tenth of a mHz of the peaks found in the corresponding Cluster data. These magnetometer observations were consistent with previously published results.
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Yeoman, T. K., T. Mukai i T. Yamamoto. "Simultaneous ionospheric and magnetospheric observations of azimuthally propagating transient features during substorms". Annales Geophysicae 16, nr 7 (31.07.1998): 754–63. http://dx.doi.org/10.1007/s00585-998-0754-x.
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Abstract. During the 6th August 1995, the CUTLASS Finland HF radar ran in a high time resolution mode, allowing measurements of line-of-sight convection velocities along a single beam with a temporal resolution of 14 s. Data from such scans, during the substorm expansion phase, revealed pulses of equatorward flow exceeding ~600 m s–1 with a duration of ~5 min and a repetition period of ~8 min. Each pulse of enhanced equatorward flow was preceded by an interval of suppressed flow and enhanced ionospheric Hall conductance. These transient features, which propagate eastwards away from local midnight, have been interpreted as ionospheric current vortices associated with field-aligned current pairs. The present study reveals that these ionospheric convection features appear to have an accompanying signature in the magnetosphere, comprising a dawnward perturbation and dipolarisation of the magnetic field and dawnward plasma flow, measured in the geomagnetic tail by the Geotail spacecraft, located at L = 10 and some four hours to the east, in the postmidnight sector. These signatures are suggested to be the consequence of the observation of the same field aligned currents in the magnetosphere. Their possible relationship with bursty Earthward plasma flow and magnetotail reconnection is discussed.Key words. Ionosphere (Auroral · ionosphere) · Magnetospheric Physics (Magnetotail; Storms and substorms)
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Streltsov, A. V., i T. R. Pedersen. "Excitation of zero-frequency magnetic field-aligned currents by ionospheric heating". Annales Geophysicae 29, nr 6 (27.06.2011): 1147–52. http://dx.doi.org/10.5194/angeo-29-1147-2011.
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Abstract. Time-dependent, three-dimensional numerical simulations of the reduced MHD model describing shear Alfvén waves in the magnetosphere provide an interesting prediction superficially similar to results of several ionospheric heating experiments conducted at high altitudes. In these experiments, heating of the ionospheric F-region with a constant/zero-frequency beam of HF waves causes luminous structures in the ionosphere in the form of a ring or a solid spot with a characteristic size comparable to the size of the heated spot. Simulations suggest that spots/rings or similar optical appearance might be associated with a magnetic field-aligned current system produced by the ionospheric heating. Two of the most interesting features of this current system are (1) strong localization across the ambient magnetic field and (2) distinctive non-symmetrical luminous signatures (ring/spot) in magnetically conjugate locations in the ionosphere.
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Simões, Fernando, Jeffrey Klenzing, Stoyan Ivanov, Robert Pfaff, Henry Freudenreich, Dieter Bilitza, Douglas Rowland i in. "Detection of ionospheric Alfvén resonator signatures in the equatorial ionosphere". Journal of Geophysical Research: Space Physics 117, A11 (listopad 2012): n/a. http://dx.doi.org/10.1029/2012ja017709.
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Wright, D. M., i T. K. Yeoman. "<i>Letter to the editor</i>CUTLASS observations of a high-m ULF wave and its consequences for the DOPE HF Doppler sounder". Annales Geophysicae 17, nr 11 (30.11.1999): 1493–97. http://dx.doi.org/10.1007/s00585-999-1493-3.
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Abstract. The CUTLASS (Co-operative UK Twin Located Auroral Sounding System) Finland HF radar, whilst operating in a high spatial and temporal resolution mode, has measured the ionospheric signature of a naturally occurring ULF wave in scatter artificially generated by the Tromsù Heater. The wave had a period of 100 s and exhibited curved phase fronts across the heated volume (about 180 km along a single radar beam). Spatial information provided by CUTLASS has enabled an m-number for the wave of about 38 to be determined. This high-m wave was not detected by the IMAGE (International Monitor for Auroral Geomagnetic Efects) network of ground magnetometers, as expected for a wave of a small spatial scale size. These observations over the first independent confirmation of the existence of the ground uncorrelated ULF wave signatures previously reported in measurements recorded from an HF Doppler sounder located in the vicinity of Tromsö. These results both demonstrate a new capability for geophysical exploration from the combined CUTLASS-EISCAT ionospheric Heater experiment, and provide a verification of the HF Doppler technique for the investigation of small scale ULF waves.Key words. Ionosphere (ionosphere – magnetosphere interactions) . Magnetospheric physics (magnetosphere – ionosphere interactions; MHD waves and instabilities)
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Tsunomura, S. "Numerical analysis of global ionospheric current system including the effect of equatorial enhancement". Annales Geophysicae 17, nr 5 (31.05.1999): 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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Juusola, L., O. Amm, H. U. Frey, K. Kauristie, R. Nakamura, C. J. Owen, V. Sergeev, J. A. Slavin i A. Walsh. "Ionospheric signatures during a magnetospheric flux rope event". Annales Geophysicae 26, nr 12 (5.12.2008): 3967–77. http://dx.doi.org/10.5194/angeo-26-3967-2008.
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Abstract. On 13 August 2002, during a substorm, Cluster encountered two earthward moving flux ropes (FR) in the central magnetotail. The first FR was observed during the expansion phase of the substorm, and the second FR during the recovery phase. In the conjugate ionospheric region in Northern Fennoscandia, the ionospheric equivalent currents were observed by the MIRACLE network and the auroral evolution was monitored by the Wideband Imaging Camera (WIC) on-board the IMAGE satellite. Extending the study of Amm et al. (2006), we examine and compare the possible ionospheric signatures associated with the two FRs. Amm et al. studied the first event in detail and found that the ionospheric footprint of Cluster coincided with a region of downward field-aligned current. They suggested that this region of downward current, together with a trailing region of upward current further southwestward, might correspond to the ends of the FR. Unlike during the first FR, however, we do not see any clear ionospheric features associated with the second one. In the GSM xy-plane, the first flux rope axis was tilted with respect to the y-direction by 29°, while the second flux rope axis was almost aligned in the y-direction, with an angle of 4° only. It is possible that due to the length and orientation of the second FR, any ionospheric signatures were simply mapped outside the region covered by the ground-based instruments. We suggest that the ground signatures of a FR depend on the orientation and the length of the structure.
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SOUTHWOOD, D. J. "Ionospheric signatures of magnetospheric boundary phenomena." Journal of geomagnetism and geoelectricity 42, nr 6 (1990): 789–99. http://dx.doi.org/10.5636/jgg.42.789.
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KIVELSON, Margaret G., i David J. SOUTHWOOD. "Ionospheric Signatures of Localized Magnetospheric Perturbations". Journal of geomagnetism and geoelectricity 43, Supplement1 (1991): 129–40. http://dx.doi.org/10.5636/jgg.43.supplement1_129.
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Hegde, Sahil, Monica G. Bobra i Philip H. Scherrer. "Classifying Signatures of Sudden Ionospheric Disturbances". Research Notes of the AAS 2, nr 3 (7.09.2018): 162. http://dx.doi.org/10.3847/2515-5172/aade47.
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Lockwood, M., S. W. H. Cowley i P. E. Sandholt. "Transient reconnection: Search for ionospheric signatures". Eos, Transactions American Geophysical Union 71, nr 20 (1990): 709. http://dx.doi.org/10.1029/eo071i020p00709-02.
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Muslim, B., Asnawi, H. Haralambous i C. Oikonomu. "Ionospheric earthquake signatures on GPS TEC". Journal of Physics: Conference Series 1170 (marzec 2019): 012070. http://dx.doi.org/10.1088/1742-6596/1170/1/012070.
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Merkin, V. G., J. G. Lyon, B. J. Anderson, H. Korth, C. C. Goodrich i K. Papadopoulos. "A global MHD simulation of an event with a quasi-steady northward IMF component". Annales Geophysicae 25, nr 6 (29.06.2007): 1345–58. http://dx.doi.org/10.5194/angeo-25-1345-2007.
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Abstract. We show results of the Lyon-Fedder-Mobarry (LFM) global MHD simulations of an event previously examined using Iridium spacecraft observations as well as DMSP and IMAGE FUV data. The event is chosen for the steady northward IMF sustained over a three-hour period during 16 July 2000. The Iridium observations showed very weak or absent Region 2 currents in the ionosphere, which makes the event favorable for global MHD modeling. Here we are interested in examining the model's performace during weak magnetospheric forcing, in particular, its ability to reproduce gross signatures of the ionospheric currents and convection pattern and energy deposition in the ionosphere both due to the Poynting flux and particle precipitation. We compare the ionospheric field-aligned current and electric potential patterns with those recovered from Iridium and DMSP observations, respectively. In addition, DMSP magnetometer data are used for comparisons of ionospheric magnetic perturbations. The electromagnetic energy flux is compared with Iridium-inferred values, while IMAGE FUV observations are utilized to verify the simulated particle energy flux.
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von Savigny, Christian, Dieter H. W. Peters i Günter Entzian. "Solar 27-day signatures in standard phase height measurements above central Europe". Atmospheric Chemistry and Physics 19, nr 3 (15.02.2019): 2079–93. http://dx.doi.org/10.5194/acp-19-2079-2019.
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Abstract. We report on the effect of solar variability at the 27-day and the 11-year timescales on standard phase height measurements in the ionospheric D region carried out in central Europe. Standard phase height corresponds to the reflection height of radio waves (for constant solar zenith distance) in the ionosphere near 80 km altitude, where NO is ionized by solar Lyman-α radiation. Using the superposed epoch analysis (SEA) method, we extract statistically highly significant solar 27-day signatures in standard phase heights. The 27-day signatures are roughly inversely correlated to solar proxies, such as the F10.7 cm radio flux or the Lyman-α flux. The sensitivity of standard phase height change to solar forcing at the 27-day timescale is found to be in good agreement with the sensitivity for the 11-year solar cycle, suggesting similar underlying mechanisms. The amplitude of the 27-day signature in standard phase height is larger during solar minimum than during solar maximum, indicating that the signature is not only driven by photoionization of NO. We identified statistical evidence for an influence of ultra-long planetary waves on the quasi 27-day signature of standard phase height in winters of solar minimum periods.
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Tang, C. L. "A plasma flow vortex in the magnetotail and its related ionospheric signatures". Annales Geophysicae 30, nr 3 (16.03.2012): 537–44. http://dx.doi.org/10.5194/angeo-30-537-2012.
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Abstract. We presented a large-scale plasma flow vortex event that occurred on 1 March 2009 observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites. During the interval, THEMIS satellites were located in the premidnight region between 11 and 16 RE downtail. Dawnward-earthward plasma flows were seen initially in the magnetotail, followed by duskward-tailward flows. This suggests that a clockwise plasma flow vortex (seen from above the equatorial plane) was observed on the dawn side of the plasma sheet. Furthermore, high energy (>1 keV) electrons were observed. Auroral images at 427.8 nm and THEMIS white light all-sky imager (ASI) at Fort Smith showed a discrete auroral patch formed at the poleward of the auroral oval, it then intensified. It extended eastward and equatorward first and followed by westward motion to form the clockwise auroral vortex. The auroral feature corresponded to the ionospheric signatures of the plasma flow vortex in the magnetotail when the Alfvén transit time between the magnetotail and the ionosphere was taken into account. We suggest that the large-scale clockwise plasma flow vortex in association with the high energy (>1 keV) electrons on the dawn side of the plasma sheet generated a downward field-aligned current (FAC) that caused the related ionospheric signatures. The plasma flow vortex had rotational flow speeds of up to 300 km s−1. The current density associated with the plasma flow vortex was estimated at 2.0 μA m−2, mapped to the ionosphere.
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Semenova, N. V., i A. G. Yahnin. "Diurnal behaviour of the ionospheric Alfvén resonator signatures as observed at high latitude observatory Barentsburg (<I>L</I>=15)". Annales Geophysicae 26, nr 8 (5.08.2008): 2245–51. http://dx.doi.org/10.5194/angeo-26-2245-2008.
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Abstract. The signature of the ionospheric Alfvén resonator (IAR), so called spectral resonant structures (SRS) in the spectra of the electromagnetic noise in the range of 0.1–10 Hz is rather frequently observed with the search coil magnetometer at observatory Barentsburg on Svalbard (L=15). In this report we discuss some peculiarities of diurnal occurrence of SRS at this high latitude station. We show that the pronounced minimum of the SRS occurrence around noon can not be explained by the diurnal variations of the solar zenith angle (illumination of ionosphere). We conclude that the SRS occurrence minimum is the result of the enhanced variability of ionospheric parameters when the observing point enters (during the Earth's rotation) the region of the ionospheric projection of the dayside cusp and its vicinity.
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Provan, G., S. E. Milan, M. Lester, T. K. Yeoman i H. Khan. "<i>Letter to the Editor</i> Simultaneous observations of the ionospheric footprint of flux transfer events and dispersed ion signatures". Annales Geophysicae 20, nr 2 (28.02.2002): 281–87. http://dx.doi.org/10.5194/angeo-20-281-2002.
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Abstract. We perform a case study of a favourable conjunction of overpasses of the DMSP F11 and F13 spacecraft with the field of view of the Hankasalmi HF coherent scatter. At the time, pulsed ionospheric flows (PIFs) were clearly observed at a high-latitude in the radar field of view. The PIFs were associated with medium spectral width values and were identified as the fossilized signatures of pulsed dayside reconnection. Simultaneously, DMSP spectrograms from the two spacecraft showed dispersed ion signatures, observed equatorwards of the PIF signatures. We identified dayside high-latitude magnetosphere boundaries; these boundaries agreed well with those defined using the algorithm on the JHU/APL auroral particle website (Haerendel et al., 1978; Newell and Meng, 1988, 1995; Newell et al., 1991a, 1991b, 1991c; Traver et al., 1991). We conclude that in this case study the dispersed ion signatures map to regions of very newly-opened flux. It is only when this flux has convected polewards that the signatures of the PIFs with medium spectral widths are observed by the HF radars. These particular PIF signatures map to regions of mantle precipitation, i.e. recently reconnected flux.Key words. Ionosphere (ionosphere-magnetosphere interaction) – Magnetospheric physics (magnetopause, cusp and boundary layers; plasma convection)
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Amm, O., R. Nakamura, H. U. Frey, Y. Ogawa, M. Kubyshkina, A. Balogh i H. Rème. "Substorm topology in the ionosphere and magnetosphere during a flux rope event in the magnetotail". Annales Geophysicae 24, nr 2 (23.03.2006): 735–50. http://dx.doi.org/10.5194/angeo-24-735-2006.
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Abstract. On 13 August 2002, at ~23:00 UT, about 10 min after a substorm intensification, Cluster observes a flux rope in the central magnetotail, followed by a localised fast flow event about oneminute later. Associated with the flux rope event, a traveling compression region (TCR) is seen by those Cluster spacecraft which reside in the lobe. In the conjugate ionospheric region in Northern Scandinavia, the MIRACLE network observes the ionospheric equivalent currents, and the electron densities and electric fields are measured by the EISCAT radar along a meridional scanning profile. Further, the auroral evolution is observed with the Wideband Imaging Camera (WIC) on the IMAGE satellite. We compare in detail the substorm evolution as observed in the ionosphere and in the magnetosphere, and examine whether topological correspondences to the flux rope event exist in the ionospheric signatures. The large-scale mapping of both the location and the direction of the flux rope to the ionosphere shows an excellent correspondence to a lens-shaped region of an auroral emission minimum. This region is bracketed by an auroral region equatorward of it which was preexisting to the substorm intensification, and a substorm-related auroral region poleward of it. It is characterised by reduced ionospheric conductances with respect to its environment, and downward field-aligned current (FAC) observed both in the magnetosphere and in the ionosphere. As determined from the ionospheric data, this downward FAC area is moving eastward with a speed of ~2 km s-1, in good agreement with the mapped plasma bulk velocity measured at the Cluster satellite closest to that area. Further southwestward to this leading downward FAC area, a trailing upward FAC area is observed that moves eastward with the same speed. The direction of the ionospheric electric field permits a current closure between these two FAC areas through the ionosphere. We speculate that these FAC areas may correspond to the ends of the flux rope in its symmetry direction.
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Vlasov, A., K. Kauristie, M. van de Kamp, J. P. Luntama i A. Pogoreltsev. "A study of Traveling Ionospheric Disturbances and Atmospheric Gravity Waves using EISCAT Svalbard Radar IPY-data". Annales Geophysicae 29, nr 11 (24.11.2011): 2101–16. http://dx.doi.org/10.5194/angeo-29-2101-2011.
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Abstract. We present a statistical study of Traveling Ionospheric Disturbances (TIDs) as observed by the EISCAT Svalbard Radar (ESR) during the continuous IPY-run (March 2007–February 2008) with field-aligned measurements. We have developed a semi-automatic routine for searching and extracting Atmospheric Gravity Wave (AGW) activity. The collected data shows that AGW-TID signatures are common in the high-latitude ionosphere especially in the field-aligned ion velocity data (244 cases of AGW-TID signatures in daily records), but they can be observed also in electron density (26 cases), electron temperature (12 cases) and ion temperature (26 cases). During the IPY campaign (in solar minimum conditions) AGW-TID events appear more frequently during summer months than during the winter months. It remains still as a topic for future studies whether the observed seasonal variation is natural or caused by seasonal variation in the performance of the observational method that we use (AGW-TID signature may be more pronounced in a dense ionosphere). In our AGW-TID dataset the distribution of the oscillation periods has two peaks, one around 0.5–0.7 h and the other around 1.1–1.3 h. The diurnal occurrence rate has a deep minimum in the region of magnetic midnight, which might be partly explained by irregular auroral activity obscuring the TID signatures from our detection routines. As both the period and horizontal phase speed estimates (as derived from the classical AGW dispersion relation) show values typical both for large scale TIDs and mesoscale TIDs it is difficult to distinguish whether the generator for high-latitude AGW-TIDs resides typically in the troposphere or in the near-Earth space. The results of our statistical analysis give anyway some valuable reference information for the future efforts to learn more about the dominating TID source mechanisms in polar cap conditions, and to improve AGW simulations.
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Friedrich, M., K. M. Torkar, W. Singer, I. Strelnikova, M. Rapp i S. Robertson. "Signatures of mesospheric particles in ionospheric data". Annales Geophysicae 27, nr 2 (18.02.2009): 823–29. http://dx.doi.org/10.5194/angeo-27-823-2009.
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Abstract. The state of the ionosphere during the 2007 ECOMA/MASS campaign is described by in-situ observations by three sounding rockets launched from the Andøya Rocket Range and by ground based observations. The ground based measurements included the incoherent scatter radar EISCAT near Tromsø (both on UHF and VHF), as well as an MF radar, a meteor radar and an imaging riometer all located in the close vicinity of the rocket range. The pronounced electron density bite-outs seen by two of the rockets could not be detected from the ground, but the associated PMSE (Polar Mesospheric Summer Echoes) provide indirect evidence of pronounced perturbations of mesospheric electron densities.
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Sutcliffe, P. R. "Modelling the Ionospheric Signatures of Geomagnetic Pulsations." Journal of geomagnetism and geoelectricity 46, nr 11 (1994): 1011–27. http://dx.doi.org/10.5636/jgg.46.1011.
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Coley, W. R., R. A. Heelis, W. B. Hanson, P. H. Reiff, J. R. Sharber i J. D. Winningham. "Ionospheric convection signatures and magnetic field topology". Journal of Geophysical Research 92, A11 (1987): 12352. http://dx.doi.org/10.1029/ja092ia11p12352.
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Bell, T. F., U. S. Inan, M. T. Danielson i S. A. Cummer. "VLF signatures of ionospheric heating by HIPAS". Radio Science 30, nr 6 (listopad 1995): 1855–67. http://dx.doi.org/10.1029/95rs02191.
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Marchaudon, A., J. C. Cerisier, J. M. Bosqued, M. W. Dunlop, J. A. Wild, P. M. E. Décréau, M. Förster, D. Fontaine i H. Laakso. "Transient plasma injections in the dayside magnetosphere: one-to-one correlated observations by Cluster and SuperDARN". Annales Geophysicae 22, nr 1 (1.01.2004): 141–58. http://dx.doi.org/10.5194/angeo-22-141-2004.
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Abstract. Conjunctions in the cusp between the four Cluster spacecraft and SuperDARN ground-based radars offer unique opportunities to compare the signatures of transient plasma injections simultaneously in the high-altitude dayside magnetosphere and in the ionosphere. We report here on such observations on 17 March 2001, when the IMF initially northward and duskward, turns southward and dawnward for a short period. The changes in the convection direction at Cluster are well correlated with the interplanetary magnetic field (IMF) By variations. Moreover, the changes in the ionosphere follow those in the magnetosphere, with a 2–3min delay. When mapped into the ionosphere, the convection velocity at Cluster is about 1.5 times larger than measured by SuperDARN. In the high-altitude cusp, field and particle observations by Cluster display the characteristic signatures of plasma injections into the magnetosphere suggestive of Flux Transfer Events (FTEs). Simultaneous impulsive and localized convection plasma flows are observed in the ionospheric cusp by the HF radars. A clear one-to-one correlation is observed for three successive injections, with a 2–3min delay between the magnetospheric and ionospheric observations. For each event, the drift velocity of reconnected flux tubes (phase velocity) has been compared in the magnetosphere and in the ionosphere. The drift velocity measured at Cluster is of the order of 400–600ms–1 when mapped into the ionosphere, in qualitative agreement with SuperDARN observations. Finally, the reconnected flux tubes are elongated in the north-south direction, with an east-west dimension of 30–60km in the ionosphere from mapped Cluster observations, which is consistent with SuperDARN observations, although slightly smaller. Key words. Ionosphere (plasma convection) – Magnetospheric physics (magnetopause, cusp, and boundary layers; magnetosphere-ionosphere interactions)
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Mabie, Justin, i Terence Bullett. "Multiple Cusp Signatures in Ionograms Associated with Rocket-Induced Infrasonic Waves". Atmosphere 13, nr 6 (12.06.2022): 958. http://dx.doi.org/10.3390/atmos13060958.
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We are interested in understanding how and when infrasonic waves propagate in the thermosphere, specifying the physical properties of those waves, and understanding how they affect radio wave propagation. We use a combination of traditional ionosonde observations and fixed frequency Doppler soundings to make high quality observations of vertically propagating infrasonic waves in the lower thermosphere/bottom side ionosphere. The presented results are the first simultaneous observations of infrasonic wave-induced deformations in ionograms and high-time-resolution observations of corresponding plasma displacements. Deformations in ionospheric echoes, which manifest as additional cusps and range variations, are shown to be caused by infrasonic wave-induced plasma displacements.
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Mahrous, Ayman. "Ionospheric response to magnetar flare: signature of SGR J1550–5418 on coherent ionospheric Doppler radar". Annales Geophysicae 35, nr 3 (7.03.2017): 345–51. http://dx.doi.org/10.5194/angeo-35-345-2017.
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Abstract. This paper presents observational evidence of frequent ionospheric perturbations caused by the magnetar flare of the source SGR J1550–5418, which took place on 22 January 2009. These ionospheric perturbations are observed in the relative change of the total electron content (ΔTEC/Δt) measurements from the coherent ionospheric Doppler radar (CIDR). The CIDR system makes high-precision measurements of the total electron content (TEC) change along ray-paths from ground receivers to low Earth-orbiting (LEO) beacon spacecraft. These measurements can be integrated along the orbital track of the beacon satellite to construct the relative spatial, not temporal, TEC profiles that are useful for determining the large-scale plasma distribution. The observed spatial TEC changes reveal many interesting features of the magnetar signatures in the ionosphere. The onset phase of the magnetar flare was during the CIDR's nighttime satellite passage. The nighttime small-scale perturbations detected by CIDR, with ΔTEC/Δt ≥ 0.05 TECU s−1, over the eastern Mediterranean on 22 January 2009 were synchronized with the onset phase of the magnetar flare and consistent with the emission of hundreds of bursts detected from the source. The maximum daytime large-scale perturbation measured by CIDR over northern Africa and the eastern Mediterranean was detected after ∼ 6 h from the main phase of the magnetar flare, with ΔTEC/Δt ≤ 0.10 TECU s−1. These ionospheric perturbations resembled an unusual poleward traveling ionospheric disturbance (TID) caused by the extraterrestrial source. The TID's estimated virtual velocity is 385.8 m s−1, with ΔTEC/Δt ≤ 0.10 TECU s−1.
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Bradshaw, E. G., i M. Lester. "SABRE observations of Pi2 pulsations: case studies". Annales Geophysicae 15, nr 1 (31.01.1997): 40–53. http://dx.doi.org/10.1007/s00585-997-0040-3.
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Abstract. The characteristics of substorm-associated Pi2 pulsations observed by the SABRE coherent radar system during three separate case studies are presented. The SABRE field of view is well positioned to observe the differences between the auroral zone pulsation signature and that observed at mid-latitudes. During the first case study the SABRE field of view is initially in the eastward electrojet, equatorward and to the west of the substorm-enhanced electrojet current. As the interval progresses, the western, upward field-aligned current of the substorm current wedge moves westward across the longitudes of the radar field of view. The westward motion of the wedge is apparent in the spatial and temporal signatures of the associated Pi2 pulsation spectra and polarisation sense. During the second case study, the complex field-aligned and ionospheric currents associated with the pulsation generation region move equatorward into the SABRE field of view and then poleward out of it again after the third pulsation in the series. The spectral content of the four pulsations during the interval indicate different auroral zone and mid-latitude signatures. The final case study is from a period of low magnetic activity when SABRE observes a Pi2 pulsation signature from regions equatorward of the enhanced substorm currents. There is an apparent mode change between the signature observed by SABRE in the ionosphere and that on the ground by magnetometers at latitudes slightly equatorward of the radar field of view. The observations are discussed in terms of published theories of the generation mechanisms for this type of pulsation. Different signatures are observed by SABRE depending on the level of magnetic activity and the position of the SABRE field of view relative to the pulsation generation region. A twin source model for Pi2 pulsation generation provides the clearest explanation of the signatures observed.
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Ogunsua, Babalola O., Xiushu Qie, Abhay Srivastava, Oladipo Emmanuel Abe, Charles Owolabi, Rubin Jiang i Jing Yang. "Ionospheric Perturbations Due to Large Thunderstorms and the Resulting Mechanical and Acoustic Signatures". Remote Sensing 15, nr 10 (15.05.2023): 2572. http://dx.doi.org/10.3390/rs15102572.
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Perturbations from thunderstorms can play a notable role in the dynamics of the ionosphere. In this work, ionospheric perturbation effects due to thunderstorms were extracted and studied. Thunderstorm-associated lightning activities and their locations were detected by the World-Wide Lightning Location Network (WWLLN). The mechanical components of ionospheric perturbations due to thunderstorms were extracted from the total electron content (TEC), which was measured at selected thunderstorm locations using the polynomial filtering method. Further analyses were conducted using wavelet analysis and Discrete Fourier Transform (DFT) to study the frequency modes and periodicities of TEC deviation. It was revealed that the highest magnitudes of TEC deviations could reach up to ~2.2 TECUs, with dominant modes of frequency in the range of ~0.2 mHz to ~1.2 mHz, falling within the gravity wave range and the second dominant mode in the acoustic range of >1 mHz to <7.5 mHz. Additionally, a 20–60 min time delay was observed between the sprite events, the other high-energy electrical discharges, and the time of occurrence at the highest peak of acoustic-gravity wave perturbations extracted from TEC deviations. The possible mechanism responsible for this phenomenon is further proposed and discussed.
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Wild, J. A., T. K. Yeoman, P. Eglitis i H. J. Opgenoorth. "Multi-instrument observations of the electric and magnetic field structure of omega bands". Annales Geophysicae 18, nr 1 (31.01.2000): 99–110. http://dx.doi.org/10.1007/s00585-000-0099-6.
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Abstract. High time resolution data from the CUTLASS Finland radar during the interval 01:30-03:30 UT on 11 May, 1998, are employed to characterise the ionospheric electric field due to a series of omega bands extending ~5° in latitude at a resolution of 45 km in the meridional direction and 50 km in the azimuthal direction. E-region observations from the STARE Norway VHF radar operating at a resolution of 15 km over a comparable region are also incorporated. These data are combined with ground magnetometer observations from several stations. This allows the study of the ionospheric equivalent current signatures and height integrated ionospheric conductances associated with omega bands as they propagate through the field-of-view of the CUTLASS and STARE radars. The high-time resolution and multi-point nature of the observations leads to a refinement of the previous models of omega band structure. The omega bands observed during this interval have scale sizes ~500 km and an eastward propagation velocity ~0.75 km s-1. They occur in the morning sector (~05 MLT), simultaneously with the onset/intensification of a substorm to the west during the recovery phase of a previous substorm in the Scandinavian sector. A possible mechanism for omega band formation and their relationship to the substorm phase is discussed..Key words. Ionosphere (auroral ionosphere; electric fields and currents) · Magnetospheric physics (magnetosphere-ionosphere interactions)
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Fagundes, P. R., Y. Sahai, I. S. Batista, M. A. Abdu, J. A. Bittencourt i H. Takahashi. "Observations of day-to-day variability in precursor signatures to equatorial F-region plasma depletions". Annales Geophysicae 17, nr 8 (31.08.1999): 1053–63. http://dx.doi.org/10.1007/s00585-999-1053-x.
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Abstract. In December 1995, a campaign was carried out to study the day-to-day variability in precursor signatures to large-scale ionospheric F-region plasma irregularities, using optical diagnostic techniques, near the magnetic equator in the Brazilian sector. Three instruments were operated simultaneously: (a) an all-sky (180° field of view) imaging system for observing the OI 630 nm nightglow emission at Alcântara (2.5°S, 44.4°W); (b) a digisonde (256-Lowell) at São Luis (2.6°S, 44.2°W); and (c) a multi-channel tilting filter-type zenith photometer for observing the OI 630 nm and mesospheric nightglow emissions at Fortaleza (3.9°S, 38.4°W). During the period December 14-18, 1995 (summer in the southern hemisphere), a good sequence of the OI 630 nm imaging observations on five consecutive nights were obtained, which are presented and discussed in this study. The observing period was geomagnetically quiet to moderate (Kp = 0+ to 5+; Dst = 18 nT to -37 nT). On four nights, out of the five observation nights, the OI 630 nm imaging pictures showed formations of transequatorial north-south aligned intensity depletions, which are the optical signatures of large-scale ionospheric F-region plasma bubbles. However, considerable day-to-day variability in the onset and development of the plasma depleted bands was observed. On one of the nights it appears that the rapid uplifting of the F-layer in the post-sunset period, in conjunction with gravity wave activity at mesospheric heights, resulted in generation of very strong plasma bubble irregularities. One of the nights showed an unusual formation of north-south depleted band in the western sector of the imaging system field of view, but the structure did not show any eastward movement, which is a normal characteristic of plasma bubbles. This type of irregularity structure, which probably can be observed only by wide-angle imaging system, needs more investigations for a better understanding of its behaviour.Key words. Atmospheric composition and structure (airglow and aurora) · Ionosphere (equatorial ionosphere; ionospheric irregularities)
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Pimenta, A. A., P. R. Fagundes, Y. Sahai, J. A. Bittencourt i J. R. Abalde. "Equatorial F-region plasma depletion drifts: latitudinal and seasonal variations". Annales Geophysicae 21, nr 12 (31.12.2003): 2315–22. http://dx.doi.org/10.5194/angeo-21-2315-2003.
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Abstract. The equatorial ionospheric irregularities have been observed in the past few years by different techniques (e.g. ground-based radar, digisonde, GPS, optical instruments, in situ satellite and rocket instrumentation), and its time evolution and propagation characteristics can be used to study important aspects of ionospheric dynamics and thermosphere-ionosphere coupling. At present, one of the most powerful optical techniques to study the large-scale ionospheric irregularities is the all-sky imaging photometer system, which normally measures the strong F-region nightglow 630 nm emission from atomic oxygen. The monochromatic OI 630 nm emission images usually show quasi-north-south magnetic field-aligned intensity depletion bands, which are the bottomside optical signatures of large-scale F-region plasma irregularities (also called plasma bubbles). The zonal drift velocities of the plasma bubbles can be inferred from the space-time displacement of the dark structures (low intensity regions) seen on the images. In this study, images obtained with an all-sky imaging photometer, using the OI 630 nm nightglow emission, from Cachoeira Paulista (22.7° S, 45° W, 15.8° S dip latitude), Brazil, have been used to determine the nocturnal monthly and latitudinal variation characteristics of the zonal plasma bubble drift velocities in the low latitude (16.7° S to 28.7° S) region. The east and west walls of the plasma bubble show a different evolution with time. The method used here is based on the western wall of the bubble, which presents a more stable behavior. Also, the observed zonal plasma bubble drift velocities are compared with the thermospheric zonal neutral wind velocities obtained from the HWM-90 model (Hedin et al., 1991) to investigate the thermosphere-ionosphere coupling. Salient features from this study are presented and discussed.Key words. Ionosphere (ionosphere-atmosphere interactions; ionospheric irregularities; instruments and techniques)
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Dudeney, J. R., K. B. Baker, P. H. Stoker i A. D. M. Walker. "The Southern Hemisphere Auroral Radar Experiment (SHARE)". Antarctic Science 6, nr 1 (marzec 1994): 123–24. http://dx.doi.org/10.1017/s0954102094000155.
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The near Earth space environment (known as Geospace) is dominated by the interaction between the solar wind and the geomagnetic field, which creates the magnetosphere. Considerable energy flows from the solar wind into the magnetosphere and ends up in the Earth's upper atmosphere (the thermosphere and ionosphere). The coupling of the geomagnetic field with that of the solar wind (known as the interplanetary magnetic field, or IMF) produces a variety of electro-dynamic responses with signatures such as electric fields and currents in the polar ionospheres. These produce, inter alia, motion of the ionospheric plasma (at altitudes between 100 and 1000kms) which can be monitored from the ground using radar techniques. Analysis of such plasma motion provides a very powerful means of investigating the nature of the interactions taking place at the boundaries between the magnetosphere and the solar wind. To do this effectively requires simultaneous measurements over as large an area (in latitude and longitude) as possible.
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Ritter, P., i H. Lühr. "Near-Earth magnetic signature of magnetospheric substorms and an improved substorm current model". Annales Geophysicae 26, nr 9 (18.09.2008): 2781–93. http://dx.doi.org/10.5194/angeo-26-2781-2008.
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Abstract. Based on a comprehensive catalogue with more than 4000 magnetospheric substorm entries from the years 2000–2005, the spatial distribution of the substorm-related magnetic signatures at mid and low latitudes around local midnight was investigated. Superposed epoch analysis of a larger number of recent observatory data from mid and low latitudes revealed a field strength increase that is consistent with the results of earlier studies. For the first time, the magnetic signature of the substorm current wedge formation is studied also in near-Earth satellite data from CHAMP. The average maximal deflection measured on board the satellite is smaller by a factor of 2 than that determined from ground observations. The recurrence frequency of substorms as well as the amplitude of their magnetic signature depends strongly on the prevailing magnetic activity. The observed average substorm-related magnetic field signatures cannot be described adequately by a simple current wedge model. A satisfactory agreement between model results and observations at satellite height and on ground can be achieved only if the current reconfiguration scenario combines four elements: (1) the gradual decrease of the tail lobe field, (2) the re-routing of a part of the cross-tail current through the ionosphere, (3) eastward ionospheric currents at low and mid latitudes driven by Region-2 field-aligned currents, and (4) a partial ring current connected to these Region-2 FACs.
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Jarmołowski, Wojciech, Anna Belehaki, Manuel Hernández Pajares, Michael Schmidt, Andreas Goss, Paweł Wielgosz, Heng Yang i in. "Combining Swarm Langmuir probe observations, LEO-POD-based and ground-based GNSS receivers and ionosondes for prompt detection of ionospheric earthquake and tsunami signatures: case study of 2015 Chile-Illapel event". Journal of Space Weather and Space Climate 11 (2021): 58. http://dx.doi.org/10.1051/swsc/2021042.
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The study investigates ionospheric electric field responses to the earthquake (EQ) of magnitude 8.3, and the related seismic activity and tsunami triggered by the mainshock in Chile-Illapel region, at 22:54 UTC, in the evening of September 16, 2015. The work is a wider review of available ground and satellite data and techniques available to detect seismically induced traveling ionospheric disturbances (TID) and irregularities of smaller scale. The data used in the experiment includes several types of ground and satellite observations from low-Earth orbit (LEO) satellites. The number of techniques applied here is also extended and includes spectral analysis of LEO along-track data and composed analysis of ground GNSS data. The timeframe of the analyses is focused on September 16 and 17, 2015 but also extended to several adjacent days, where an enhanced seismic activity has been recorded. Several examples of seismically triggered TIDs are shown, as detected by combined observations from more than one source and applying different methods, including spectral analysis. These disturbances occur before the mainshock, just after, or in time following this large EQ, and can be found in close neighborhood of Chile-Illapel or far away from the epicenter. The objective of the work was to demonstrate an increasing number of available data and techniques, which can be limited when applied alone, but their combination can provide many advantages in the analysis of seismically disturbed ionosphere. The combination of LEO satellite data reaching all regions of the globe with local but dense ground-based GNSS data and ionospheric HF sounders looks promising, especially in view of the nearby availability of CubeSat constellations equipped with instruments for ionosphere sounding. An important conclusion coming from the study is a need for spectral analysis techniques in the processing of LEO along-track data and the requirement of the validation of LEO observations with separate LEO data or ground-based data. A general but key finding refers to the complementarities of different observations of the ionospheric electric field, which is critically important in the case of analyzing ionospheric irregularities in the extended and composed ionosphere, especially if not every sounding direction can successfully find it.
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Peñano, J. R., i G. Ganguli. "Ionospheric Source for Low-Frequency Broadband Electromagnetic Signatures". Physical Review Letters 83, nr 7 (16.08.1999): 1343–46. http://dx.doi.org/10.1103/physrevlett.83.1343.
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Engebretson, M. J., B. J. Anderson, L. J. Cahill, R. L. Arnoldy, T. J. Rosenberg, D. L. Carpenter, W. B. Gail i R. H. Eather. "Ionospheric signatures of cusp latitude Pc 3 pulsations". Journal of Geophysical Research 95, A3 (1990): 2447. http://dx.doi.org/10.1029/ja095ia03p02447.
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Inan, U. S., T. F. Bell, V. P. Pasko, D. D. Sentman, E. M. Wescott i W. A. Lyons. "VLF signatures of ionospheric disturbances associated with sprites". Geophysical Research Letters 22, nr 24 (15.12.1995): 3461–64. http://dx.doi.org/10.1029/95gl03507.
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Sutcliffe, Peter R. "Amplitude modulation in ionospheric signatures of ULF pulsations". Planetary and Space Science 40, nr 7 (lipiec 1992): 965–71. http://dx.doi.org/10.1016/0032-0633(92)90136-c.
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