Journal articles on the topic 'Auroral plasma'
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1
Titova, E. E., A. G. Yahnin, O. Santolík, D. A. Gurnett, F. Jirícek, J. L. Rauch, F. Lefeuvre, L. A. Frank, J. B. Sigwarth, and M. M. Mogilevsky. "The relationship between auroral hiss at high altitudes over the polar caps and the substorm dynamics of aurora." Annales Geophysicae 23, no. 6 (September 15, 2005): 2117–28. http://dx.doi.org/10.5194/angeo-23-2117-2005.
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Abstract. Strong variations of intensity and cutoff frequency of the auroral hiss were observed by INTERBALL-2 and POLAR satellites at high altitudes, poleward from the auroral oval. The hiss intensifications are correlated with the auroral activations during substorms and/or pseudo-breakups. The low cutoff frequency of auroral hiss increases with the distance between the aurora and the satellite footprint. Multicomponent wave measurements of the hiss emissions on board the POLAR spacecraft show that the horizontal component of the Poynting flux of auroral hiss changes its direction in good accordance with longitudinal displacements of the bright auroras. The vertical component of the Poynting flux is directed upward from the aurora region, indicating that hiss could be generated by upgoing electron beams. This relationship between hiss and the aurora dynamics means that the upgoing electron beams are closely related to downgoing electron beams which produce the aurora. During the auroral activations the upgoing and downgoing beams move and change their intensities simultaneously. Keywords. Magnetospheric physics (Auroral phenomena; Plasma waves and instabilities; Storms and substorms)
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Blomberg, L. G., J. A. Cumnock, I. I. Alexeev, E. S. Belenkaya, S. Yu Bobrovnikov, and V. V. Kalegaev. "Transpolar aurora: time evolution, associated convection patterns, and a possible cause." Annales Geophysicae 23, no. 5 (July 28, 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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Miyashita, Yukinaga, and Akimasa Ieda. "Revisiting substorm events with preonset aurora." Annales Geophysicae 36, no. 5 (October 19, 2018): 1419–38. http://dx.doi.org/10.5194/angeo-36-1419-2018.
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Abstract. Nishimura et al. (2010) proposed a new plasma intrusion or preonset aurora scenario of substorm triggering. In this scenario, a substorm is triggered by a fast earthward flow generated at the distant neutral line which corresponds to a preonset auroral streamer or arc in the ionosphere propagating from the auroral poleward boundary to the initial auroral brightening site, i.e., “preonset aurora”. In the present paper, we revisited three substorm events reported as being triggered by such a mechanism related to preonset auroras, based on THEMIS ground-based all-sky imager data. Unlike previous studies, we examined the arrival timing of the preonset aurora relative to the three steps of auroral onset arc development (initial brightening, enhancement of the wave-like structure, and poleward expansion) to make the role of the preonset aurora in the auroral steps clearer. Our detailed timing analysis found that preonset auroral streamers reached the auroral onset arc but away from the initial brightening site after initial brightening for two events, while no preonset aurora reaching the initial brightening site could be identified for the other event. This result suggests that the processes associated with auroral streamers are unlikely to affect at least initial brightening, even if we consider not only the presence and arrival timing and location of the auroral streamers but also the scale of the corresponding flow and flow vortices. We list a series of open questions for testing the preonset aurora scenario further in future studies. Keywords. Magnetospheric physics (storms and substorms; auroral phenomena; magnetotail)
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Sandholt, P. E., and C. J. Farrugia. "Monitoring magnetosheath-magnetosphere interconnection topology from the aurora." Annales Geophysicae 20, no. 5 (May 31, 2002): 629–37. http://dx.doi.org/10.5194/angeo-20-629-2002.
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Abstract. A strong southward rotation of the IMF (BZ from 5 to -6 nT in ~ 20 s) on 4 January 1995 caused an abrupt reconfiguration of midday aurorae and plasma convection consisting of the following: (1) the red-line aurora associated with magnetosheath plasma transfer at the low-latitude magnetopause appeared at the same time that (2) the green-line aurora from precipitating energetic plasma sheet particles equatorward of the cusp (near the open-closed field line boundary) weakened visibly and shifted equatorward, (3) the high-latitude aurora during the previous northward IMF, which is associated with lobe reconnection, persisted briefly (3 min) and brightened, before it disappeared from the field-of-view, (4) the activation of a strong convection bay (DPY current) at cusp and sub-cusp latitudes when the field turned strongly south, (5) a distinct wave motion of the plasma sheet outer boundary, as inferred from the aurora, which correlates closely with Pc 5 magnetic pulsations. Our interpretation of the dramatic reconfiguration is that reconnection poleward of the cusp coexisted briefly with reconnection at sub-cusp latitudes. The latter provided a magnetic field connection which enabled, on the one hand, magnetosheath particles to enter and cause the red-line cusp aurora, and on the other hand, allowed for magnetospheric energetic particles to escape and weaken the outer plasma sheet source of the green-line emission. The coexistence of the two cusp auroras reflects the time required for one field line topology to replace another, which, under the prevailing high speed wind ( ~ 650 km/s), lasts ~ 3–4 min. The motion of open flux tubes propagating from equator to pole during this transition is traced in the aurora by a poleward moving form. The waves on the outer boundary of the plasma sheet are most likely due to the Kelvin-Helmholtz instability. The study illustrates the ability of local auroral observations to monitor even a global change in magnetospheric magnetic topology.Key words. Magnetospheric Physics (auroral phenomena; magnetopause, cusp, and boundary layers; solar wind-magnethoshere interactions)
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Blixt, E. M., M. J. Kosch, and J. Semeter. "Relative drift between black aurora and the ionospheric plasma." Annales Geophysicae 23, no. 5 (July 27, 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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Yahnin, A. G., V. A. Sergeev, B. B. Gvozdevsky, and S. Vennerstrøm. "Magnetospheric source region of discrete auroras inferred from their relationship with isotropy boundaries of energetic particles." Annales Geophysicae 15, no. 8 (August 31, 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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De Keyser, J., and M. Echim. "Auroral and sub-auroral phenomena: an electrostatic picture." Annales Geophysicae 28, no. 2 (February 23, 2010): 633–50. http://dx.doi.org/10.5194/angeo-28-633-2010.
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Abstract. Many auroral and sub-auroral phenomena are manifestations of an underlying magnetosphere-ionosphere coupling. In the electrostatic perspective the associated auroral current circuit describes how the generator (often in the magnetosphere) is connected to the load (often in the ionosphere) through field-aligned currents. The present paper examines the generic properties of the current continuity equation that characterizes the auroral circuit. The physical role of the various elements of the current circuit is illustrated by considering a number of magnetospheric configurations, various auroral current-voltage relations, and different types of behaviour of the ionospheric conductivity. Based on realistic assumptions concerning the current-voltage relation and the ionospheric conductivity, a comprehensive picture of auroral and sub-auroral phenomena is presented, including diffuse aurora, discrete auroral arcs, black aurora, and subauroral ion drift. The electrostatic picture of field-aligned potential differences, field-aligned currents, ionospheric electric fields and plasma drift, and spatial scales for all these phenomena is in qualitative agreement with observations.
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Borodkova, N. L., A. G. Yahnin, K. Liou, J. A. Sauvaud, A. O. Fedorov, V. N. Lutsenko, M. N. Nozdrachev, and A. A. Lyubchich. "Plasma sheet fast flows and auroral dynamics during substorm: a case study." Annales Geophysicae 20, no. 3 (March 31, 2002): 341–47. http://dx.doi.org/10.5194/angeo-20-341-2002.
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Abstract. Interball-1 observations of a substorm development in the mid-tail on 16 December 1998 are compared with the auroral dynamics obtained from the Polar UV imager. Using these data, the relationship between plasma flow directions in the tail and the location of the auroral activation is examined. Main attention is given to tailward and earth-ward plasma flows, interpreted as signatures of a Near Earth Neutral Line (NENL). It is unambiguously shown that in the mid-plasma sheet the flows were directed tailward when the auroral bulge developed equatorward of the spacecraft ionospheric footprint. On the contrary, when active auroras moved poleward of the Interball-1 projection, earthward fast flow bursts were observed. This confirms the concept that the NENL (or flow reversal region) is the source of auroras forming the poleward edge of the auroral bulge. The observed earthward flow bursts have all typical signatures of Bursty Bulk Flows (BBFs), described by Angelopolous et al. (1992). These BBFs are related to substorm activations starting at the poleward edge of the expanded auroral bulge. We interpret the BBFs as a result of reconnection pulses occurring tail-ward of Interball-1. In addition, some non-typically observed phenomena were detected in the plasma sheet during this substorm: (i) tailward/earthward flows were superimposed on a very strong duskward flow, and (ii) wavy structures of both magnetic field and plasma density were registered. The latter observation is probably linked to the filamentary structure of the current sheet.Key words. Magnetospheric physics (auroral phenomena; plasma sheet; storms and substorms)
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Cumnock, J. A., and L. G. Blomberg. "Transpolar arc evolution and associated potential patterns." Annales Geophysicae 22, no. 4 (April 2, 2004): 1213–31. http://dx.doi.org/10.5194/angeo-22-1213-2004.
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Abstract. We present two event studies encompassing detailed relationships between plasma convection, field-aligned current, auroral emission, and particle precipitation boundaries. We illustrate the influence of the Interplanetary Magnetic Field By component on theta aurora development by showing two events during which the theta originates on both the dawn and dusk sides of the auroral oval. Both theta then move across the entire polar region and become 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. Utilizing satellite data as inputs, the Royal Institute of Technology model provides the high-latitude ionospheric electrostatic potential pattern calculated at different times during the evolution of the theta aurora, resulting from a variety of field-aligned current configurations associated with the changing global aurora. Key words. Ionosphere (auroral ionosphere; electric fields and currents). Magnetospheric physics (magnetosphereionosphere interactions)
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Rycroft, M. J. "Auroral plasma dynamics." Journal of Atmospheric and Terrestrial Physics 57, no. 13 (November 1995): 1668. http://dx.doi.org/10.1016/0021-9169(95)90035-7.
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Sandholt, P. E., and C. J. Farrugia. "The role of external triggers in flow shear arcs in the dayside aurora." Annales Geophysicae 26, no. 8 (August 4, 2008): 2159–77. http://dx.doi.org/10.5194/angeo-26-2159-2008.
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Abstract. In case studies we relate dayside auroral transients to IMF By-distorted plasma convection cells based on high-resolution observations from the ground. We selected three days representing positive and negative IMF By conditions when SuperDARN returned reliable dayside convection patterns in the sector of our optical observations from Ny Ålesund, Svalbard (76° MLAT). We combine two perspectives on the dayside aurora, the local and the global. In the first we derive the fine-structure of dayside precipitation/convection as a function of magnetic latitude (MLAT) and magnetic local time (MLT), which is necessary to understand the local M-I coupling processes (Birkeland current structure). The larger perspective (quasi-global dayside aurora) may be used to shed light on the solar wind-magnetosphere interconnection topology. The auroral morphology consists of brightening events and poleward moving auroral forms (PMAFs) in the pre- and postnoon sectors longitudinally separated by a band of strongly attenuated aurora near noon. We find that the MLT-dependent spatial structure in the dayside aurora (PMAFs/prenoon – "midday gap aurora" – PMAFs/postnoon) which is present during stable IMF conditions is altered by temporal structure during intervals of IMF/solar wind plasma transients. The focus is on the PMAF substructure (so-called "rebrightening forms") which we identify as dynamical plasma flow shear arcs (FSAs) in By-distorted dawn- and dusk-centered convection cells in the close vicinity of the cusp.
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Yahnin, A. G., I. V. Despirak, A. A. Lubchich, B. V. Kozelov, N. P. Dmitrieva, M. A. Shukhtina, and H. K. Biernat. "Indirect mapping of the source of the oppositely directed fast plasma flows in the plasma sheet onto the auroral display." Annales Geophysicae 24, no. 2 (March 23, 2006): 679–87. http://dx.doi.org/10.5194/angeo-24-679-2006.
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Abstract. Data from Polar and Geotail spacecraft are combined to investigate the relationship between locations of active auroras and the magnetotail plasma sheet region where reversed fast plasma flows are generated during substorms. Using the magnetospheric magnetic field model, it is shown that at the beginning of the tailward fast flow the ionospheric footprint of the spacecraft measuring the flow tends to be located poleward of the auroral bulge. The spacecraft within the earthward flow is mapped equatorward of the poleward edge of the auroral bulge. We conclude that a source of the fast plasma flows is conjugated with the poleward edge of the auroral bulge. Analysis of the behavior of the plasma and the magnetic field in the vicinity of the source of the diverging flows allows us to conclude that the source region, interpreted as the magnetic reconnection site, coincides with the region of the cross-tail current reduction, and the tailward propagation of the region is associated with the tailward propagation of the current disruption front.
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Sandholt, P. E., C. J. Farrugia, and W. F. Denig. "Detailed dayside auroral morphology as a function of local time for southeast IMF orientation: implications for solar wind-magnetosphere coupling." Annales Geophysicae 22, no. 10 (November 3, 2004): 3537–60. http://dx.doi.org/10.5194/angeo-22-3537-2004.
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Abstract. In two case studies we elaborate on spatial and temporal structures of the dayside aurora within 08:00-16:00 magnetic local time (MLT) and discuss the relationship of this structure to solar wind-magnetosphere interconnection topology and the different stages of evolution of open field lines in the Dungey convection cycle. The detailed 2-D auroral morphology is obtained from continuous ground observations at Ny Ålesund (76° magnetic latitude (MLAT)), Svalbard during two days when the interplanetary magnetic field (IMF) is directed southeast (By>0; Bz<0). The auroral activity consists of the successive activations of the following forms: (i) latitudinally separated, sunward moving, arcs/bands of dayside boundary plasma sheet (BPS) origin, in the prenoon (08:00-11:00 MLT) and postnoon (12:00-16:00 MLT) sectors, within 70-75° MLAT, (ii) poleward moving auroral forms (PMAFs) emanating from the pre- and postnoon brightening events, and (iii) a specific activity appearing in the 07:00-10:00 MLT/75-80° MLAT during the prevailing IMF By>0 conditions. The pre- and postnoon activations are separated by a region of strongly attenuated auroral activity/intensity within the 11:00-12:00 MLT sector, often referred to as the midday gap aurora. The latter aurora is attributed to the presence of component reconnection at the subsolar magnetopause where the stagnant magnetosheath flow lead to field-aligned currents (FACs) which are of only moderate intensity. The much more active and intense aurorae in the prenoon (07:00-11:00 MLT) and postnoon (12:00-16:00 MLT) sectors originate in magnetopause reconnection events that are initiated well away from the subsolar point. The high-latitude auroral activity in the prenoon sector (feature iii) is found to be accompanied by a convection channel at the polar cap boundary. The associated ground magnetic deflection (DPY) is a Svalgaard-Mansurov effect. The convection channel is attributed to effective momentum transfer from the solar wind-magnetosphere dynamo in the high-latitude boundary layer (HBL), on the downstream side of the cusp.
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Sandholt, P. E., J. Moen, C. J. Farrugia, S. W. H. Cowley, M. Lester, S. E. Milan, C. Valladares, W. F. Denig, and S. Eriksson. "Multi-site observations of the association between aurora and plasma convection in the cusp/polar cap during a southeastward(<i>B<sub>y</sub></i> <u>~</u> |<i>B<sub>z</sub></i>|) IMF orientation." Annales Geophysicae 21, no. 2 (February 28, 2003): 539–58. http://dx.doi.org/10.5194/angeo-21-539-2003.
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Abstract. In a case study we demonstrate the spatiotemporal structure of aurora and plasma convection in the cusp/polar cap when the interplanetary magnetic field (IMF) Bz < 0 and By ~ | Bz | (clock angle in GSM Y - Z plane: ~ 135°). This IMF orientation elicited a response different from that corresponding to strongly northward and southward IMF. Our study of this "intermediate state" is based on a combination of ground observations of optical auroral emissions and ionospheric plasma convection. Utilizing all-sky cameras at NyAlesund, Svalbard and Heiss Island (Russian arctic), we are able to monitor the high-latitude auroral activity within the ~10:00–15:00 MLT sector. Information on plasma convection is obtained from the SuperDARN radars, with emphasis placed on line of sight observations from the radar situated in Hankasalmi, Finland (Cutlass). A central feature of the auroral observations in the cusp/polar cap region is a ~ 30-min long sequence of four brightening events, some of which consists of latitudinally and longitudinally separated forms, which are found to be associated with pulsed ionospheric flows in merging and lobe convection cells. The auroral/convection events may be separated into different forms/cells and phases, reflecting a spatiotem-poral evolution of the reconnection process on the dayside magnetopause. The initial phase consists of a brightening in the postnoon sector (~ 12:00–14:00 MLT) at ~ 73° MLAT, accompanied by a pulse of enhanced westward convection in the postnoon merging cell. Thereafter, the event evolution comprises two phenomena which occur almost simultaneously: (1) westward expansion of the auroral brightening (equatorward boundary intensification) across noon, into the ~ 10:00–12:00 MLT sector, where the plasma convection subsequently turns almost due north, in the convection throat, and where classical poleward moving auroral forms (PMAFs) are observed; and (2) auroral brightening at slightly higher latitudes (~ 75° MLAT) in the postnoon lobe cell, with expansion towards noon, giving rise to a clear cusp bifurcation. The fading phase of PMAFs is accompanied by a "patch" of enhanced (~ 1 km/s) poleward-directed merging cell convection at high latitudes (75–82° MLAT), e.g. more than 500 km poleward of the cusp equatorward boundary. The major aurora/convection events are recurring at ~ 5–10 min intervals. Key words. Magnetospheric physics (auroral phenomena; magnetopause, cusp, and boundary layers; plasma convection)
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Sandholt, P. E., and C. J. Farrugia. "Poleward moving auroral forms (PMAFs) revisited: responses of aurorae, plasma convection and Birkeland currents in the pre- and postnoon sectors under positive and negative IMF <I>B<sub>y</sub></I> conditions." Annales Geophysicae 25, no. 7 (July 30, 2007): 1629–52. http://dx.doi.org/10.5194/angeo-25-1629-2007.
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Abstract. Using five case studies, we investigate the dynamical evolution of dayside auroral precipitation in relation to plasma convection, classifying it by the IMF By component and position with respect to noon. Auroral observations were made by meridian scanning photometers (MSPs) and an all-sky camera (ASC) in Ny Ålesund, Svalbard at 76° MLAT, while the spatial structure of the ionospheric plasma convection is inferred from SuperDARN radars and ion drift observations from spacecraft in polar orbit. The IMF configuration of major interest here is one pointing southward and with a dominant east-west component. Our emphasis is on the auroral phenomenon of PMAFs (poleward moving auroral forms), which are ionospheric signatures of pulsed reconnection at the magnetopause. We distinguish between PMAFs/prenoon and PMAFs/postnoon. These two activities are found to be separated by an auroral form around noon with attenuated emission at 630.0 nm. We document for the first time that this "midday gap aurora" appears in the form of a midday auroral brightening sequence (MABS). We study the PMAF activity consisting of an initial brightening phase and the later stages of PMAF evolution in relation to plasma convection cells, flow vorticity, and precipitation boundaries in the prenoon and postnoon sectors for both By polarities. Flow channels (PIFs) associated with PMAFs are strengthened by polarization effects at auroral boundaries. Addressing the implications of our proposed, extended perspective on dayside auroral morphology under southeast/west IMF for M-I coupling associated with pulsed magnetopause reconnection (FTEs), we draw inferences on the MLT-dependent geoeffectiveness (Birkeland current/auroral intensity) of magnetopause FTEs (subsolar region versus flanks).
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Sandholt, P. E., C. J. Farrugia, and W. F. Denig. "Dayside aurora and the role of IMF ∣<i>B<sub>y</sub></i>∣/∣<i>B<sub>z</sub></i>∣: detailed morphology and response to magnetopause reconnection." Annales Geophysicae 22, no. 2 (January 1, 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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Grono, Eric, and Eric Donovan. "Differentiating diffuse auroras based on phenomenology." Annales Geophysicae 36, no. 3 (June 20, 2018): 891–98. http://dx.doi.org/10.5194/angeo-36-891-2018.
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Abstract. There is mounting evidence which suggests that pulsating auroral patches often move with convection. This study is an initial step at identifying the differences between patches that move with convection and those that do not. While many properties of pulsating patches vary, here we outline criteria for separating pulsating auroral patches into three categories based on two properties: their structural stability and the spatial extent of their pulsations. Patchy aurora is characterized by stable structures whose pulsations are limited to small regions. Patchy pulsating aurora consists of stable patches whose pulsations are far less subtle and occur throughout much of their area. Amorphous pulsating auroral structures are unstable – very rapidly evolving – and can pulsate over their entire area. The speed with which amorphous pulsating aurora evolves makes their motion difficult to ascertain and seems unrelated to the E×B drifting of cold, equatorial plasma.
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Simmons, D. A. R., F. Sigernes, and K. Henriksen. "Geomagnetic storm and substorm aurora observed from Spitsbergen." Polar Record 31, no. 179 (October 1995): 375–88. http://dx.doi.org/10.1017/s0032247400027352.
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ABSTRACTThe present study confirms that the auroras of the nightside oval population consist of two main types, namely storm and substorm aurora. Storm-type aurora, which is relatively infrequent, results from bombardment of the upper ionosphere by fast particle streams generated in the solar wind by cataclysmic solar events related to coronal mass ejections. The associated turbulent plasma that is injected into the magnetosphere produces great magnetic storms of world-wide dimensions that may last for days. In contrast, substorm aurora is a frequent (almost daily) occurrence that lasts for an hour or two around geomagnetic midnight. It is generated by the impulsive release of stored magnetospheric energy from the substorm onset region in the Earth's magnetotail and is associated with localised negative magnetic bays in the H (horizontal)-component of the Earth's magnetic field in the vicinity of the auroral oval.
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Keiling, A., V. Angelopoulos, J. M. Weygand, O. Amm, E. Spanswick, E. Donovan, S. Mende, et al. "THEMIS ground-space observations during the development of auroral spirals." Annales Geophysicae 27, no. 11 (November 27, 2009): 4317–32. http://dx.doi.org/10.5194/angeo-27-4317-2009.
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Abstract. A simultaneous observation of an auroral spiral and its generator region in the near-Earth plasma sheet is rather unlikely. Here we present such observations using the THEMIS spacecraft as well as the THEMIS ground network of all-sky imagers and magnetometers. Two consecutive auroral spirals separated by approximately 14 min occurred during a substorm on 19 February 2008. The spirals formed during the expansion phase and a subsequent intensification, and were among the brightest features in the aurora with diameters of 200–300 km. The duration for the formation and decay of each spiral was less than 60 s. Both spirals occurred shortly after the formation of two oppositely rotating plasma flow vortices in space, which were also accompanied by dipolarizations and ion injections, at ~11 RE geocentric distance. Observations and model calculations also give evidence for a magnetic-field-aligned current generation of approximately 0.1 MA via the flow vortices, connecting the generator region of the spirals with the ionosphere, during the formation of both spirals. In the ionosphere, a pair of equivalent ionospheric current (EIC) vortices with opposite rotations (corresponding to upward and downward currents) was present during both auroral spirals with enhanced EICs and ionospheric flows at the locations of the auroral spirals and along the auroral arcs. The combined ground and space observations suggest that each auroral spiral was powered by two oppositely rotating plasma flow vortices that caused a current enhancement in the substorm current wedge.
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Partamies, N., K. Kauristie, E. Donovan, E. Spanswick, and K. Liou. "Meso-scale aurora within the expansion phase bulge." Annales Geophysicae 24, no. 8 (September 13, 2006): 2209–18. http://dx.doi.org/10.5194/angeo-24-2209-2006.
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Abstract. We present ground-based optical, riometer and magnetometer recordings together with Polar UVI and GOES magnetic field observations of a substorm that occurred over Canada on 24 November 1997. This event involved a clear optical onset followed by poleward motion of the aurora as a signature of an expanding auroral bulge. During the expansion phase, there were three distinct types of meso-scale (10–1000 km) auroral structures embedded in the bulge: at first a series of equatorward moving auroral arcs, followed by a well-defined spiral pair, and finally north-south directed aurora (a streamer). The spirals occurred several minutes after the onset, and indicate a shear in the field-aligned current. The north-south aligned aurora that formed about 10 min after the onset suggest bursty bulk flow type flows taking place in the central plasma sheet. Polar UVI observations of the polar cap location indicate that the southward drifting arcs were associated with magnetospheric activity within closed field lines, while the auroral streamer was launched by the bulge reaching the polar cap boundary, i.e. the mid-tail reconnection starting on the open field lines. The riometer data imply high energy electron precipitation in the vicinity of the the poleward moving edge of the auroral bulge, starting at the onset and continuing until the formation of the north-south structure. In this paper, we examine this evolving auroral morphology within the context of substorm theories.
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Waite, J. Hunter, John T. Clarke, R. J. Walker, John E. P. Connerney, D. McComas, P. Riley, and 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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Sergienko, T., I. Sandahl, B. Gustavsson, L. Andersson, U. Brändström, and Å. Steen. "A study of fine structure of diffuse aurora with ALIS-FAST measurements." Annales Geophysicae 26, no. 11 (October 21, 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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Kozlovsky, A., V. Safargaleev, N. Østgaard, T. Turunen, A. Koustov, J. Jussila, and A. Roldugin. "On the motion of dayside auroras caused by a solar wind pressure pulse." Annales Geophysicae 23, no. 2 (February 28, 2005): 509–21. http://dx.doi.org/10.5194/angeo-23-509-2005.
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Abstract. Global ultraviolet auroral images from the IMAGE satellite were used to investigate the dynamics of the dayside auroral oval responding to a sudden impulse (SI) in the solar wind pressure. At the same time, the TV all-sky camera and the EISCAT radar on Svalbard (in the pre-noon sector) allowed for detailed investigation of the auroral forms and the ionospheric plasma flow. After the SI, new discrete auroral forms appeared in the poleward part of the auroral oval so that the middle of the dayside oval moved poleward from about 70° to about 73° of the AACGM latitude. This poleward shift first occurred in the 15 MLT sector, then similar shifts were observed in the MLT sectors located more westerly, and eventually the shift was seen in the 6 MLT sector. Thus, the auroral disturbance "propagated" westward (from 15 MLT to 6 MLT) at an apparent speed of the order of 7km/s. This motion of the middle of the auroral oval was caused by the redistribution of the luminosity within the oval and was not associated with the corresponding motion of the poleward boundary of the oval. The SI was followed by an increase in the northward plasma convection velocity. Individual auroral forms showed poleward progressions with velocities close to the velocity of the northward plasma convection. The observations indicate firstly a pressure disturbance propagation through the magnetosphere at a velocity of the order of 200km/s which is essentially slower than the velocity of the fast Alfvén (magnetosonic) wave, and secondly a potential (curl-free) electric field generation behind the front of the propagating disturbance, causing the motion of the auroras. We suggest a physical explanation for the slow propagation of the disturbance through the magnetosphere and a model for the electric field generation. Predictions of the model are supported by the global convection maps produced by the SuperDARN HF radars. Finally, the interchange instability and the eigenmode toroidal Alfvén oscillations are discussed as possible generation mechanisms for the dayside auroral forms launched by the SI.
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Falthammar, C. G. "Physics of the Aurora." Geofísica Internacional 30, no. 4 (October 1, 1991): 197–211. http://dx.doi.org/10.22201/igeof.00167169p.1991.30.4.1227.
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La aurora, ya de por si fascinante por su belleza y la multitud de sus formas, ha resultado aún más fascinante en términos de la física que se puede aprender de su estudio científico. Observaciones in situ de la aurora y de los fenómenos relacionados con ella han sacado a la luz los procesos de la física de plasmas, cuya presencia tiene un profundo impacto en nuestro concepto del espacio que nos rodea. La mayoría de estos procesos están relacionados con procesos de aceleraci6n auroral. Aunque se sabe desde hace tiempo que la aurora es causada por electrones de unos cuantos keV que inciden sobre la atmósfera superior, la forma en que estos electrones obtienen su energía ha sido un asunto crucial y controvertido. Actualmente existe una concordancia casi universal en que los campos eléctricos alineados a los campos magnéticos juegan un papel clave, lo que confirma lo predicho por Hannes Alfven hace más de tres décadas. Se han reconocido tres mecanismos principales que hacen posible la existencia de tales campos. Es probable que todos ellos operen en la región de aceleración auroral, pero los papeles que cada uno de ellos juega aún no están determinados. También ha quedado claro que existe una intrincada relación entre estos campos y las diversas formas de interacci6n onda-partícula que involucran campos eléctricos dependientes del tiempo en un amplio rango de frecuencias. Los campos eléctricos alineados con los campos magnéticos tienen consecuencias importantes sobre el comportamiento de un plasma, no solo porque energetizan las partículas sino porque afectan también al comportamiento del plasma mismo, mediante la violación de la "condición de campo congelado". Por lo tanto, el entendimiento de estos campos constituye también una base importante para la comprensi6n de los plasmas cósmicos en general. Las mismas fuerzas que lanzan a los electrones aurorales hacia abajo también lanzan iones positives hacia arriba, al interior de la magnetosfera. Esta expulsión puede ser tan abundante que en ocasiones una gran parte de la magnetosfera está dominada por plasma de oxígeno proveniente de la propia ionosfera de la Tierra, en vez de estarlo por el plasma de hidrógeno del viento solar. Por razones que apenas empezamos a entender, la expulsión es altamente selectiva. En otras palabras, constituye un mecanismo eficiente de separación química, cuya mera existencia era completamente inesperada hasta hace muy poco. tiempo. Un mecanismo similar de separación puede operar en otros plasmas astrofísicos, de modo que la importancia de este descubrimiento podría tener un largo alcance. Alguien ha enfatizado que las lecciones aprendidas en las regiones accesibles del espacio de plasma requieren un "cambia de paradigma" que afecte a toda la astrofísica, la cosmología y la cosmogonía. La mayoría de estas lecciones han provenido del estudio de los problemas relacionados con la aurora, y es posible que aún surjan más.doi: Sin doi
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Volosevich, A. V., and Y. I. Galperin. "Nonlinear wave structures in collisional plasma of auroral E-region ionosphere." Annales Geophysicae 15, no. 7 (July 31, 1997): 890–98. http://dx.doi.org/10.1007/s00585-997-0890-8.
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Abstract. Studies of the auroral plasma with small-scale inhomogenieties producing the VHF-radar reflections (radar aurora) when observed in conditions of the saturated Farley-Buneman instability within the auroral E region, show strong nonlinear interactions and density fluctuations of 5–15%. Such nonlinearity and high fluctation amplitudes are inconsistent with the limitations of the weak turbulence theory, and thus a theory for arbitrary amplitudes is needed. To this end, a nonlinear theory is described for electrostatic MHD moving plasma structures of arbitrary amplitude for conditions throughout the altitude range of the collisional auroral E region. The equations are derived, from electron and ion motion self-consistent with the electric field, for the general case of the one-dimensional problem. They take into account nonlinearity, electron and ion inertia, diffusion, deviation from quasi-neutrality, and dynamical ion viscosity. The importance of the ion viscosity for dispersion is stressed, while deviation from the quasi-neutrality can be important only at rather low plasma densities, not typical for the auroral E region. In a small amplitude limit these equations have classical nonlinear solutions of the type of "electrostatic shock wave" or of knoidal waves. In a particular case these knoidal waves degrade to a dissipative soliton. A two-dimensional case of a quasi-neutral plasma is considered in the plane perpendicular to the magnetic field by way of the Poisson brackets, but neglecting the nonlinearity and ion inertia. It is shown that in these conditions an effective saturation can be achieved at the stationary turbulence level of order of 10%.
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Grono, Eric, Eric Donovan, and Kyle R. Murphy. "Tracking patchy pulsating aurora through all-sky images." Annales Geophysicae 35, no. 4 (July 3, 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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Safargaleev, V., T. Sergienko, H. Nilsson, A. Kozlovsky, S. Massetti, S. Osipenko, and A. Kotikov. "Combined optical, EISCAT and magnetic observations of the omega bands/Ps6 pulsations and an auroral torch in the late morning hours: a case study." Annales Geophysicae 23, no. 5 (July 28, 2005): 1821–38. http://dx.doi.org/10.5194/angeo-23-1821-2005.
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Abstract. We present here the results of multi-instrument observations of auroral torch and Ps6 magnetic pulsations, which are assumed to be the magnetic signature of the spatially periodic optical auroras known as omega bands. Data from TV and ASC cameras in Barentsburg and Ny Ålesund, EISCAT radars in Longyearbyen and Tromsø, as well as IMAGE network were used in this study. The auroral phenomenon which was considered differed from that previously discussed, as it occurred both in an unusual place (high latitudes) and at an unusual time (late morning hours). We show that this might occur due to specific conditions in the interplanetary medium, causing the appropriate deformation of the magnetosphere. In such a case, the IMF turned out to be an additional factor in driving the regime of Ps6/omega bands, namely, only by acting together could a substorm onset in the night sector and Bz variations result in their generation. Since the presumable source of Ps6/omega bands does not co-locate with convection reversal boundaries, we suggest the interpretation of the phenomena in the frame of the interchange instability instead of the Kelvin-Helmholtz instability that is widely discussed in the literature in connection with omega auroras. Some numerical characteristics of the auroral torch were obtained. We also emphasize to the dark hole in the background luminosity and the short-lived azimuthally-restricted auroral arc, since their appearance could initiate the auroral torch development. Keywords. Magnetospheric physics (Auroral phenomena; Plasma convection; Solar wind-magnetosphere interaction)
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Miller, Steven, Hoanh An Lam, and Jonathan Tennyson. "What astronomy has learned from observations of." Canadian Journal of Physics 72, no. 11-12 (November 1, 1994): 760–71. http://dx.doi.org/10.1139/p94-100.
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We review the uses to which [Formula: see text] astronomical observations have been put, in the five years since the first detection of this molecular ion in the aurorae of Jupiter. Spectroscopy of Jupiter shows that the ionospheric temperature is high (~1000 K) compared with the lower atmosphere and that auroral column densities are between 1012 and 1013 cm−2. The molecule is also distributed widely across the planet in concentrations about 10% of peak auroral densities. Imaging in [Formula: see text]-sensitive wavelengths links the aurorae to open magnetic field lines, rather than those encompassing the Io plasma torus. Uranus shows [Formula: see text] emission intensities of a few percent of the peak jovian auroral lines, with somewhat weaker emission from Saturn. On Uranus, [Formula: see text] is distributed fairly evenly across the planet, with some indication that auroral enhancement is not more than a factor of two. Saturnine [Formula: see text] appears to be concentrated more towards the magnetic poles. The identification of [Formula: see text] in the spectrum of the type II supernova SN1987a constrains models of the explosion to avoid microscopic mixing during much of the first year of the event. So far no reliable detection of [Formula: see text] in the interstellar medium has been reported.
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Milan, S. E., T. K. Yeoman, M. Lester, J. Moen, and P. E. Sandholt. "Post-noon two-minute period pulsating aurora and their relationship to the dayside convection pattern." Annales Geophysicae 17, no. 7 (July 31, 1999): 877–91. http://dx.doi.org/10.1007/s00585-999-0877-8.
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Abstract. Poleward-moving auroral forms, as observed by meridian-scanning photometers, in the vicinity of the cusp region are generally assumed to be the optical signature of flux transfer events. Another class of quasi-continuous, short period (1-2 min) wave-like auroral emission has been identified, closely co-located with the convection reversal boundary in the post-noon sector, which is similar in appearance to such cusp aurora. It is suggested that these short period wave-like auroral emissions, the optical signature of boundary plasma sheet precipitation in the region 1 field-aligned current system, are associated with ULF magnetohydrodynamic wave activity, which is observed simultaneously by ground magnetometer stations. This association with ULF wave activity is strengthened by the observation of several harmonic frequencies in the pulsation spectrum, each an overtone of the fundamental standing wave resonance frequency.Key words. Magnetospheric physics (auroral phenomena; magnetopause · cusp · and boundary layers; MHD waves and instabilities)
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McWilliams, K. A., T. K. Yeoman, J. B. Sigwarth, L. A. Frank, and M. Brittnacher. "The dayside ultraviolet aurora and convection responses to a southward turning of the interplanetary magnetic field." Annales Geophysicae 19, no. 7 (July 31, 2001): 707–21. http://dx.doi.org/10.5194/angeo-19-707-2001.
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Abstract. We examine the large-scale ultraviolet aurora and convection responses to a series of flux transfer events that immediately followed a sharp and isolated southward turning of the IMF. During the interval of interest, SuperDARN was monitoring the plasma convection in the dayside northern ionosphere, while the VIS Earth Camera and the Far Ul-traviolet Imager (UVI) were monitoring the northern hemisphere’s ultraviolet aurora. Reconnection signatures were seen in the SuperDARN HF radar data in the postnoon sector following a sharp southward turning of the IMF. The presence of flux transfer events is supported by measurements of a classic dispersed ion signature in the low-altitude cusp from the DMSP spacecraft. Subsequent to the onset of reconnection, the postnoon convection and ultraviolet aurora expanded in concert, reaching 18 MLT in half an hour. The auroral oval was found to move equatorward at the convection speed in the 16–18 MLT sector, implying that it was related directly to an adiaroic magnetospheric boundary. In the present study, we have estimated the field-aligned current response to magnetic reconnection in terms of the vorticity of the ionospheric plasma convection velocity. The convection velocities were obtained using two methods: (a) direct reconstruction of the full vector velocities from bistatic measurements of the convection by the SuperDARN HF radars in a relatively small region of the auroral zone, and (b) from global-scale spherical harmonic fits to the SuperDARN velocities deduced from the map potential model. Regions of high vorticity, which were predicted to be an estimate of a component of the total field-aligned current, agree extremely well with the images of the dayside UV aurora, indicating that, in this case, the plasma vorticity is an excellent estimator of the morphology of dayside field-aligned currents (FACs). The morphology of the aurora and ionospheric electric field in the postnoon sector supports the existence of a dayside current wedge induced in response to dayside reconnection.Key words. Magnetospheric physics (auroral phenomena; magnetosphere-ionosphere interactions; solar wind magne-tosphere interactions)
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Kletzing, C. A., and J. D. Scudder. "Auroral-plasma sheet electron anisotropy." Geophysical Research Letters 26, no. 7 (April 1, 1999): 971–74. http://dx.doi.org/10.1029/1999gl900092.
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Hilgers, A. "The auroral radiating plasma cavities." Geophysical Research Letters 19, no. 3 (February 7, 1992): 237–40. http://dx.doi.org/10.1029/91gl02938.
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Feldstein, Y. I., L. I. Gromova, M. Förster, and A. E. Levitin. "Spiral structures and regularities in magnetic field variations and auroras." History of Geo- and Space Sciences 3, no. 1 (February 21, 2012): 1–31. http://dx.doi.org/10.5194/hgss-3-1-2012.
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Abstract. The conception of spiral shaped precipitation regions, where solar corpuscles penetrate the upper atmosphere, was introduced into geophysics by C. Störmer and K. Birkeland at the beginning of the last century. Later, in the course of the XX-th century, spiral distributions were disclosed and studied in various geophysical phenomena. Most attention was devoted to spiral shapes in the analysis of regularities pertaining to the geomagnetic activity and auroras. We review the historical succession of perceptions about the number and positions of spiral shapes, that characterize the spatial-temporal distribution of magnetic disturbances. We describe the processes in the upper atmosphere, which are responsible for the appearance of spiral patterns. We considered the zones of maximal aurora frequency and of maximal particle precipitation intensity, as offered in the literature, in their connection with the spirals. We discuss the current system model, that is closely related to the spirals and that appears to be the source for geomagnetic field variations during magnetospheric substorms and storms. The currents in ionosphere and magnetosphere constitute together with field-aligned (along the geomagnetic field lines) currents (FACs) a common 3-D current system. At ionospheric heights, the westward and eastward electrojets represent characteristic elements of the current system. The westward electrojet covers the longitudinal range from the morning to the evening hours, while the eastward electrojet ranges from afternoon to near-midnight hours. The polar electrojet is positioned in the dayside sector at cusp latitudes. All these electrojets map along the magnetic field lines to certain plasma structures in the near-Earth space. The first spiral distribution of auroras was found based on observations in Antarctica for the nighttime-evening sector (N-spiral), and later in the nighttime-evening (N-spiral) and morning (M-spiral) sectors both in the Northern and Southern Hemispheres. The N- and M-spirals drawn in polar coordinates form an oval, along which one observes most often auroras in the zenith together with a westward electrojet. The nature of spiral distributions in geomagnetic field variations was unabmibuously interpreted after the discovery of the spiral's existence in the auroras had been established and this caused a change from the paradigm of the auroral zone to the paradigm of the auroral oval. Zenith forms of auroras are found within the boundaries of the auroral oval. The oval is therefore the region of most frequent precipitations of corpuscular fluxes with auroral energy, where anomalous geophysical phenomena occur most often and with maximum intensity. S. Chapman and L. Harang identified the existence of a discontinuity at auroral zone latitudes (Φ ∼ 67°) around midnight between the westward and eastward electrojets, that is now known as the Harang discontinuity. After the discovery of the auroral oval and the position of the westward electrojet along the oval, it turned out, that there is no discontinuity at a fixed latitude between the opposite electrojets, but rather a gap, the latitude of which varies smoothly between Φ ∼ 67° at midnight and Φ ∼ 73° at 20:00 MLT. In this respect the term ''Harang discontinuity'' represents no intrinsic phenomenon, because the westward electrojet does not experience any disruption in the midnight sector but continues without breaks from dawn to dusk hours.
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Moser, Chrystal, James LaBelle, and Iver H. Cairns. "High bandwidth measurements of auroral Langmuir waves with multiple antennas." Annales Geophysicae 40, no. 2 (April 22, 2022): 231–45. http://dx.doi.org/10.5194/angeo-40-231-2022.
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Abstract. The High-Bandwidth Auroral Rocket (HIBAR) was launched from Poker Flat, Alaska, on 28 January 2003 at 07:50 UT towards an apogee of 382 km in the nightside aurora. The flight was unique in having three high-frequency (HF) receivers using multiple antennas parallel and perpendicular to the ambient magnetic field, as well as very low-frequency (VLF) receivers using antennas perpendicular to the magnetic field. These receivers observed five short-lived Langmuir wave bursts lasting from 0.1–0.2 s, consisting of a thin plasma line with frequencies in the range of 2470–2610 kHz that had an associated diffuse feature occurring 5–10 kHz above the plasma line. Both of these waves occurred slightly above the local plasma frequency with amplitudes between 1–100 µV m−1. The ratio of the parallel to perpendicular components of the plasma line and diffuse feature were used to determine the angle of propagation of these waves with respect to the background magnetic field. These angles were found to be comparable to the theoretical Z-infinity angle that these waves would resonate at. The VLF receiver detected auroral hiss throughout the flight at 5–10 kHz, a frequency matching the difference between the plasma line and the diffuse feature. A dispersion solver, partially informed with measured electron distributions, and associated frequency- and wavevector-matching conditions were employed to determine if the diffuse features could be generated by a nonlinear wave–wave interaction of the plasma line with the lower-frequency auroral hiss waves/lower-hybrid waves. The results show that this interpretation is plausible.
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Milan, S. E., M. Lester, N. Sato, H. Takizawa, and J. P. Villain. "Investigation of the relationship between optical auroral forms and HF radar E region backscatter." Annales Geophysicae 18, no. 6 (June 30, 2000): 608–17. http://dx.doi.org/10.1007/s00585-000-0608-7.
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Abstract. The SuperDARN HF radars have been employed in the past to investigate the spectral characteristics of coherent backscatter from L-shell aligned features in the auroral E region. The present study employs all-sky camera observations of the aurora from Husafell, Iceland, and the two SuperDARN radars located on Iceland, Þykkvibær and Stokkseyri, to determine the optical signature of such backscatter features. It is shown that, especially during quiet geomagnetic conditions, the backscatter region is closely associated with east-west aligned diffuse auroral features, and that the two move in tandem with each other. This association between optical and radar aurora has repercussions for the instability mechanisms responsible for generating the E region irregularities from which radars scatter. This is discussed and compared with previous studies investigating the relationship between optical and VHF radar aurora. In addition, although it is known that E region backscatter is commonly observed by SuperDARN radars, the present study demonstrates for the first time that multiple radars can observe the same feature to extend over at least 3 h of magnetic local time, allowing precipitation features to be mapped over large portions of the auroral zone.Key words: Ionosphere (particle precipitation; plasma waves and instabilities)
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Feldstein, Y. I., R. D. Elphinstone, D. J. Hearn, J. S. Murphree, and L. L. Cogger. "Mapping of the statistical auroral distribution into the magnetosphere." Canadian Journal of Physics 72, no. 5-6 (May 1, 1994): 266–69. http://dx.doi.org/10.1139/p94-039.
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Statistical auroral distributions are used in combination with an empirical model of the Earth's magnetic field in an attempt to determine the large-scale magnetospheric source regions for various types of auroral luminosity. The narrow ring of structured auroral emissions during magnetically quiet intervals appears to be associated with the inner region of the nightside central plasma sheet and the dayside entry layer. Under active conditions these discrete structures expand to fill the entire central plasma sheet. The high-altitude boundary plasma sheet on the other hand is more likely to be related to diffuse auroral emissions poleward of this "oval" and to high-latitude polar auroral arcs. Under this scenario, the region of the magnetosphere bounded by the inner edge of the tail current sheet, the plasmasphere, and the dayside entry layer is the source region for the most equatorward diffuse auroral precipitation.
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Uritsky, V. M., E. Donovan, A. J. Klimas, and E. Spanswick. "Collective dynamics of bursty particle precipitation initiating in the inner and outer plasma sheet." Annales Geophysicae 27, no. 2 (February 16, 2009): 745–53. http://dx.doi.org/10.5194/angeo-27-745-2009.
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Abstract. Using multiscale spatiotemporal analysis of bursty precipitation events in the nighttime aurora as seen by the POLAR UVI instrument, we report a set of new statistical signatures of high- and low-latitude auroral activity, signaling a strongly non-uniform distribution of dissipation mechanism in the plasma sheet. We show that small-scale electron emission events that initiate in the equatorward portion of the nighttime auroral oval (scaling mode A1) have systematically steeper power-law slopes of energy, power, area, and lifetime probability distributions compared to the events that initiate at higher latitudes (mode B). The low-latitude group of events also contain a small but energetically important subpopulation of substorm-scale disturbances (mode A2) described by anomalously low distribution exponents characteristic of barely stable thermodynamic systems that are prone to large-scale sporadic reorganization. The high latitude events (mode
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Badman, S. V., S. W. H. Cowley, J. C. Gérard, and D. Grodent. "A statistical analysis of the location and width of Saturn's southern auroras." Annales Geophysicae 24, no. 12 (December 21, 2006): 3533–45. http://dx.doi.org/10.5194/angeo-24-3533-2006.
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Abstract. A selection of twenty-two Hubble Space Telescope images of Saturn's ultraviolet auroras obtained during 1997–2004 has been analysed to determine the median location and width of the auroral oval, and their variability. Limitations of coverage restrict the analysis to the southern hemisphere, and to local times from the post-midnight sector to just past dusk, via dawn and noon. It is found that the overall median location of the poleward and equatorward boundaries of the oval with respect to the southern pole are at ~14° and ~16° co-latitude, respectively, with a median latitudinal width of ~2°. These median values vary only modestly with local time around the oval, though the poleward boundary moves closer to the pole near noon (~12.5°) such that the oval is wider in that sector (median width ~3.5°) than it is at both dawn and dusk (~1.5°). It is also shown that the position of the auroral boundaries at Saturn are extremely variable, the poleward boundary being located between 2° and 20° co-latitude, and the equatorward boundary between 6° and 23°, this variability contrasting sharply with the essentially fixed location of the main oval at Jupiter. Comparison with Voyager plasma angular velocity data mapped magnetically from the equatorial magnetosphere into the southern ionosphere indicates that the dayside aurora lie poleward of the main upward-directed field-aligned current region associated with corotation enforcement, which maps to ~20°–24° co-latitude, while agreeing reasonably with the position of the open-closed field line boundary based on estimates of the open flux in Saturn's tail, located between ~11° and ~15°. In this case, the variability in location can be understood in terms of changes in the open flux present in the system, the changes implied by the Saturn data then matching those observed at Earth as fractions of the total planetary flux. We infer that the broad (few degrees) diffuse auroral emissions and sub-corotating auroral patches observed in the dayside sector at Saturn result from precipitation from hot plasma sub-corotating in the outer magnetosphere in a layer a few Saturn radii wide adjacent to the magnetopause, probably having been injected either by Dungey-cycle or Vasyliunas-cycle dynamics on the nightside.
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Yeoman, T. K., J. A. Davies, N. M. Wade, G. Provan, and S. E. Milan. "Combined CUTLASS, EISCAT and ESR observations of ionospheric plasma flows at the onset of an isolated substorm." Annales Geophysicae 18, no. 9 (September 30, 2000): 1073–87. http://dx.doi.org/10.1007/s00585-000-1073-z.
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Abstract. On August 21st 1998, a sharp southward turning of the IMF, following on from a 20 h period of northward directed magnetic field, resulted in an isolated substorm over northern Scandinavia and Svalbard. A combination of high time resolution and large spatial scale measurements from an array of coherent scatter and incoherent scatter ionospheric radars, ground magnetometers and the Polar UVI imager has allowed the electrodynamics of the impulsive substorm electrojet region during its first few minutes of evolution at the expansion phase onset to be studied in great detail. At the expansion phase onset the substorm onset region is characterised by a strong enhancement of the electron temperature and UV aurora. This poleward expanding auroral structure moves initially at 0.9 km s-1 poleward, finally reaching a latitude of 72.5°. The optical signature expands rapidly westwards at ~6 km s-1, whilst the eastward edge also expands eastward at ~0.6 km s-1. Typical flows of 600 m s-1 and conductances of 2 S were measured before the auroral activation, which rapidly changed to ~100 m s-1 and 10-20 S respectively at activation. The initial flow response to the substorm expansion phase onset is a flow suppression, observed up to some 300 km poleward of the initial region of auroral luminosity, imposed over a time scale of less than 10 s. The high conductivity region of the electrojet acts as an obstacle to the flow, resulting in a region of low-electric field, but also low conductivity poleward of the high-conductivity region. Rapid flows are observed at the edge of the high-conductivity region, and subsequently the high flow region develops, flowing around the expanding auroral feature in a direction determined by the flow pattern prevailing before the substorm intensification. The enhanced electron temperatures associated with the substorm-disturbed region extended some 2° further poleward than the UV auroral signature associated with it.Key words: Ionosphere (auroral ionosphere) - Magnetospheric physics (magnetosphere - ionosphere interactions; storms and substorms)
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Feldstein, Y. I., L. I. Gromova, J. Woch, I. Sandahl, L. Blomberg, G. Marklund, and C. I. Meng. "Structure of the auroral precipitation region in the dawn sector: relationship to convection reversal boundaries and field-aligned currents." Annales Geophysicae 19, no. 5 (May 31, 2001): 495–519. http://dx.doi.org/10.5194/angeo-19-495-2001.
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Abstract. Abstract. Simultaneous DMSP F7 and Viking satellite measurements of the dawnside high-latitude auroral energy electron and ion precipitation show that the region of the low and middle altitude auroral precipitation consists of three characteristic plasma regimes. The recommendation of the IAGA Working Group IIF/III4 at the IAGA Assembly in Boulder, July 1995 to decouple the nomenclature of ionospheric populations from magnetospheric population is used for their notation. The most equatorial regime is the Diffuse Auroral Zone (DAZ) of diffuse spatially unstructured precipitating electrons. It is generated by the plasma injection to the inner magnetosphere in the nightside and the subsequent drift plasma to the dawnside around the Earth. Precipitating particles have a hard spectrum with typical energies of electrons and ions of more than 3 keV. In the DAZ, the ion pitch-angle distribution is anisotropic, with the peak near 90°. The next part is the Auroral Oval (AO), a structured electron regime which closely resembles the poleward portion of the night-side auroral oval. The typical electron energy is several keV, and the ion energy is up to 10 keV. Ion distributions are pre-dominantly isotropic. In some cases, this plasma regime may be absent in the pre-noon sector. Poleward of the Auroral Oval, there is the Soft Small Scale Luminosity (SSSL) regime. It is caused by structured electron and ion precipitation with typical electron energy of about 0.3 keV and ion energy of about 1 keV. The connection of these low-altitude regimes with plasma domains of the distant magnetosphere is discussed. For mapping of the plasma regimes to the equatorial plane of the magnetosphere, the empirical model by Tsyganenko (1995) and the conceptual model by Alexeev et al. (1996) are used. The DAZ is mapped along the magnetic field lines to the Remnant Layer (RL), which is located in the outer radiation belt region; the zone of structured electrons and isotropic ion precipitation (AO) is mapped to the dawn periphery of the Central Plasma Sheet (CPS); the soft small scale structured precipitation (SSSL) is mapped to the outer magnetosphere close to the magnetopause, i.e. the Low Latitude Boundary Layer (LLBL). In the near-noon sector, earthward fluxes of soft electrons, which cause the Diffuse Red Aurora (DRA), are observed. The ion energies decrease with increasing latitude. The plasma spectra of the DRA regime are analogous to the spectra of the Plasma Mantle (PM). In the dawn sector, the large-scale field-aligned currents flow into the ionosphere at the SSSL latitudes (Region 1) and flow out at the AO or DAZ latitudes (Region 2). In the dawn and dusk sectors, the large-scale Region 1 and Region 2 FAC generation occurs in different plasma domains of the distant magnetosphere. The dawn and dusk FAC connection to the traditional Region 1 and Region 2 has only formal character, as FAC generating in various magnetospheric plasma domains integrate in the same region (Region 1 or Region 2). In the SSSL, there is anti-sunward convection; in the DAZ and the AO, there is the sunward convection. At PM latitudes, the convection is controlled by the azimuthal IMF component (By ). It is suggested to extend the notation of the plasma pattern boundaries, as proposed by Newell et al. (1996), for the nightside sector of the auroral oval to the dawn sector.Key words. Magnetospheric physics (current systems; magnetospheric configuration and dynamics; plasma convection)
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Danielides, M. A., and A. Kozlovsky. "Rocket-borne investigation of auroral patches in the evening sector during substorm recovery." Annales Geophysicae 21, no. 3 (March 31, 2003): 719–28. http://dx.doi.org/10.5194/angeo-21-719-2003.
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Abstract. On 11 February 1997 at 08:36 UT after a substorm onset the Auroral Turbulence 2 sounding rocket was launched from Poker Flat Research Range, Alaska into a moderately active auroral region. This experiment has allowed us to investigate evening (21:00 MLT) auroral forms at the substorm recovery, which were discrete multiple auroral arcs stretched to, the east and southeast from the breakup region, and bright auroral patches propagating westward along the arcs like a luminosity wave, which is a typical feature of the disturbed arc. The rocket crossed an auroral arc of about 40 km width, stretched along southeast direction. Auroral patches and associated electric fields formed a 200 km long periodical structure, which propagated along the arc westward at a velocity of 3 km/s, whereas the ionospheric plasma velocity inside the arc was 300 m/s westward. The spatial periodicity in the rocket data was found from optical ground-based observations, from electric field in situ measurements, as well as from ground-based magnetic observations. The bright patches were co-located with equatorward plasma flow across the arc of the order of 200 m/s in magnitude, whereas the plasma flow tended to be poleward at the intervals between the patches, where the electric field reached the magnitude of up to 20 mV/m, and these maxima were co-located with the peaks in electron precipitations indicated by the electron counter on board the rocket. Pulsations of a 70-s period were observed on the ground in the eastern component of the magnetic field and this is consistent with the moving auroral patches and the north-south plasma flows associated with them. The enhanced patch-associated electric field and fast westward propagation suggest essential differences between evening auroral patches and those occurring in the morning ionosphere. We propose the wave that propagates along the plasma sheet boundary to be a promising mechanism for the evening auroral patches.Key words. Ionosphere (auroral ionosphere; electric fields and currents)
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Despirak, I. V., T. V. Kozelova, B. V. Kozelov, and A. A. Lubchich. "OBSERVATIONS OF SUBSTORM ACTIVITY FROM THE DATA OF MAIN CAMERA SYSTEM AND THD SATELLITE IN THE PLASMA SHEET." PHYSICS OF AURORAL PHENOMENA 44 (2021): 16–19. http://dx.doi.org/10.51981/2588-0039.2021.44.003.
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We investigated an interesting case of the space-time dynamics of substorm activations (AL ~ 800 nT) on December 24, 2014, when there were simultaneous observations on the THEMIS D satellite in the plasma sheet (|X| ~ 6.2 RE) and ground-based observations on the Kola Peninsula. The development of the substorm activity in the interval of ~ 19:00 to ~ 20:00 UT was considered. In this interval, at Lovozero station (LOZ), three peaks in the Pi1B pulsations were recorded, associated with the brightening of arcs near LOZ. The first peak was observed in connection with the appearance of beads structures in the auroras along the growth phase arc to the south from LOZ latitude. The second and third peaks in Pi1B pulsations were associated with the expansion phase, when three dipolarization fronts (DFs) were registered according THD data. DFs and injection of energetic electrons into the magnetosphere were observed near the moments of sudden intensification of auroras: brightening of arcs, breakup in aurora. Besides, it was shown that the development of substorm occurs near the Harang discontinuity (HD) according to the IMAGE magnetometers data. In this case, we can follow the development of aurora around the HD according to the data of the all sky camera in Apatity. It was shown that the pre-onset auroral forms were moved accordingly the two-cell ionospheric convection developed during the growth phase of the substorm.
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Amm, O., A. Pajunpää, and U. Brandström. "Spatial distribution of conductances and currents associated with a north-south auroral form during a multiple-substorm period." Annales Geophysicae 17, no. 11 (November 30, 1999): 1385–96. http://dx.doi.org/10.1007/s00585-999-1385-6.
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Abstract. Using the method of characteristics to invert ground-based data of the ground magnetic field disturbance and of the ionospheric electric field, we obtain spatial distributions of ionospheric conductances, currents, and field-aligned currents (FACs) associated with a north-south auroral form that drifts westwards over northern Scandinavia around 2200 UT on December 2, 1977. This auroral form is one in a sequence of such north-south structures observed by all-sky cameras, and appears 14 min after the last of several breakups during that extremely disturbed night. Our analysis shows that the ionospheric Hall conductance reaches values above 200 S in the center of the form, and upward flowing FACs of up to 25 µA/m2 are concentrated near its westward and equatorward edge. The strong upward flowing FACs are fed by an area of more distributed, but still very strong downward-flowing FACs northeastward of the auroral form. In contrast to the conductances, the electric field is only slightly affected by the passage of the form. We point out similarities and differences of our observations and results to previously reported observations and models of 'auroral fingers', 'north-south aurora', and 'auroral streamers' which are suggested to be ionospheric manifestations of bursty bulk flows in the plasma sheet.Key words. Ionosphere (auroral ionosphere; electric fields and currents) · Magnetospheric physics (magnetosphere · ionosphere interactions)
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Kozlovsky, A. E., V. V. Safargaleev, J. R. T. Jussila, and A. V. Koustov. "Pre-noon high-latitude auroral arcs as a manifestation of the interchange instability." Annales Geophysicae 21, no. 12 (December 31, 2003): 2303–14. http://dx.doi.org/10.5194/angeo-21-2303-2003.
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Abstract. On 7 December 2000, TV ASC camera in Barentsburg (Svalbard) observed pre-noon (at 09:00–10:00 MLT) rayed auroral arcs, which occurred at the pole-ward edge of the auroral oval after an IMF transition from By -dominated (By = + 8.8, Bz = + 4.3) to strongly northward dominated (By = + 2.7, Bz = + 8.6). The arcs appeared from the area of enhanced luminosity seen in the western (nightside) horizon, and developed to the east, progressing at a velocity of about 1.5 km/s. Simultaneously, the arcs were drifting poleward at a velocity of 300–500 m/s, whose value was equal to the F-region ionospheric plasma drift velocity observed by the Incoherent Scatter Radar (ESR). The arc appearance and motion corresponded well to the poleward expansion of the auroral oval following the IMF shift, which was observed by the UVI on board the Polar satellite. The observed auroras were associated with closed LLBL indicated by the particle precipitation data from DMSP satellites showing also several-keV electrons of PS origin. The observations allow us to suggest that the arcs arise due to the interchange instability that starts to develop at the boundary between the magnetospheric plasma and the magnetosheath flux tubes entering the closed magnetosphere due to the reconnection beyond the cusp after the IMF changes. The interchange instability can be suggested as a possible mechanism for the formation of the LLBL. Key words. Magnetospheric physics (auroral phenomena; magnetopause, cusp and boundary layers; magnetospheric configuration and dynamics)
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Nikolaev, A. V., V. A. Sergeev, N. A. Tsyganenko, M. V. Kubyshkina, H. Opgenoorth, H. Singer, and V. Angelopoulos. "A quantitative study of magnetospheric magnetic field line deformation by a two-loop substorm current wedge." Annales Geophysicae 33, no. 4 (April 29, 2015): 505–17. http://dx.doi.org/10.5194/angeo-33-505-2015.
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Abstract. Substorm current wedge (SCW) formation is associated with global magnetic field reconfiguration during substorm expansion. We combine a two-loop model SCW (SCW2L) with a background magnetic field model to investigate distortion of the ionospheric footpoint pattern in response to changes of different SCW2L parameters. The SCW-related plasma sheet footprint shift results in formation of a pattern resembling an auroral bulge, the poleward expansion of which is controlled primarily by the total current in the region 1 sense current loop (I1). The magnitude of the footprint latitudinal shift may reach ∼ 10° corrected geomagnetic latitude (CGLat) during strong substorms (I1= 2 MA). A strong helical magnetic field around the field-aligned current generates a surge-like region with embedded spiral structures, associated with a westward traveling surge (WTS) at the western end of the SCW. The helical field may also contribute to rotation of the ionospheric projection of narrow plasma streams (auroral streamers). Other parameters, including the total current in the second (region 2 sense) loop, were found to be of secondary importance. Analyzing two consecutive dipolarizations on 17 March 2010, we used magnetic variation data obtained from a dense midlatitude ground network and several magnetospheric spacecraft, as well as the adaptive AM03 model, to specify SCW2L parameters, which allowed us to predict the magnitude of poleward auroral expansion. Auroral observations made during the two substorm activations demonstrate that the SCW2L combined with the AM03 model nicely describes the azimuthal progression and the observed magnitude of the auroral expansion. This finding indicates that the SCW-related distortions are responsible for much of the observed global development of bright auroras.
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Figueiredo, S., G. T. Marklund, T. Karlsson, T. Johansson, Y. Ebihara, M. Ejiri, N. Ivchenko, P. A. Lindqvist, H. Nilsson, and A. Fazakerley. "Temporal and spatial evolution of discrete auroral arcs as seen by Cluster." Annales Geophysicae 23, no. 7 (October 14, 2005): 2531–57. http://dx.doi.org/10.5194/angeo-23-2531-2005.
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Abstract. Two event studies are presented in this paper where intense convergent electric fields, with mapped intensities up to 1350 mV/m, are measured in the auroral upward current region by the Cluster spacecraft, at altitudes between 3 and 5 Earth radii. Both events are from May 2003, Southern Hemisphere, with equatorward crossings by the Cluster spacecraft of the pre-midnight auroral oval. Event 1 occurs during the end of the recovery phase of a strong substorm. A system of auroral arcs associated with convergent electric field structures, with a maximum perpendicular potential drop of about ~10 kV, and upflowing field-aligned currents with densities of 3 µA/m2 (mapped to the ionosphere), was detected at the boundary between the Plasma Sheet Boundary Layer (PSBL) and the Plasma Sheet (PS). The auroral arc structures evolve in shape and in magnitude on a timescale of tens of minutes, merging, broadening and intensifying, until finally fading away after about 50 min. Throughout this time, both the PS region and the auroral arc structure in its poleward part remain relatively fixed in space, reflecting the rather quiet auroral conditions during the end of the substorm. The auroral upward acceleration region is shown for this event to extend beyond 3.9 Earth radii altitude. Event 2 occurs during a more active period associated with the expansion phase of a moderate substorm. Images from the Defense Meteorological Satellite Program (DMSP) F13 spacecraft show that the Cluster spacecraft crossed the horn region of a surge-type aurora. Conjugated with the Cluster spacecraft crossing above the surge horn, the South Pole All Sky Imager recorded the motion and the temporal evolution of an east-west aligned auroral arc, 30 to 50 km wide. Intense electric field variations are measured by the Cluster spacecraft when crossing above the auroral arc structure, collocated with the density gradient at the PS poleward boundary, and coupled to intense upflowing field-aligned currents with mapped densities of up to 20 µA/m2. The surge horn consists of multiple arc structures which later merge into one structure and intensify at the PS poleward boundary. The surge horn and the associated PS region moved poleward with a velocity at the ionospheric level of 0.5 km/s, following the large-scale poleward expansion of the auroral oval associated with the substorm expansion phase. Keywords. Ionosphere (Ionosphere-magnetosphere interacctions; Electric fields and currents; Particle acceleration)
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Janhunen, P., A. Olsson, and H. Laakso. "Altitude dependence of plasma density in the auroral zone." Annales Geophysicae 20, no. 11 (November 30, 2002): 1743–50. http://dx.doi.org/10.5194/angeo-20-1743-2002.
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Abstract. We study the altitude dependence of plasma depletions above the auroral region in the 5000–30 000 km altitude range using five years of Polar spacecraft potential data. We find that besides a general decrease of plasma density with altitude, there frequently exist additional density depletions at 2–4 RE radial distance, where RE is the Earth radius. The position of the depletions tends to move to higher altitude when the ionospheric footpoint is sunlit as compared to darkness. Apart from these cavities at 2–4 RE radial distance, separate cavities above 4 RE occur in the midnight sector for all Kp and also in the morning sector for high Kp. In the evening sector our data remain inconclusive in this respect. This holds for the ILAT range 68–74. These additional depletions may be substorm-related. Our study shows that auroral phenomena modify the plasma density in the auroral region in such a way that a nontrivial and interesting altitude variation results, which reflects the nature of the auroral acceleration processes.Key words. Magnetospheric physics (auroral phenomena; magnetosphere–ionosphere interactions)
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Willis, D. M., G. M. Armstrong, C. E. Ault, and F. R. Stephenson. "Identification of possible intense historical geomagnetic storms using combined sunspot and auroral observations from East Asia." Annales Geophysicae 23, no. 3 (March 30, 2005): 945–71. http://dx.doi.org/10.5194/angeo-23-945-2005.
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Abstract. Comprehensive catalogues of ancient sunspot and auroral observations from East Asia are used to identify possible intense historical geomagnetic storms in the interval 210 BC-AD 1918. There are about 270 entries in the sunspot catalogue and about 1150 entries in the auroral catalogue. Special databases have been constructed in which the scientific information in these two catalogues is placed in specified fields. For the purposes of this study, an historical geomagnetic storm is defined in terms of an auroral observation that is apparently associated with a particular sunspot observation, in the sense that the auroral observation occurred within several days of the sunspot observation. More precisely, a selection criterion is formulated for the automatic identification of such geomagnetic storms, using the oriental records stored in the sunspot and auroral databases. The selection criterion is based on specific assumptions about the duration of sunspot visibility with the unaided eye, the likely range of heliographic longitudes of an energetic solar feature, and the likely range of transit times for ejected solar plasma to travel from the Sun to the Earth. This selection criterion results in the identification of nineteen putative historical geomagnetic storms, although two of these storms are spurious in the sense that there are two examples of a single sunspot observation being associated with two different auroral observations separated by more than half a (synodic) solar rotation period. The literary and scientific reliabilities of the East Asian sunspot and auroral records that define the nineteen historical geomagnetic storms are discussed in detail in a set of appendices. A possible time sequence of events is presented for each geomagnetic storm, including possible dates for both the central meridian passage of the sunspot and the occurrence of the energetic solar feature, as well as likely transit times for the ejected solar plasma. European telescopic sunspot drawings from the seventeenth century are also used to assess the credibility of some of the later historical geomagnetic storms defined solely by the East Asian sunspot and auroral records. These drawings cast doubt on a few of the associations between sunspot and auroral observations based entirely on the oriental records, at least to the extent that the occidental drawings provide a more realistic date for central meridian passage of the sunspot actually associated with a particular auroral observation. Nevertheless, on those occasions for which European sunspot drawings are available, the dates of all the pertinent East Asian sunspot and auroral observations are corroborated, apart from just one Chinese sunspot observation. The ancient historical observations of sunspots and aurorae are discussed briefly in terms of modern observations of great geomagnetic storms.
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Sakaguchi, K., K. Shiokawa, A. Ieda, R. Nomura, A. Nakajima, M. Greffen, E. Donovan, I. R. Mann, H. Kim, and M. Lessard. "Fine structures and dynamics in auroral initial brightening at substorm onsets." Annales Geophysicae 27, no. 2 (February 9, 2009): 623–30. http://dx.doi.org/10.5194/angeo-27-623-2009.
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Abstract. We show four auroral initial brightening events at substorm onsets focusing on fine structures and their longitudinal dynamics, which were observed by all-sky TV cameras (30-Hz sampling) on January 2008, in Canada. For two initial brightenings started in the field of views of the cameras, we found that they started at longitudinal segments with a size of less than ~30–60 km. One brightening expanded with wavy structures and the other expanded as a straight arc. Although the two events had different structures, both brightening auroras expanded with an average speed of ~20 km/s in the first 10 s, and ~10 km/s in the following 10 s. The other two events show that brightening auroras developed with periodic structures, with longitudinal wavelengths of ~100–200 km. Assuming that the brightening auroras are mapped to the physical processes occurring in the plasma sheet, we found that the scale size (30–60 km) and the expanding speed (20 km/s) of brightening auroras correspond to the order of ion gyro radii (~500–1400 km) and Alfvén speed or fast ion-flow speed (~400 km/s), respectively, in the plasma sheet.
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Olsson, A., P. Janhunen, and W. K. Peterson. "Ion shell distributions as free energy source for plasma waves on auroral field lines mapping to plasma sheet boundary layer." Annales Geophysicae 22, no. 6 (June 14, 2004): 2115–33. http://dx.doi.org/10.5194/angeo-22-2115-2004.
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Abstract. Ion shell distributions are hollow spherical shells in velocity space that can be formed by many processes and occur in several regions of geospace. They are interesting because they have free energy that can, in principle, be transmitted to ions and electrons. Recently, a technique has been developed to estimate the original free energy available in shell distributions from in-situ data, where some of the energy has already been lost (or consumed). We report a systematic survey of three years of data from the Polar satellite. We present an estimate of the free energy available from ion shell distributions on auroral field lines sampled by the Polar satellite below 6 RE geocentric radius. At these altitudes the type of ion shells that we are especially interested in is most common on auroral field lines close to the polar cap (i.e. field lines mapping to the plasma sheet boundary layer, PSBL). Our analysis shows that ion shell distributions that have lost some of their free energy are commonly found not only in the PSBL, but also on auroral field lines mapping to the boundary plasma sheet (BPS), especially in the evening sector auroral field lines. We suggest that the PSBL ion shell distributions are formed during the so-called Velocity Dispersed Ion Signatures (VDIS) events. Furthermore, we find that the partly consumed shells often occur in association with enhanced wave activity and middle-energy electron anisotropies. The maximum downward ion energy flux associated with a shell distribution is often 10mWm-2 and sometimes exceeds 40mWm-2 when mapped to the ionosphere and thus may be enough to power many auroral processes. Earlier simulation studies have shown that ion shell distributions can excite ion Bernstein waves which, in turn, energise electrons in the parallel direction. It is possible that ion shell distributions are the link between the X-line and the auroral wave activity and electron acceleration in the energy transfer chain for stable auroral arcs.