Articles de revues : « Auroral electrons »

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Kozelov, Boris V., et Elena E. Titova. « Conjunction Ground Triangulation of Auroras and Magnetospheric Processes Observed by the Van Allen Probe Satellite near 6 Re ». Universe 9, n^o 8 (29 juillet 2023) : 353. http://dx.doi.org/10.3390/universe9080353.

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Conjunction observations of auroras with electron distributions and broadband electrostatic fluctuations on Van Allen Probe A satellite in the equatorial region are considered. Using triangulation measurements, the energy spectra of the precipitating electrons in the rayed auroral structures were determined for the 17 March 2015 event. A comparison of the spectra of precipitating electrons in the auroral rays with satellite measurements of electrons in the equatorial region related to the aurora showed their agreement. The concomitance between Van Allen Probe A broadband electric waves and auroral variations measured by the ground-based auroral camera was observed on 17 March 2015. This suggests that broadband electrostatic waves may be responsible for electron precipitation, leading to the formation of rayed structures in the aurora.

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Vichare, Geeta, Ankush Bhaskar, Rahul Rawat, Virendra Yadav, Wageesh Mishra, Dorje Angchuk et Anand Kumar Singh. « Low-latitude Auroras : Insights from 2023 April 23 Solar Storm ». Astrophysical Journal 977, n^o 2 (1 décembre 2024) : 171. https://doi.org/10.3847/1538-4357/ad8dd3.

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Abstract In 2023 April, a low-latitude aurora observed by the all-sky camera at Hanle, Ladakh, India (33°14’N geographic latitude), generated significant interest. This was the first such aurora recorded from the Indian region in the space era and occurred during a moderate solar storm. This study explores this low-latitude auroral sighting, which happened during the sheath-region passage of an interplanetary coronal mass ejection. We analyze in situ multispacecraft particle measurements and geomagnetic field observations from both ground-based and satellite-based magnetometers. The auroral observations at Hanle coincided with intense substorm activity. Our findings indicate that the aurora did not actually reach India; the equatorward boundary was beyond 50°N geographic latitude. Enhanced electron fluxes with energies below 100 eV were detected at 54°N geographic latitude at about 830 km altitude in the predawn sector (4–5 hr local time). In the midnight sector, the equatorward boundary is estimated to be around 52°N geographic latitude, based on Hanle observations and considering emission altitudes of 600–650 km due to low-energy electrons. Thus, the low-latitude red aurora observed from India resulted from the emissions at higher altitudes due to low-energy electron precipitation in the auroral oval and a slight equatorward expansion of the auroral oval. The low-energy electrons likely originated from the plasma sheet and were precipitated due to enhanced wave–particle interactions from strong magnetosphere compression during high solar wind pressure. This study is crucial in understanding low-latitude auroras in the modern space era.

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Samara, M., R. G. Michell, K. Asamura, M. Hirahara, D. L. Hampton et H. C. Stenbaek-Nielsen. « Ground-based observations of diffuse auroral structures in conjunction with Reimei measurements ». Annales Geophysicae 28, n^o 3 (26 mars 2010) : 873–81. http://dx.doi.org/10.5194/angeo-28-873-2010.

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Abstract. We present results from ground-based auroral observations coordinated with the Japanese satellite, Reimei, that took place during the winters of 2006, 2007 and 2008 at Poker Flat, Alaska. Comparable temporal and spatial resolution for the optical and in situ particle data, allowed for investigation of small scale and/or rapidly time-varying auroral structures. Four satellite passes through diffuse auroral structures were identified. The structures within the aurora, whether stationary or time-varying (pulsating aurora), were most closely correlated with the highest energy precipitating electrons measured by these detectors (8 to 12 keV). This relation is found to be consistent across all four examples, revealing that the electron precipitation responsible for these diffuse auroral structures is primarily that of the ≥8 keV electrons.

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Yahnin, A. G., V. A. Sergeev, B. B. Gvozdevsky et S. Vennerstrøm. « Magnetospheric source region of discrete auroras inferred from their relationship with isotropy boundaries of energetic particles ». Annales Geophysicae 15, n^o 8 (31 août 1997) : 943–58. http://dx.doi.org/10.1007/s00585-997-0943-z.

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Abstract. According to observations, the discrete auroral arcs can sometimes be found, either deep inside the auroral oval or at the poleward border of the wide (so-called double) auroral oval, which map to very different regions of the magnetotail. To find common physical conditions for the auroral-arc generation in these magnetotail regions, we study the spatial relationship between the diffuse and discrete auroras and the isotropic boundaries (IBs) of the precipitating energetic particles which can be used to characterise locally the equatorial magnetic field in the tail. From comparison of ground observation of auroral forms with meridional profiles of particle flux measured simultaneously by the low-altitude NOAA satellites above the ground observation region, we found that (1) discrete auroral arcs are always situated polewards from (or very close to) the IB of >30-keV electrons, whereas (2) the IB of the >30-keV protons is often seen inside the diffuse aurora. These relationships hold true for both quiet and active (substorm) conditions in the premidnight-nightside (18-01-h) MLT sector considered. In some events the auroral arcs occupy a wide latitudinal range. The most equatorial of these arcs was found at the poleward edge of the diffuse auroras (but anyway in the vicinity of the electron IB), the most poleward arcs were simultaneously observed on the closed field lines near the polar-cap boundary. These observations disagree with the notion that the discrete aurora originate exclusively in the near-Earth portion of plasma sheet or exclusively on the PSBL field lines. Result (1) may imply a fundamental feature of auroral-arc formation: they originate in the current-sheet regions having very curved and tailward-stretched magnetic field lines.

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Blixt, E. M., M. J. Kosch et J. Semeter. « Relative drift between black aurora and the ionospheric plasma ». Annales Geophysicae 23, n^o 5 (27 juillet 2005) : 1611–21. http://dx.doi.org/10.5194/angeo-23-1611-2005.

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Abstract. Black auroras are recognized as spatially well-defined regions within uniform diffuse aurora where the optical emission is significantly reduced. Although a well studied phenomenon, there is no generally accepted theory for black auroras. One theory suggests that black regions are formed when energetic magnetospheric electrons no longer have access to the loss cone. If this blocking mechanism drifts with the source electron population in the magnetosphere, black auroras in the ionosphere should drift eastward with a velocity that increases with the energy of the precipitating electrons in the surrounding aurora, since the gradient-B curvature drift is energy dependent. It is the purpose of this paper to test this hypothesis. To do so we have used simultaneous measurements by the European Incoherent Scatter (EISCAT) radar and an auroral TV camera at Tromsø, Norway. We have analyzed 8 periods in which a black aurora occurred frequently to determine their relative drift with respect to the ionospheric plasma. The black aurora was found to drift eastward with a velocity of 1.5–4km/s, which is in accordance with earlier observations. However, one case was found where a black patch was moving westward, this being the first report of such behaviour in the literature. In general, the drift was parallel to the ionospheric flow but at a much higher velocity. This suggests that the generating mechanism is not of ionospheric origin. The characteristic energy of the precipitating electron population was estimated through inversion of E-region plasma density profiles. We show that the drift speed of the black patches increased with the energy of the precipitating electrons in a way consistent with the gradient-B curvature drift, suggesting a magnetospheric mechanism for the black aurora. As expected, a comparison of the drift speeds with a rudimentary dipole field model of the gradient-B curvature drift speed only yields order-of-magnitude agreement, which most likely is due to the nightside disturbed magnetosphere being significantly stretched. Keywords. Auroral ionosphere; MI interaction; Energetic particles, precipitating

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Sergienko, T., I. Sandahl, B. Gustavsson, L. Andersson, U. Brändström et Å. Steen. « A study of fine structure of diffuse aurora with ALIS-FAST measurements ». Annales Geophysicae 26, n^o 11 (21 octobre 2008) : 3185–95. http://dx.doi.org/10.5194/angeo-26-3185-2008.

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Abstract. We present results of an investigation of the fine structure of the night sector diffuse auroral zone, observed simultaneously with optical instruments (ALIS) from the ground and the FAST electron spectrometer from space 16 February 1997. Both the optical and particle data show that the diffuse auroral zone consisted of two regions. The equatorward part of the diffuse aurora was occupied by a pattern of regular, parallel auroral stripes. The auroral stripes were significantly brighter than the background luminosity, had widths of approximately 5 km and moved southward with a velocity of about 100 m/s. The second region, located between the region with auroral stripes and the discrete auroral arcs to the north, was filled with weak and almost homogeneous luminosity, against which short-lived auroral rays and small patches appeared chaotically. From analysis of the electron differential fluxes corresponding to the different regions of the diffuse aurora and based on existing theories of the scattering process we conclude the following: Strong pitch angle diffusion by electron cyclotron harmonic waves (ECH) of plasma sheet electrons in the energy range from a few hundred eV to 3–4 keV was responsible for the electron precipitation, that produced the background luminosity within the whole diffuse zone. The fine structure, represented by the auroral stripes, was created by precipitation of electrons above 3–4 keV as a result of pitch angle diffusion into the loss cone by whistler mode waves. A so called "internal gravity wave" (Safargaleev and Maltsev, 1986) may explain the formation of the regular spatial pattern formed by the auroral stripes in the equatorward part of the diffuse auroral zone.

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Blomberg, L. G., J. A. Cumnock, I. I. Alexeev, E. S. Belenkaya, S. Yu Bobrovnikov et V. V. Kalegaev. « Transpolar aurora : time evolution, associated convection patterns, and a possible cause ». Annales Geophysicae 23, n^o 5 (28 juillet 2005) : 1917–30. http://dx.doi.org/10.5194/angeo-23-1917-2005.

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Abstract. We present two event studies illustrating the detailed relationships between plasma convection, field-aligned currents, and polar auroral emissions, as well as illustrating the influence of the Interplanetary Magnetic Field's y-component on theta aurora development. The transpolar arc of the theta aurorae moves across the entire polar region and becomes part of the opposite side of the auroral oval. Electric and magnetic field and precipitating particle data are provided by DMSP, while the POLAR UVI instrument provides measurements of auroral emissions. Ionospheric electrostatic potential patterns are calculated at different times during the evolution of the theta aurora using the KTH model. These model patterns are compared to the convection predicted by mapping the magnetopause electric field to the ionosphere using the Paraboloid Model of the magnetosphere. The model predicts that parallel electric fields are set up along the magnetic field lines projecting to the transpolar aurora. Their possible role in the acceleration of the auroral electrons is discussed. Keywords. Ionosphere (Plasma convection; Polar ionosphere) – Magnetospheric physics (Magnetosphereionosphere interactions)

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Kong, Wanqiu, Zejun Hu, Jiaji Wu, Tan Qu et Gwanggil Jeon. « A Comparative Study of Estimating Auroral Electron Energy from Ground-Based Hyperspectral Imagery and DMSP-SSJ5 Particle Data ». Remote Sensing 12, n^o 14 (14 juillet 2020) : 2259. http://dx.doi.org/10.3390/rs12142259.

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Aurora, the spectacular phenomenon commonly occurring in high latitudes, is caused by the precipitation of energetic particles penetrating the Earth’s atmosphere. Being the result of solar-terrestrial interactions, electron precipitation significantly contributes to auroral production. To evaluate its magnitude, a physical quantity describing the characteristics of precipitating auroral electrons—their characteristic energy—is adopted. In this paper, this quantity is derived from joint data observed by the ground-based auroral spectroscopic imager located in Antarctica Zhongshan Station and the particle detectors “Special Sensor J5 (SSJ5)” on the Defense Meteorological Satellite Program (DMSP) satellites. A postprocessing scheme of ground-based spectral data is proposed to infer the characteristic energy that successively uses classical brute-force, recursive brute-force and self-consistent approximation strategies for step-up speed improvement. Then, the inferred characteristic energies are compared to the average energies calibrated from the relevant electron data detected by SSJ5 to confirm whether this inference is valid. Regarding DMSP F18/SSJ5, these two energy estimations about auroral electrons deviate slightly from each other and show a strong linear relationship. It sheds light on further applications of the valuable aurora spectral data.

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Lummerzheim, D., et J. Lilensten. « Electron transport and energy degradation in the ionosphere : evaluation of the numerical solution, comparison with laboratory experiments and auroral observations ». Annales Geophysicae 12, n^o 10/11 (31 août 1994) : 1039–51. http://dx.doi.org/10.1007/s00585-994-1039-7.

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Abstract. Auroral electron transport calculations are a critical part of auroral models. We evaluate a numerical solution to the transport and energy degradation problem. The numerical solution is verified by reproducing simplified problems to which analytic solutions exist, internal self-consistency tests, comparison with laboratory experiments of electron beams penetrating a collision chamber, and by comparison with auroral observations, particularly the emission ratio of the N2 second positive to N+2 first negative emissions. Our numerical solutions agree with range measurements in collision chambers. The calculated N22P to N+21N emission ratio is independent of the spectral characteristics of the incident electrons, and agrees with the value observed in aurora. Using different sets of energy loss cross sections and different functions to describe the energy distribution of secondary electrons that emerge from ionization collisions, we discuss the uncertainties of the solutions to the electron transport equation resulting from the uncertainties of these input parameters.

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Milan, S. E., A. Grocott, C. Forsyth, S. M. Imber, P. D. Boakes et B. Hubert. « A superposed epoch analysis of auroral evolution during substorm growth, onset and recovery : open magnetic flux control of substorm intensity ». Annales Geophysicae 27, n^o 2 (11 février 2009) : 659–68. http://dx.doi.org/10.5194/angeo-27-659-2009.

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Abstract. We perform two superposed epoch analyses of the auroral evolution during substorms using the FUV instrument on the Imager for Magnetopause-to-Aurora Global Explorer (IMAGE) spacecraft. The larger of the two studies includes nearly 2000 substorms. We subdivide the substorms by onset latitude, a measure of the open magnetic flux in the magnetosphere, and determine average auroral images before and after substorm onset, for both electron and proton aurora. Our results indicate that substorms are more intense in terms of auroral brightness when the open flux content of the magnetosphere is larger, and that magnetic flux closure is more significant. The increase in auroral brightness at onset is larger for electrons than protons. We also show that there is a dawn-dusk offset in the location of the electron and proton aurora that mirrors the relative locations of the region 1 and region 2 current systems. Superposed epoch analyses of the solar wind, interplanetary magnetic field, and geomagnetic indices for the substorms under study indicate that dayside reconnection is expected to occur at a faster rate prior to low latitude onsets, but also that the ring current is enhanced for these events.

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Moen, J., J. A. Holtet, A. Pedersen, B. Lybekk, K. Svenes, K. Oksavik, W. F. Denig, E. Lucek, F. Søraas et M. André. « Cluster boundary layer measurements and optical observations at magnetically conjugate sites ». Annales Geophysicae 19, n^o 10/12 (30 septembre 2001) : 1655–68. http://dx.doi.org/10.5194/angeo-19-1655-2001.

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Abstract. The Cluster spacecraft experienced several boundary layer encounters when flying outbound from the magnetosphere to the magnetosheath in the dusk sector on 14 January 2001. The dayside boundary layer was populated by magnetosheath electrons, but not with quite as high densities as in the magnetosheath itself. The Cluster ground track was calculated using the Tsyganenko-96 model which appears to be a strong tool for combining high-altitude satellite and ground observations, given that the solar wind conditions are known. This paper focuses on identifying auroral responses corresponding to boundary layer dynamics observed by Cluster. The first boundary layer encounter studied was a brief visit into a closed LLBL, most likely due to a boundary wave that travelled tailward over the spacecraft. A corresponding equatorward and eastward movement was seen in the post-noon aurora between Greenland and Svalbard. The second boundary encounter was in a high-latitude cusp, and occurred as a consequence of a transient reconfiguration of the cusp. The cusp expanded duskward over the spacecraft into the late post-noon sector. NOAA-12 probed the 16:30 MLT sector of this auroral activity, and measured a 1.4 keV electron beam located poleward of the 30 keV electron-trapping boundary. A sequence of three moving auroral forms emanating from this active region are likely candidates for flux transfer events. The auroral signatures are discussed in relation to earlier observations, and appear to be an example of accelerated electrons/discrete post-noon aurora on open magnetic field lines.Key words. Ionosphere (particle precipitation) Magnetospheric physics (auroral phenomena; magnetopause, cusp and boundary layers)

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Fukizawa, Mizuki, Takeshi Sakanoi, Yoshimasa Tanaka, Yasunobu Ogawa, Keisuke Hosokawa, Björn Gustavsson, Kirsti Kauristie et al. « Reconstruction of precipitating electrons and three-dimensional structure of a pulsating auroral patch from monochromatic auroral images obtained from multiple observation points ». Annales Geophysicae 40, n^o 4 (12 juillet 2022) : 475–84. http://dx.doi.org/10.5194/angeo-40-475-2022.

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Abstract. In recent years, aurora observation networks using high-sensitivity cameras have been developed in the polar regions. These networks allow dimmer auroras, such as pulsating auroras (PsAs), to be observed with a high signal-to-noise ratio. We reconstructed the horizontal distribution of precipitating electrons using computed tomography with monochromatic PsA images obtained from three observation points. The three-dimensional distribution of the volume emission rate (VER) of the PsA was also reconstructed. The characteristic energy of the reconstructed precipitating electron flux ranged from 6 to 23 keV, and the peak altitude of the reconstructed VER ranged from 90 to 104 km. We evaluated the results using a model aurora and compared the model's electron density with the observed one. The electron density was reconstructed correctly to some extent, even after a decrease in PsA intensity. These results suggest that the horizontal distribution of precipitating electrons associated with PsAs can be effectively reconstructed from ground-based optical observations.

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Waite, J. Hunter, John T. Clarke, R. J. Walker, John E. P. Connerney, D. McComas, P. Riley et William S. Lewis. « Jupiter’s Aurora : Solar Wind and Rotational Influences ». Highlights of Astronomy 12 (2002) : 606. http://dx.doi.org/10.1017/s1539299600014362.

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AbstractJovian auroral emissions are observed at infrared, visible, ultraviolet, and x-ray wavelengths. As at Earth, pitch-angle scattering of energetic particles into the atmospheric loss cone and the acceleration of current-carrying electrons in field-aligned currents both play a role in exciting the auroral emissions. The x-ray aurora is believed to result principally from heavy ion precipitation, while the ultraviolet aurora is produced predominantly by precipitating energetic electrons. The magnetospheric processes responsible for the aurora are driven primarily by planetary rotation. Acceleration of Iogenic plasma by rotationally-induced electric fields results in both the formation of the energetic ions that are scattered and the formation of strong, field-aligned currents that communicate the torques from the ionosphere. In addition to rotation-driven processes, solar-wind-modulated processes in the outer magnetosphere may lead to highly, time-dependent acceleration and thus also contribute to jovian auroral activity. Observational evidence for both sources will be presented. See Waite et al. (2001, Nat., 410, 787).

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Stoker, P. H. « Elektronpresipitasie in die bo-atmosfeer te Sanae ». Suid-Afrikaanse Tydskrif vir Natuurwetenskap en Tegnologie 14, n^o 3 (10 juillet 1995) : 78–84. http://dx.doi.org/10.4102/satnt.v14i3.613.

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Auroral light emissions are due to collisional processes of electrons with atmospheric constituents and occur primarily above 100 km in the ionospheric E- and F-layers, according to studies on excitation of emission lines. The absorption of about ten metre wavelength cosmic radio noise energy, as observed by riometers, occurs mainly below 90 km due to an increase of electron concentration in the ionospheric D-layer. Because auroral luminosity and auroral absorption are produced mainly by electrons in different energy regions, simultaneous observations will provide information on the energy spectrum of the incident electrons and on spectral changes of these precipitating electrons during auroral events.

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Klimov, P. A., K. F. Sigaeva et V. V. Kalegaev. « Registration of the auroral near-UV emission by the orbital detector TUS ». Izvestiâ Akademii nauk SSSR. Seriâ fizičeskaâ 88, n^o 2 (14 octobre 2024) : 327–30. http://dx.doi.org/10.31857/s0367676524020291.

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The TUS detector is a highly sensitive orbital telescope. Due to the polar orbit of the spacecraft, the detector made observations of the UV luminosity of the atmosphere above the aurora oval. Events with intensity variations characteristic of pulsating auroras have been registered. The events are located along the equatorial boundary of the auroral oval and occur during long-term geomagnetic disturbances. Comparison with data from charged particle detectors shows the presence of an increased flux of precipitating high-energy electrons (with energies above 100 keV) simultaneously with UV pulsations.

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Shi, R., D. Han, B. Ni, Z. J. Hu, C. Zhou et X. Gu. « Intensification of dayside diffuse auroral precipitation : contribution of dayside Whistler-mode chorus waves in realistic magnetic fields ». Annales Geophysicae 30, n^o 9 (3 septembre 2012) : 1297–307. http://dx.doi.org/10.5194/angeo-30-1297-2012.

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Abstract. Compared to the recently improved understanding of nightside diffuse aurora, the mechanism(s) responsible for dayside diffuse aurora remains poorly understood. While dayside chorus has been thought as a potential major contributor to dayside diffuse auroral precipitation, quantitative analyses of the role of chorus wave scattering have not been carefully performed. In this study we investigate a dayside diffuse auroral intensification event observed by the Chinese Arctic Yellow River Station (YRS) all-sky imagers (ASI) on 7 January 2005 and capture a substantial increase in diffuse auroral intensity at the 557.7 nm wavelength that occurred over almost the entire ASI field-of-view near 09:24 UT, i.e., ~12:24 MLT. Computation of bounce-averaged resonant scattering rates by dayside chorus emissions using realistic magnetic field models demonstrates that dayside chorus scattering can produce intense precipitation losses of plasma sheet electrons on timescales of hours (even approaching the strong diffusion limit) over a broad range of both energy and pitch angle, specifically, from ~1 keV to 50 keV with equatorial pitch angles from the loss cone to up to ~85° depending on electron energy. Subsequent estimate of loss cone filling index indicates that the loss cone can be substantially filled, due to dayside chorus driven pitch angle scattering, at a rate of ≥0.8 for electrons from ~500 eV to 50 keV that exactly covers the precipitating electrons for the excitation of green-line diffuse aurora. Estimate of electron precipitation flux at different energy levels, based on loss cone filling index profile and typical dayside electron distribution observed by THEMIS spacecraft under similar conditions, gives a total precipitation electron energy flux of the order of 0.1 erg cm−2 s−1 with ~1 keV characteristic energy (especially when using T01s), which can be very likely to cause intense green-line diffuse aurora activity on the dayside. Therefore, dayside chorus scattering in the realistic magnetic field can greatly contribute to the YRS ASI observed intensification of dayside green-line aurora. Besides wave induced scattering and changes in the ambient magnetic field, variations in associated electron flux can also contribute to enhanced diffuse aurora emissions, the possibility of which we cannot exactly rule out due to lack of simultaneous observations of magnetospheric particles. Since the geomagnetic activity level was rather low during the period of interest, it is reasonable to infer that changes in the associated electron flux in the magnetosphere should be small, and consequently its contribution to the observed enhanced diffuse auroral activity should be small as well. Our results support the scenario that dayside chorus could play a major role in the production of dayside diffuse aurora, and also demonstrate that changes in magnetospheric magnetic field should be considered to reasonably interpret observations of dayside diffuse aurora.

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Treumann, R. A., R. Nakamura et W. Baumjohann. « Downward auroral currents from the reconnection Hall-region ». Annales Geophysicae 29, n^o 4 (14 avril 2011) : 679–85. http://dx.doi.org/10.5194/angeo-29-679-2011.

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Abstract. We present a simple (stationary) mechanism capable of generating the auroral downward field-aligned electric field that is needed for accelerating the ionospheric electron component up into the magnetosphere and confining the ionospheric ions at low latitudes (as is required by observation of an ionospheric cavity in the downward auroral current region). The lifted ionospheric electrons carry the downward auroral current. Our model is based on the assumption of collisionless reconnection in the tail current sheet. It makes use of the dynamical difference between electrons and ions in the ion inertial region surrounding the reconnection X-line which causes Hall currents to flow. We show that the spatial confinement of the Hall magnetic field and flux to the ion inertial region centred on the X-point generates a spatially variable electromotive force which is positive near the outer inflow boundaries of the ion inertial region and negative in the central inflow region. Looked at from the ionosphere it functions like a localised meso-scale electric potential. The positive electromotive force gives rise to upward electron flow from the ionosphere during substorms (causing "black aurorae"). A similar positive potential is identified on the earthward side of the fast reconnection outflow region which has the same effect, explaining the observation that auroral upward currents are flanked from both sides by narrow downward currents.

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Norberg, O., J. D. Winningham, H. Lauche, W. Keith, W. Puccio, J. Olsen, K. Lundin et J. Scherrer. « The MEDUSA electron and ion spectrometer and the PIA ultraviolet photometers on Astrid-2 ». Annales Geophysicae 19, n^o 6 (30 juin 2001) : 593–600. http://dx.doi.org/10.5194/angeo-19-593-2001.

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Abstract. The miniature electron and ion spectrometer MEDUSA on Astrid-2 consists of two "top-hat"-type spherical electrostatic analyzers, sharing a common top-hat. Fast energy sweeps (16 electron sweeps and 8 ion sweeps per second) allow for very high temporal resolution measurements of a two-dimensional slice of the particle distribution function. The energy range covered, is in the case of electrons, 4 eV to 22 keV and, in the case of ions, 2 eV to 12 keV. MEDUSA is mounted with its aperture close to the spin plane of Astrid-2, which allows for good pitch-angle coverage when the local magnetic field is in the satellite spin plane. The PIA-1/2 spin-scanning ultraviolet photometers measure auroral emissions. Using the spacecraft spin and orbital motion, it is possible to create two-dimensional images from the data. Spin-scanning photometers, such as PIA, are low-cost, low mass alternatives to auroral imagers, but place constraints on the satellite attitude. Data from MEDUSA are used to study processes in the auroral region, in particular, electrodynamics of aurora and "black aurora". MEDUSA is also a technological development, paving the way for highly capable, miniaturized particle spectrometers.Key words. Ionosphere (instruments and techniques) – Magnetospheric physics (auroral phenomena; instruments and techniques)

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Tesema, Fasil, Noora Partamies, Daniel K. Whiter et Yasunobu Ogawa. « Types of pulsating aurora : comparison of model and EISCAT electron density observations ». Annales Geophysicae 40, n^o 1 (4 janvier 2022) : 1–10. http://dx.doi.org/10.5194/angeo-40-1-2022.

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Abstract. Energetic particle precipitation associated with pulsating aurora (PsA) can reach down to lower mesospheric altitudes and deplete ozone. It is well documented that pulsating aurora is a common phenomenon during substorm recovery phases. This indicates that using magnetic indices to model the chemistry induced by PsA electrons could underestimate the energy deposition in the atmosphere. Integrating satellite measurements of precipitating electrons in models is considered to be an alternative way to account for such an underestimation. One way to do this is to test and validate the existing ion chemistry models using integrated measurements from satellite and ground-based observations. By using satellite measurements, an average or typical spectrum of PsA electrons can be constructed and used as an input in models to study the effects of the energetic electrons in the atmosphere. In this study, we compare electron densities from the EISCAT (European Incoherent Scatter scientific radar system) radars with auroral ion chemistry and the energetics model by using pulsating aurora spectra derived from the Polar Operational Environmental Satellite (POES) as an energy input for the model. We found a good agreement between the model and EISCAT electron densities in the region dominated by patchy pulsating aurora. However, the magnitude of the observed electron densities suggests a significant difference in the flux of precipitating electrons for different pulsating aurora types (structures) observed.

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Milan, S. E., N. Sato, M. Lester, T. K. Yeoman, Y. Murata, H. Doi et T. Saemundsson. « The spectral characteristics of E-region radar echoes co-located with and adjacent to visual auroral arcs ». Annales Geophysicae 20, n^o 6 (30 juin 2002) : 795–805. http://dx.doi.org/10.5194/angeo-20-795-2002.

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Abstract. Simultaneous all-sky camera and HF radar observations of the visual and E-region radar aurora in the west-ward electrojet suggest a close relationship between a pair of parallel east-west-aligned auroral arcs, separated by ~ 30 km, and a region of strong radar backscatter. Poleward of this a broader region of radar backscatter is observed, though the spectral characteristics of the echoes in these two regions differ considerably. We suggest that the visual aurorae and their radar counterparts are produced in a region of upward field-aligned current (FAC), whereas the backscatter poleward of this is associated with downward FAC. Relatively low electric fields ( ~ 10 mV m-1) are observed in the vicinity of the arc system, suggesting that in this case, two-stream waves are not directly generated through the electrodynamics of the arc. Rather, the generation of irregularities is most probably associated with the gradient drift instability operating within horizontal electron density gradients produced by the filamentary nature of the arc FAC system. The observation of high Doppler shift echoes superimposed on slow background flow within the region of backscatter poleward of the visual aurora is argued to be consistent with previous suggestions that the ion-acoustic instability threshold is reduced in the presence of upwelling thermal electrons carrying downward FAC.Key words. Ionosphere (auroral ionosphere; ionospheric irregularities; particle precipitation)

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Kumar, S., S. K. Dixit et A. K. Gwal. « Electron cyclotron waves in the presence of parallel electric fields in the Earth's auroral plasma ». Annales Geophysicae 15, n^o 1 (31 janvier 1997) : 24–28. http://dx.doi.org/10.1007/s00585-997-0024-3.

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Abstract. The electron cyclotron waves that originate at low altitudes (<0.5 RE) and observed by ground facilities have been studied in the presence of a weak parallel electric field in auroral magnetoplasma consisting of trapped energetic auroral electrons and cold background electrons of ionospheric origin. The model distribution for auroral trapped electrons is taken as Maxwellian ring distribution. An expression for the growth rate has been obtained in the presence of parallel electric field assuming that the real frequency in the whistler mode is not affected by the presence of the electric field. The results show that waves grow (or damp) in amplitude for a parallel (or antiparallel) electric field. The influence of the electric field is more pronounced at a shorter wavelength spectrum. An increase in population of energetic electrons increases the growth rate and thus, plays a significant role in the wave excitation process in the auroral regions.

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Tanaka, Y. M., T. Aso, B. Gustavsson, K. Tanabe, Y. Ogawa, A. Kadokura, H. Miyaoka, T. Sergienko, U. Brändström et I. Sandahl. « Feasibility study on Generalized-Aurora Computed Tomography ». Annales Geophysicae 29, n^o 3 (15 mars 2011) : 551–62. http://dx.doi.org/10.5194/angeo-29-551-2011.

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Abstract. Aurora Computed Tomography (ACT) is a method for retrieving the three-dimensional (3-D) distribution of the volume emission rate from monochromatic auroral images obtained simultaneously by a multi-point camera network. We extend this method to a Generalized-Aurora Computed Tomography (G-ACT) that reconstructs the energy and spatial distributions of precipitating electrons from multi-instrument data, such as ionospheric electron density from incoherent scatter radar, cosmic noise absorption (CNA) from imaging riometers, as well as the auroral images. The purpose of this paper is to describe the reconstruction algorithm involved in this method and to test its feasibility by numerical simulation. Based on a Bayesian model with prior information as the smoothness of the electron energy spectra, the inverse problem is formulated as a maximization of posterior probability. The relative weighting of each instrument data is determined by the cross-validation method. We apply this method to the simulated data from real instruments, the Auroral Large Imaging System (ALIS), the European Incoherent Scatter (EISCAT) radar at Tromsø, and the Imaging Riometer for Ionospheric Study (IRIS) at Kilpisjärvi. The results indicate that the differential flux of the precipitating electrons is well reconstructed from the ALIS images for the low-noise cases. Furthermore, we demonstrate in a case study that the ionospheric electron density from the EISCAT radar is useful for improving the reconstructed electron flux. On the other hand, the incorporation of CNA data into this method is difficult at this stage, because the extension of energy range to higher energy causes a difficulty in the reconstruction of the low-energy electron flux. Nevertheless, we expect that this method may be useful in analyzing multi-instrument data and, in particular, 3-D data, which will be obtained in the upcoming EISCAT_3D.

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Simmons, D. A. R., et K. Henriksen. « Daytime aurora observed from Spitsbergen ». Polar Record 30, n^o 173 (avril 1994) : 85–96. http://dx.doi.org/10.1017/s003224740002129x.

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AbstractDaytime (or dayside cleft) aurora is almost a permanent feature of the midday skies over Spitsbergen during the continuous darkness of the polar night It was observed in one or other of its characteristic forms around geomagnetic noon on 58 of 59 clear day sduring the wintersof 1987/1988, 1990/1991, and 1992/1993. The three types of day time aurora were studied by visual, colour photographic, and interference-filter techniques to confirm the precise nature of the observed emissions. Prenoon aurora, which is characterised by diffuse, patchy, green aurora at 557.7 nm, was observed on 42 occasions. It is generated by low-energy electrons of less than 300 eV coming through the entry layer of the dayside cleft. Noontime aurora, which consists largely of pure red emissions at 630.0/636.4 nm, was observed on 50 occasions. It is generated by high-flux, very low-energy electrons of 10–50 eV flowing directly from the solar wind through the polar cusp. Postnoon aurora, which is characterised by discrete, green auroral arcs at 557.7 nm, was also observed on 42 occasions. Like prenoon aurora, it is generated by low-energy electrons of less than 300 eV derived from the entry layer of the cleft Occasionally, some background or diffuse aurora is also observed, generated by high fluxes of low-energy proton precipitation and characterised by the hydrogen lines Hα and Hβ. On the one exceptional day on which daytime aurora was not observed, magnetic activity was exceptionally low.These ground-based observations complement satellite studies of analogous auroral events. In particular, the visual characteristics of the different types of daytime aurora may be explained in terms of the flux rates and energy profiles of the electrons that have been mapped in the different regions of the dayside cleft by satellite-borne detectors.

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Axelsson, K., T. Sergienko, H. Nilsson, U. Brändström, Y. Ebihara, K. Asamura et M. Hirahara. « Spatial characteristics of wave-like structures in diffuse aurora obtained using optical observations ». Annales Geophysicae 30, n^o 12 (14 décembre 2012) : 1693–701. http://dx.doi.org/10.5194/angeo-30-1693-2012.

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Abstract. We present the results of a statistical study using optical images from ALIS (Auroral Large Imaging System) to investigate the spatial and temporal variations of structures in diffuse aurora. Analysis of conjugate Reimei data shows that such fine structures are a result of modulation of high-energy precipitating electrons. Pitch angle diffusion into the loss cone due to interaction of whistler mode waves with plasma sheet electrons is the most feasible mechanism leading to high-energy electron precipitation. This suggests that the fine structure is an indication of modulations of the efficiency of the wave–particle interaction. The scale sizes and variations of these structures, mapped to the magnetosphere, can give us information about the characteristics of the modulating wave activity. We found the scale size of the auroral stripes and the spacing between them to be on average 13–14 km, which corresponds to 3–4 ion gyro radii for protons with an energy of 7 keV. The structures move southward with a speed close to zero in the plasma convection frame.

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Lanchester, B. S., M. H. Rees, K. J. F. Sedgemore, J. R. Palmer, H. U. Frey et K. U. Kaila. « Ionospheric response to variable electric fields in small-scale auroral structures ». Annales Geophysicae 16, n^o 10 (31 octobre 1998) : 1343–54. http://dx.doi.org/10.1007/s00585-998-1343-8.

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Abstract. High time and space resolution optical and radar measurements have revealed the influence of electric fields on E-region electron density profiles in small-scale auroral structures. Large electric fields are present adjacent to auroral filaments produced by monoenergetic electron fluxes. The ionisation profiles measured within and beside the auroral filaments show the effects of plasma convection due to electric fields as well as the consequences of the response time to large and dynamic fluxes of energetic electrons. Without high-resolution optical measurements, the interpretation of the radar data is limited.Key words. Auroral ionosphere · Ionosphere-magnetosphere interactions · EISCAT

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Hamrin, M., P. Norqvist, K. Rönnmark et D. Fellgård. « The importance of solar illumination for discrete and diffuse aurora ». Annales Geophysicae 23, n^o 11 (21 décembre 2005) : 3481–86. http://dx.doi.org/10.5194/angeo-23-3481-2005.

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Abstract. We present a comprehensive overview of the occurrence of discrete and diffuse aurora in the nightside Northern Hemisphere at invariant latitudes 55°-75°. Twenty-one months of Freja observations (1 January 1993 to 30 September 1994) from the Northern Hemisphere, obtained at altitude, are included in this investigation. We investigate the importance of seasonal effects, solar illumination and geomagnetic activity for the auroral precipitation. The seasonal variations in the occurrence of discrete aurora are separated from the dependence on solar illumination of the ionosphere. When the effects of sunlight are eliminated, aurora is found to be more common during the summer. The occurrence of diffuse, as well as discrete aurora, is suppressed by solar illumination of the ionosphere. This dependence of diffuse auroral precipitation on ionospheric conditions is not predicted by theories that attribute diffuse aurora to equatorial pitch-angle diffusion of hot magnetospheric electrons.

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Blixt, E. M., T. Grydeland, N. Ivchenko, T. Hagfors, C. La Hoz, B. S. Lanchester, U. P. Løvhaug et T. S. Trondsen. « Dynamic rayed aurora and enhanced ion-acoustic radar echoes ». Annales Geophysicae 23, n^o 1 (31 janvier 2005) : 3–11. http://dx.doi.org/10.5194/angeo-23-3-2005.

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Abstract. The generation mechanism for naturally enhanced ion-acoustic echoes is still debated. One important issue is how these enhancements are related to auroral activity. All events of enhanced ion-acoustic echoes observed simultaneously with the EISCAT Svalbard Radar (ESR) and with high-resolution narrow field-of-view auroral imagers have been collected and studied. Characteristic of all the events is the appearance of very dynamic rayed aurora, and some of the intrinsic features of these auroral displays are identified. Several of these identified features are directly related to the presence of low energy (10-100eV) precipitating electrons in addition to the higher energy population producing most of the associated light. The low energy contribution is vital for the formation of the enhanced ion-acoustic echoes. We argue that this type of aurora is sufficient for the generation of naturally enhanced ion-acoustic echoes. In one event two imagers were used to observe the auroral rays simultaneously, one from the radar site and one 7km away. The data from these imagers shows that the auroral rays and the strong backscattering filaments (where the enhanced echoes are produced) are located on the same field line, which is in contrast to earlier statements in the litterature that they should be separated.

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Dashkevich, Zhanna, et Vladimir Ivanov. « Estimating the average energy of auroral electrons from 427.8 nm emission intensity measurements ». Solnechno-Zemnaya Fizika 10, n^o 4 (18 décembre 2024) : 72–78. https://doi.org/10.12737/szf-104202408.

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We propose a method for estimating average energy of precipitating electrons from 427.8 nm emission intensity measurements. This method is based on the experimental dependence of the ratio of λ630.0 and λ427.8 nm emission intensities on the λ427.8 emission intensity and model calculations of the dependence of the average auroral electron energy on the I₆₃₀.₀/I₄₂₇.₈ ratio. We present numerical estimates of the influence of three factors on this dependence: the shape of the auroral electron energy spectrum, the atomic oxygen concentration, and the NO concentration. The dependence of the average energy of the auroral electron flux on the 427.8 nm emission intensity is obtained and its analytical approximation is presented.

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Dashkevich, Zhanna, et Vladimir Ivanov. « Estimating the average energy of auroral electrons from 427.8 nm emission intensity measurements ». Solar-Terrestrial Physics 10, n^o 4 (18 décembre 2024) : 65–71. https://doi.org/10.12737/stp-104202408.

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We propose a method for estimating average energy of precipitating electrons from 427.8 nm emission intensity measurements. This method is based on the experimental dependence of the ratio of λ630.0 and λ427.8 nm emission intensities on the λ427.8 emission intensity and model calculations of the dependence of the average auroral electron energy on the I₆₃₀.₀/I₄₂₇.₈ ratio. We present numerical estimates of the influence of three factors on this dependence: the shape of the auroral electron energy spectrum, the atomic oxygen concentration, and the NO concentration. The dependence of the average energy of the auroral electron flux on the 427.8 nm emission intensity is obtained and its analytical approximation is presented.

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Grono, Eric, Eric Donovan et Kyle R. Murphy. « Tracking patchy pulsating aurora through all-sky images ». Annales Geophysicae 35, n^o 4 (3 juillet 2017) : 777–84. http://dx.doi.org/10.5194/angeo-35-777-2017.

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Abstract. Pulsating aurora is frequently observed in the evening and morning sector auroral oval. While the precipitating electrons span a wide range of energies, there is increasing evidence that the shape of pulsating auroral patches is controlled by structures in near-equatorial cold plasma; these patches appear to move with convection, for example. Given the tremendous and rapidly increasing amount of auroral image data from which the velocity of these patches can be inferred, it is timely to develop and implement techniques for the automatic identification of pulsating auroral patch events in these data and for the automatic determination of the velocity of individual patches from that data. As a first step towards this, we have implemented an automatic technique for determining patch velocities from sequences of images from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imager (ASI) and applied it to many pulsating aurora events. Here we demonstrate the use of this technique and present the initial results, including a comparison between ewograms (east–west keograms) and time series of patch position as determined by the algorithm. We discuss the implications of this technique for remote sensing convection in the inner magnetosphere.

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Feldstein, Y. I., L. I. Gromova, J. Woch, I. Sandahl, L. Blomberg, G. Marklund et 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, n^o 5 (31 mai 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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Kosch, M. J., T. Pedersen, J. Hughes, R. Marshall, E. Gerken, A. Senior, D. Sentman, M. McCarrick et F. T. Djuth. « Artificial optical emissions at HAARP for pump frequencies near the third and second electron gyro-harmonic ». Annales Geophysicae 23, n^o 5 (27 juillet 2005) : 1585–92. http://dx.doi.org/10.5194/angeo-23-1585-2005.

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Abstract. High-power high-frequency radio waves beamed into the ionosphere cause plasma turbulence, which can accelerate electrons. These electrons collide with the F-layer neutral oxygen causing artificial optical emissions identical to natural aurora. Pumping at electron gyro-harmonic frequencies has special significance as many phenomena change their character. In particular, artificial optical emissions become strongly reduced for the third and higher gyro-harmonics. The High frequency Active Auroral Research Program (HAARP) facility is unique in that it can select a frequency near the second gyro-harmonic. On 25 February 2004, HAARP was operated near the third and passed through the second gyro-harmonic for the first time in a weakening ionosphere. Two novel observations are: firstly, a strong enhancement of the artificial optical emission intensity near the second gyro-harmonic, which is opposite to higher gyro-harmonics; secondly, the optical enhancement maximum occurs for frequencies just above the second gyro-harmonic. We provide the first experimental evidence for these effects, which have been predicted theoretically. In addition, irregular optical structures were created when the pump frequency was above the ionospheric critical frequency.Keywords. Active experiments – Auroral ionosphere – Wave-particle interactions

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Bisikalo, Dmitri, Valery Shematovich et Benoit Hubert. « The Kinetic Monte Carlo Model of the Auroral Electron Precipitation into N2-O2 Planetary Atmospheres ». Universe 8, n^o 8 (22 août 2022) : 437. http://dx.doi.org/10.3390/universe8080437.

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Auroral events are the prominent manifestation of solar/stellar forcing on planetary atmospheres. They are closely related to the energy deposition by and evolution of planetary atmospheres, and their observations are widely used to analyze the composition, structure, and chemistry of the atmosphere under study, as well as energy fluxes of the precipitating particles that affect the atmosphere. A numerical kinetic Monte Carlo model had been developed, allowing us to study the processes of precipitation of high-energy auroral electrons into the N2-O2 atmospheres of the rocky planets in the Solar and exosolar planetary systems. This model describes on a molecular level the collisions of auroral electrons and atmospheric gas, taking into account the stochastic nature of collisional scattering at high kinetic energies. The current status of the kinetic model is illustrated in the applications to the auroral events on the Earth such as the production of suprathermal nitrogen atoms due to the electron impact dissociation of N2. It was found that electron impact dissociation of N2 can potentially be an important source of suprathermal N atoms in the auroral regions of the N2-O2 atmosphere of terrestrial-type planets. Such research will allow us to study the odd nitrogen chemistry as an atmospheric marker of the N2-O2 atmosphere of rocky exoplanets.

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Janhunen, P., A. Olsson, A. Vaivads et W. K. Peterson. « Generation of Bernstein waves by ion shell distributions in the auroral region ». Annales Geophysicae 21, n^o 4 (30 avril 2003) : 881–91. http://dx.doi.org/10.5194/angeo-21-881-2003.

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Abstract. Hot ion shell distributions could possibly contain enough free energy for waves that could power electron energisation above auroral inverted-V regions. Using both linear theory (WHAMP) and two-dimensional electrostatic simulations, we show that ion shell distributions can cause unstable ion Bernstein mode emissions with high temporal growth rates, as well as perpendicular and parallel e-folding distances, that are in accordance with the tranverse dimensions of auroral arcs and the parallel size of the energisation region, respectively. The phase velocities of the waves are in the proper range to give parallel energisation to electrons with a Landau resonance. The simulation shows that about 90% of the energy goes into electrons and 10% goes into cold ion perpendicular heating. An electron heating rate of ~ 80 eV/s is obtained.Key words. Ionosphere (auroral phenomena) – Space plasma physics (numerical simulation studies; wave-particle interactions)

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Riazanteseva, Maria O., Elizaveta E. Antonova, Marina V. Stepanova, Boris V. Marjin, Ilia A. Rubinshtein, Vera O. Barinova et Nikita V. Sotnikov. « Relative positions of the polar boundary of the outer electron radiation belt and the equatorial boundary of the auroral oval ». Annales Geophysicae 36, n^o 4 (21 août 2018) : 1131–40. http://dx.doi.org/10.5194/angeo-36-1131-2018.

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Abstract. Finding the position of the polar boundary of the outer electron radiation belt, relative to the position of the auroral oval, is a long-standing problem. Here we analyze it using the data of the METEOR-M1 auroral satellite for the period from 11 November 2009 to 27 March 2010. The geomagnetic conditions during the analyzed period were comparatively quiet. METEOR-M1 has a polar solar-synchronous circular orbit with an altitude of ≈832 km, a period of 101.3 min, and an inclination of 98∘. We analyze flux observations of auroral electrons with energies between 0.03 and 16 keV, and electrons with energies >100 keV, measured simultaneously by the GGAK-M set of instruments, composed of semiconductors, scintillator detectors, and electrostatic analyzers. We assume that in the absence of geomagnetic storms the polar boundary of the outer radiation belt can be identified as a decrease in the count rate of precipitating energetic electrons to the background level. It was found that this boundary can be located both inside the auroral oval or equatorward of the equatorial boundary of the auroral precipitation. It was also found that for slightly disturbed geomagnetic conditions the polar boundary of the outer radiation belt is almost always located inside the auroral oval. We observe that the difference between the position of the polar boundary of the outer radiation belt and the position of the equatorial boundary of the auroral precipitation depend on the AE and PC indices of geomagnetic activity. The implications of these results in the analysis of the formation of the outer radiation belt are discussed.

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Nasrin, S., et M. Bose. « Effect of Two Different Electron Temperatures in Auroral Ionosphere ». Ukrainian Journal of Physics 67, n^o 2 (1 avril 2022) : 136. http://dx.doi.org/10.15407/ujpe67.2.136.

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We have investigated the effect of two different electron temperatures in an auroral ionosphere in the presence of ions and obtained a modified electron-acoustic and modified lower-hybrid drift dissipative modes which will not be affected much due to the presence of cold electrons. However, in the drift dissipative case, the growth rate of the electron-acoustic wave depends on the number density of cold electrons.

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Tanaka, Yoshimasa, Yasunobu Ogawa, Akira Kadokura, Takehiko Aso, Björn Gustavsson, Urban Brändström, Tima Sergienko, Genta Ueno et Satoko Saita. « Application of generalized aurora computed tomography to the EISCAT_3D project ». Annales Geophysicae 42, n^o 1 (29 mai 2024) : 179–90. http://dx.doi.org/10.5194/angeo-42-179-2024.

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Abstract. EISCAT_3D is a project to build a multi-site phased-array incoherent scatter radar system in northern Fenno-Scandinavia. We demonstrate via numerical simulation how useful monochromatic images taken by a multi-point imager network are for auroral research in the EISCAT_3D project. We apply the generalized aurora computed tomography (G-ACT) method to modelled observational data from real instruments, such as the Auroral Large Imaging System (ALIS) and the EISCAT_3D radar. G-ACT is a method for reconstructing the three-dimensional (3D) distribution of auroral emissions and ionospheric electron density (corresponding to the horizontal two-dimensional (2D) distribution of energy spectra of precipitating electrons) from multi-instrument data. It is assumed that the EISCAT_3D radar scans an area of 0.8° in geographic latitude and 3° in longitude at an altitude of 130 km with 10 × 10 beams from the radar core site at Skibotn (69.35° N, 20.37° E). Two neighboring discrete arcs are assumed to appear in the observation region of the EISCAT_3D radar. The reconstruction results from G-ACT are compared with those from the normal ACT as well as the ionospheric electron density from the radar. It is found that G-ACT can interpolate the ionospheric electron density at a much higher spatial resolution than that observed by the EISCAT_3D radar. Furthermore, the multiple arcs reconstructed by G-ACT are more precise than those by ACT. In particular, underestimation of the ionospheric electron density and precipitating electrons' energy fluxes inside the arcs is significantly improved by G-ACT including the EISCAT_3D data. Even when the ACT reconstruction is difficult due to the unsuitable locations of the imager sites relative to the discrete arcs and/or a small number of available images, G-ACT allows us to obtain better reconstruction results.

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Shue, J. H., P. T. Newell, K. Liou, C. I. Meng, M. R. Hairston et F. J. Rich. « Ionospheric characteristics of the dusk-side branch of the two-cell aurora ». Annales Geophysicae 24, n^o 1 (7 mars 2006) : 203–14. http://dx.doi.org/10.5194/angeo-24-203-2006.

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Abstract. The two-cell aurora is characterized by azimuthally elongated regions of enhanced auroral brightness over extended local times in the dawn and dusk sectors. Its association with the convection, particle precipitation, and field-aligned currents under various phases of substorms has not been fully understood. With Polar Ultraviolet Imager auroral images in conjunction with Defense Meteorological Satellite Program (DMSP) F12 spacecraft on the dusk-side branch of the two-cell aurora, we are able to investigate an association of the auroral emissions with the electric fields, field-aligned currents, and energy flux of electrons. Results show that the substorm expansion onset does not significantly change the orientation of the dusk-side branch of the two-cell aurora. Also, the orientation of the magnetic deflection vector produced by the region 1 field-aligned current changed from 73±1° to the DMSP trajectory during the substorm growth phase, to 44±6° to the DMSP trajectory during the substorm expansion phase. With a comparison between the orientation of the dusk-side branch of the two-cell aurora and the orientation of the magnetic deflection vector, it is found that the angular difference between the two orientations is 28±5° during the substorm growth phase, and 13±6° during the substorm expansion phase.

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Milan, S. E., K. Hosokawa, M. Lester, N. Sato, H. Yamagishi et F. Honary. « D region HF radar echoes associated with energetic particle precipitation and pulsating aurora ». Annales Geophysicae 26, n^o 7 (14 juillet 2008) : 1897–904. http://dx.doi.org/10.5194/angeo-26-1897-2008.

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Abstract. Milan et al. (2001) identified a class of narrow, slow-moving HF radar backscatter echoes which originate between altitudes of 80 and 100 km, the ionospheric D- and lower E-regions. These echoes appeared to be associated with the occurrence of pulsating aurora, which are known to be created by energetic electrons capable of penetrating to D region altitudes. In this study we show that these echoes are observed in tandem with enhancements in cosmic noise absorption (auroral absorption), additional evidence that energetic (>30 keV) particle precipitation is responsible for generating the irregularities from which a radar can scatter. In addition, we show that the D region backscatter echoes occur predominantly in the post-midnight sector during substorm recovery phase, in common with auroral absorption events and pulsating aurora.

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Bryant, D. A., A. C. Cook, Z. S. Wang, U. de Angelis et C. H. Perry. « Turbulent acceleration of auroral electrons ». Journal of Geophysical Research : Space Physics 96, A8 (1 août 1991) : 13829–39. http://dx.doi.org/10.1029/91ja01105.

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Margot, Joëlle, et A. G. McNamara. « Comparison of plasma-density and electron-temperature profiles during the auroral modelling campaign ARIES ». Canadian Journal of Physics 69, n^o 8-9 (1 août 1991) : 950–58. http://dx.doi.org/10.1139/p91-150.

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Plasma-density and electron-temperature profiles were measured during the auroral modelling campaign ARIES. This campaign consisted of two rockets launched in the auroral E region under different geophysical conditions. The plasma-density and electron-temperature behaviours were tentatively related to the energy and intensity of the ionizing primary-electron fluxes. It is concluded that the plasma-density height distribution can be used to estimate the primary-electrons energy. The set of data presented is sufficiently complete to allow, when used together with other types of experiments such as the height distribution of the optical intensity and the high-energy electron spectra, the achievement of the objective of the ARIES multi-instrument campaign, i.e., refinement of the auroral model.

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Coumans, V., J. C. Gérard, B. Hubert, M. Meurant et S. B. Mende. « Global auroral conductance distribution due to electron and proton precipitation from IMAGE-FUV observations ». Annales Geophysicae 22, n^o 5 (8 avril 2004) : 1595–611. http://dx.doi.org/10.5194/angeo-22-1595-2004.

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Abstract. The Far Ultraviolet (FUV) imaging system on board the IMAGE satellite provides a global view of the north auroral region in three spectral channels, including the SI12 camera sensitive to Doppler shifted Lyman-α emission. FUV images are used to produce instantaneous maps of electron mean energy and energy fluxes for precipitated protons and electrons. We describe a method to calculate ionospheric Hall and Pedersen conductivities induced by auroral proton and electron ionization based on a model of interaction of auroral particles with the atmosphere. Different assumptions on the energy spectral distribution for electrons and protons are compared. Global maps of ionospheric conductances due to instantaneous observation of precipitating protons are calculated. The contribution of auroral protons in the total conductance induced by both types of auroral particles is also evaluated and the importance of proton precipitation is evaluated. This method is well adapted to analyze the time evolution of ionospheric conductances due to precipitating particles over the auroral region or in particular sectors. Results are illustrated with conductance maps of the north polar region obtained during four periods with different activity levels. It is found that the proton contribution to conductance is relatively higher during quiet periods than during substorms. The proton contribution is higher in the period before the onset and strongly decreases during the expansion phase of substorms. During a substorm which occurred on 28 April 2001, a region of strong proton precipitation is observed with SI12 around 14:00MLT at ~75° MLAT. Calculation of conductances in this sector shows that neglecting the protons contribution would produce a large error. We discuss possible effects of the proton precipitation on electron precipitation in auroral arcs. The increase in the ionospheric conductivity, induced by a former proton precipitation can reduce the potential drop along field lines in the upward field-aligned currents by creating an opposite polarization electric field. This feedback mechanism possibly reduces the electron acceleration. Key words. Ionosphere (auroral ionosphere; ionospheremagnetosphere interactions; particle precipitation)

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Vorobjev, V. G., O. I. Yagodkina, E. E. Antonova et I. P. Kirpichev. « The Influence of Extreme Levels of the Solar Wind Dynamic Pressure on the Structure of Nightside Auroral Precipitation ». Geomagnetism and Aeronomy 62, n^o 6 (décembre 2022) : 704–10. http://dx.doi.org/10.1134/s0016793222060160.

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Abstract The data from the DMSP spacecraft were used to study the characteristics of ion and electron precipitation in the nightside sector of the auroral zone during magnetically quiet periods at extreme values of the solar wind dynamic pressure (Psw). It was shown that the ion pressure at the isotropy boundary (IB) increases with Psw and can reach a level of 4–6 nPa at Psw = 20–22 nPa. The latitude profiles of the ion pressure obtained at different levels of Psw indicate that the increase in Psw is accompanied by an expansion of the ion precipitation region and a shift of the IB to lower latitudes. At 〈Psw〉 = 0.5 nPa, the IB latitude is ~70.4° CGL, while at 〈Psw〉 = 16.3 nPa, it shifts toward the equator to ~64.6° CGL. As the Psw level decreases, the energy fluxes of precipitating electrons decrease significantly. At Psw < ~2.0 nPa, auroras in the region of the auroral oval can be considered subvisual. At extremely low values of dynamic pressure, Psw= ~0.2 nPa, it becomes very problematic to identify the zone of electron and ion precipitation.

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Mishra, R. K., A. Gautam, P. Poudel, N. Parajuli, A. Silwal, B. Adhikari, B. R. Tiwari et S. P. Gautam. « Geomagnetically Quiet Period Analysis of Relativistic Electrons, Auroral Precipitation, Joule Heating, and Ring Current During the Years of 1999, 2000 and 2004 ». Journal of Nepal Physical Society 7, n^o 2 (6 août 2021) : 126–37. http://dx.doi.org/10.3126/jnphyssoc.v7i2.38633.

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This work presents the study of the quietest time variation in relativistic electrons, auroral precipitation, ring current, and joule heating during 1999, 2000, and 2004. Geostationary Operational Environmental Satellite (GOES) data on relativistic electrons with energies above 0.6 MeV, 2 MeV, and 4 MeV were analyzed. The time-series analysis of the relativistic electrons over a 24-hour averaged interval reveals a precise 24-hour modulation of the relativistic electron population during all seasons for energies above 0.6 MeV and 2 MeV, and during the winter season for higher energies above 4 MeV. In addition, relativistic electron fluxes at energies above 0.6 MeV and above 2 MeV were higher during the descending phase of the solar cycle compared to the ascending and solar-maximum phases. The cross-correlation analysis presented a strong correlation of Joule heating, ring current, and auroral precipitation with the relativistic electron population in three energy bands considered, as indicated by the zero-time lag. Studying the quiet time variation of relativistic electrons will lead to more complete ionospheric models, which were previously limited to the geomagnetically disturbed period.

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Sandholt, P. E., C. J. Farrugia et W. F. Denig. « Dayside aurora and the role of IMF ∣By∣/∣Bz∣ : detailed morphology and response to magnetopause reconnection ». Annales Geophysicae 22, n^o 2 (1 janvier 2004) : 613–28. http://dx.doi.org/10.5194/angeo-22-613-2004.

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Abstract. We document the detailed spatio-temporal structure of the dayside aurora during intervals of ongoing dayside magnetopause reconnection, primarily during interplanetary magnetic field (IMF) Bz≤0 conditions. The present study is based on ground auroral observations in combination with particle precipitation data from a DMSP spacecraft. We describe auroral forms corresponding to the following particle precipitation regimes identified by Newell and Meng (1994): (i) central plasma sheet (CPS), (ii) precipitation void, (iii) dayside boundary plasma sheet (BPS), and (iv) cusp (LLBL/cusp/mantle). Two distinctly different auroral configurations are observed, corresponding to different regimes of the IMF clock angle (θ) and the ∣By∣/∣Bz∣ ratio. Two regimes are defined. In regime (I) θ lies within ∼ 90–135° and ∣By∣/∣Bz∣>1 (By-dominated), while in regime (II) θ is in the range 135°–180° and ∣By∣/∣Bz∣<1 (Bz-dominated). Within regime (I) the auroral response to reconnection events typically progresses from lower to higher latitudes in stages as indicated below: (A) equatorward boundary intensifications (EBIs): sequential brightenings of closely spaced, fragmented, rayed bands (BPS aurora) within the ∼08:00–15:00 MLT sector, each of which are moving noonward/sunward, (B) poleward moving auroral forms (PMAFs): forms expanding westward from the postnoon side (By>0) and later appearing as a poleward expanding form in the convection throat in the ∼09:00–12:00 MLT sector, with a fading phase in the regime of mantle precipitation. During strongly southward IMF conditions (regime II), the intense PMAF activity is replaced by a more latitudinally restricted, but longitudinally wide aurora of moderate intensity. The latter auroral state is accompanied by a 2-cell convection pattern which is rather symmetrical about noon. This state is very different from the convection/FAC configuration present during IMF regime (I), with its strong zonal flows (convection current), more intense FAC sheets and PMAF activity in the midday sector. The strong IMF regulation of the dayside BPS aurora, consisting of keV electrons, and its location with respect to the green line auroral gap (precipitation void), indicate that it is an important signature of the reconnection process, located on open boundary layer field lines. The observed longitudinal bifurcation of the auroral brightenings (EBIs) preceding PMAFs is consistent with antiparallel magnetopause reconnection. Key words. Magnetospheric physics (auroral phenomena magnetopause, cusp, and boundary layers: solar windmagnetosphere interactions) – Ionosphere (particle precipitation)

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Cowley, S. W. H. « Current-voltage and kinetic energy flux relations for relativistic field-aligned acceleration of auroral electrons ». Annales Geophysicae 24, n^o 1 (7 mars 2006) : 325–38. http://dx.doi.org/10.5194/angeo-24-325-2006.

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Abstract. Recent spectroscopic observations of Jupiter's "main oval" auroras indicate that the primary auroral electron beam is routinely accelerated to energies of ~100 keV, and sometimes to several hundred keV, thus approaching the relativistic regime. This suggests the need to re-examine the classic non-relativistic theory of auroral electron acceleration by field-aligned electric fields first derived by Knight (1973), and to extend it to cover relativistic situations. In this paper we examine this problem for the case in which the source population is an isotropic Maxwellian, as also assumed by Knight, and derive exact analytic expressions for the field-aligned current density (number flux) and kinetic energy flux of the accelerated population, for arbitrary initial electron temperature, acceleration potential, and field strength beneath the acceleration region. We examine the limiting behaviours of these expressions, their regimes of validity, and their implications for auroral acceleration in planetary magnetospheres (and like astrophysical systems). In particular, we show that for relativistic accelerating potentials, the current density increases as the square of the minimum potential, rather than linearly as in the non-relativistic regime, while the kinetic energy flux then increases as the cube of the potential, rather than as the square.

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Saka, Osuke. « Ionospheric control of space weather ». Annales Geophysicae 39, n^o 3 (17 mai 2021) : 455–60. http://dx.doi.org/10.5194/angeo-39-455-2021.

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Abstract. As proposed by Saka (2019), plasma injections arising out of the auroral ionosphere (ionospheric injection) are a characteristic process of the polar ionosphere at substorm onset. The ionospheric injection is triggered by westward electric fields transmitted from the convection surge in the magnetosphere at field line dipolarization. Localized westward electric fields result in local accumulation of ionospheric electrons and ions, which produce local electrostatic potentials in the auroral ionosphere. Field-aligned electric fields are developed to extract excess charges from the ionosphere. This process is essential to the equipotential equilibrium of the auroral ionosphere. Cold electrons and ions that evaporate from the auroral ionosphere by ionospheric injection tend to generate electrostatic parallel potential below an altitude of 10 000 km. This is a result of charge separation along the mirror fields introduced by the evaporated electrons and ions moving earthward in phase space.

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Morooka, M., T. Mukai et H. Fukunishi. « Current-voltage relationship in the auroral particle acceleration region ». Annales Geophysicae 22, n^o 10 (3 novembre 2004) : 3641–55. http://dx.doi.org/10.5194/angeo-22-3641-2004.

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Abstract. The current-voltage relationship in the auroral particle acceleration region has been studied statistically by the Akebono (EXOS-D) satellite in terms of the charge carriers of the upward field-aligned current. The Akebono satellite often observed field-aligned currents which were significantly larger than the model value predicted by Knight (1973). We compared the upward field-aligned current estimated by three different methods, and found that low-energy electrons often play an important role as additional current carriers, together with the high-energy primary electrons which are expected from Knight's relation. Such additional currents have been observed especially at high and middle altitudes of the particle acceleration region. Some particular features of electron distribution functions, such as "cylindrical distribution functions" and "electron conics", have often been observed coinciding with the additional currents. They indicated time variability of the particle acceleration region. Therefore, we have concluded that the low-energy electrons within the "forbidden" region of electron phase space in the stationary model often contribute to charge carriers of the current because of the rapid time variability of the particle acceleration region. "Cylindrical distribution functions" are expected to be found below the time-varying potential difference. We statistically examined the locations of "cylindrical distribution function", and found that their altitudes are related to the location where the additional currents have been observed. This result is consistent with the idea that the low-energy electrons can also carry significant current when the acceleration region changes in time.

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Mura, A., A. Adriani, J. E. P. Connerney, S. Bolton, F. Altieri, F. Bagenal, B. Bonfond et al. « Juno observations of spot structures and a split tail in Io-induced aurorae on Jupiter ». Science 361, n^o 6404 (5 juillet 2018) : 774–77. http://dx.doi.org/10.1126/science.aat1450.

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Jupiter’s aurorae are produced in its upper atmosphere when incoming high-energy electrons precipitate along the planet’s magnetic field lines. A northern and a southern main auroral oval are visible, surrounded by small emission features associated with the Galilean moons. We present infrared observations, obtained with the Juno spacecraft, showing that in the case of Io, this emission exhibits a swirling pattern that is similar in appearance to a von Kármán vortex street. Well downstream of the main auroral spots, the extended tail is split in two. Both of Ganymede’s footprints also appear as a pair of emission features, which may provide a remote measure of Ganymede’s magnetosphere. These features suggest that the magnetohydrodynamic interaction between Jupiter and its moon is more complex than previously anticipated.

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Antonova, E. E., V. G. Vorobjev, O. I. Yagodkina, N. V. Sotnikov, I. P. Kirpichev, I. L. Ovchinnikov, D. Yu Naiko et al. « MAGNETOSPHERIC SUBSTORMS AND RELATIVISTIC ELECTRONS ». PHYSICS OF AURORAL PHENOMENA 46, n^o 1 (2023) : 7–10. http://dx.doi.org/10.51981/2588-0039.2023.46.001.

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The most recent findings on the dynamics of the outer radiation belt (ORB) and the physics of magnetospheric substorms are examined. Specifically, we investigate the relationship between storm time substorms and the energetic electron population that forms the ORB. Traditionally, storm time substorms have been considered as the primary source of energetic electrons, which are further accelerated during storms to contribute to the formation of the ORB. However, several observations have demonstrated that large magnetospheric substorms can generate high-energy electrons even in the absence of magnetic storms. Substorms introduce dispersionless injections of energetic electrons deep into the magnetosphere from the geosynchronous orbit during storm times. The injected electrons undergo additional acceleration via the betatron mechanism during the storm recovery phase, thus increasing the ORB population. To gain a better understanding of this process, it is crucial to study plasma sheet turbulence, substorm onset processes, and the brightening of auroral arcs. By analyzing the aforementioned findings, this study aims to highlight the need for reanalyzing of the role of auroral processes in the formation of the ORB.

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