Plasmaspheric Electrons Research Articles

The Role of Plasmasphere in the Formation of Electron Heat Fluxes

AbstractPlasmasphere plays an important role in the magnetospheric physics, defining many important inputs to the ionosphere from the middle to the auroral latitudes. Among them are electron thermal heat fluxes resulting from the Coulomb interaction of superthermal electrons (SE) and cold plasmaspheric electrons. These fluxes define the electron temperature at the upper ionospheric altitudes and are the input to the global ionospheric modeling networks. As was previously found from the calculation of lower energy SE and thermal heat fluxes, the knowledge of field‐aligned cold plasma distribution in the plasmasphere is a very sensitive parameter that introduces the most uncertainties in the calculation of these values. To verify the previously used SuperThermal Electron Transport code assumptions regarding plasmaspheric field‐aligned density structure ∼[B(s)/Bo]a, we used the latest version of 3D plasmaspheric model developed by Pierrard, Botek, and Darrouzet (2021, https://doi.org/10.3389/fspas.2021.681401). Such an assumption is found to be very reasonable in the calculations of electron thermal heat fluxes entering upper ionospheric altitudes and the associated electron temperature formation for the two selected dayside and nightside electron precipitation events driven by whistler‐mode wave activity.

Multi‐Event Analysis of Plasma and Field Variations in Source of Stable Auroral Red (SAR) Arcs in Inner Magnetosphere During Non‐Storm‐Time Substorms

AbstractStable auroral red (SAR) arcs are optical events with dominant 630.0‐nm emission caused by low‐energy electron heat flux into the topside ionosphere from the inner magnetosphere. SAR arcs are observed at subauroral latitudes and often occur during the recovery phase of magnetic storms and substorms. Past studies concluded that these low‐energy electrons were generated in the spatial overlap region between the outer plasmasphere and ring‐current ions and suggested that Coulomb collisions between plasmaspheric electrons and ring‐current ions are more feasible for the SAR‐arc generation mechanism rather than Landau damping by electromagnetic ion cyclotron waves or kinetic Alfvén waves. This work studies three separate SAR‐arc events with conjunctions, using all‐sky imagers and inner magnetospheric satellites (Arase and Radiation Belt Storm Probes [RBSP]) during non‐storm‐time substorms on December 19, 2012 (event 1), January 17, 2015 (event 2), and November 4, 2019 (event 3). We evaluated for the first time the heat flux via Coulomb collision using full‐energy‐range ion data obtained by the satellites. The electron heat fluxes due to Coulomb collisions reached ∼109 eV/cm2/s for events 1 and 2, indicating that Coulomb collisions could have caused the SAR arcs. RBSP‐A also observed local enhancements of 7–20‐mHz electromagnetic wave power above the SAR arc in event 2. The heat flux for the freshly detached SAR arc in event 3 reached ∼108 eV/cm2/s, which is insufficient to have caused the SAR arc. In event 3, local flux enhancement of electrons (<200 eV) and various electromagnetic waves were observed, these are likely to have caused the freshly detached SAR arc.

Plasma and Field Observations in the Magnetospheric Source Region of a Stable Auroral Red (SAR) Arc by the Arase Satellite on 28 March 2017

AbstractA stable auroral red (SAR) arc is an aurora with a dominant 630 nm emission at subauroral latitudes. SAR arcs have been considered to occur due to the spatial overlap between the plasmasphere and the ring‐current ions. In the overlap region, plasmaspheric electrons are heated by ring‐current ions or plasma waves, and their energy is then transferred down to the ionosphere where it causes oxygen red emission. However, there have been no study conducted so far that quantitatively examined plasma and electromagnetic fields in the magnetosphere associated with SAR arc. In this paper, we report the first quantitative evaluation of conjugate measurements of a SAR arc observed at 2204 UT on 28 March 2017 and investigate its source region using an all‐sky imager at Nyrölä (magnetic latitude: 59.4°N), Finland, and the Arase satellite. The Arase observation shows that the SAR arc appeared in the overlap region between a plasmaspheric plume and the ring‐current ions and that electromagnetic ion cyclotron waves and kinetic Alfven waves were not observed above the SAR arc. The SAR arc was located at the ionospheric trough minimum identified from a total electron content map obtained by the GNSS receiver network. The Swarm satellite flying in the ionosphere also passed the SAR arc at ~2320 UT and observed a decrease in electron density and an increase in electron temperature during the SAR‐arc crossing. These observations suggest that the heating of plasmaspheric electrons via Coulomb collision with ring‐current ions is the most plausible mechanism for the SAR‐arc generation.

Cold Plasmaspheric Electrons Affected by ULF Waves in the Inner Magnetosphere: A Van Allen Probes Statistical Study

AbstractSix years of Van Allen Probes data are used to investigate cold plasmaspheric electrons affected by ultralow‐frequency (ULF) waves in the inner magnetosphere (L<7) including spatial distributions, occurrence conditions, and resonant energy range. Events exhibit a global distribution within L= 4–7 but preferentially occur at L∼5.5–7 in the dayside, while there is higher occurrence rate in the duskside than dawnside. They can occur under different geomagnetic activities and solar wind velocities (VS), but the occurrence rates are increasing with larger AE, |SYMH|, and VS. These features are closely associated with the generation and propagation of ULF waves in Pc4 (45–150 s) and Pc5 (150–600 s) bands. Combined with electron observations from HOPE instrument, the resonant energies inferred from wave power indicate that cold electrons at ones to hundreds of electron volts can be affected by ULF waves. This study may shed new light on further investigations on the acceleration and transportation of cold plasmaspheric particles that would affect plasmaspheric material release to the Earth's magnetosphere and instabilities for exciting various waves.

A Comparative Study of ULF Waves' Role in the Dynamics of Charged Particles in the Plasmasphere: Van Allen Probes Observation

AbstractBy analyzing observations from Van Allen Probes in its inbound and outbound orbits, we present evidence of coherent enhancement of cold plasmaspheric electrons and ions due to drift‐bounce resonance with ultralow frequency (ULF) waves. From 18:00 UT on 28 May 2017 to 10:00 UT on 29 May 2017, newly formed poloidal mode standing ULF waves with significant electric field oscillations were observed in two consecutive orbits when Probe B was traveling inbound. In contrast to observations during outbound orbits, the cold (<150 eV) electrons measured by the HOPE instrument were characterized by flux enhancements several times larger and bidirectional pitch angle distributions during inbound orbits. The electron number density inferred from upper hybrid waves is twice as larger as during inbound orbits, which were also confirmed by an increase of spacecraft potential. The observed ULF waves are identified as second harmonic modes that satisfy the drift‐bounce resonant condition of N = 1 with cold electrons. An enhancement of the plasmaspheric ion number density to restore charge neutrality of plasmas in inbound orbits is observed, which is associated with an increase of ULF wave periods. The observations suggest that the dynamics of plasmaspheric electrons is modified by ULF waves through drift‐bounce resonance and that plasmaspheric ions are indirectly impacted.

VLF waves from ground‐based transmitters observed by the Van Allen Probes: Statistical model and effects on plasmaspheric electrons

AbstractWhistler mode very low frequency (VLF) waves from powerful ground‐based transmitters can resonantly scatter energetic plasmaspheric electrons and precipitate them into the atmosphere. A comprehensive 4 year statistics of Van Allen Probes measurements is carried out to assess their consequences on the dynamics of the inner radiation belt and slot region. Statistical models of the measured wave electric field power and of the inferred full wave magnetic amplitude are provided as a function of L, magnetic local time, season, and Kp over L = 1–3, revealing the localization of VLF wave intensity and its variation with geomagnetic activity over 2012–2016. Since this VLF wave model can be directly used together with existing hiss and lightning‐generated wave models in radiation belt simulation codes, we perform numerical calculations of the corresponding quasi‐linear pitch angle diffusion rates, allowing us to demonstrate the crucial role played by VLF waves from transmitters in energetic electron loss at L < 2.5.

Corotating drift-bounce resonance of plasmaspheric electron with poloidal ULF waves

The purpose of this paper is to understand how low energy plasmaspheric electrons respond to ULF waves excited by interplanetary shocks impinging on magnetosphere. It is found that both energy and pitch angle dispersed plasmaspheric electrons with energy of a few eV to tens of eV can be generated simultaneously by the interplanetary shock. The subsequent period of successive dispersion signatures is around 40 s and is consistent with the ULF wave period (third harmonic). By tracing back the energy and pitch angle dispersion signatures, the position of the electron injection region is found to be off-equator at around –32° in the southern hemisphere. This can be explained as the result of injected electrons being accelerated by higher harmonic ULF waves (e.g. third harmonic) which carry a larger amplitude electric field off-equator. The dispersion signatures are due to the flux modulations (or accelerations) of local” plasmaspheric electrons rather than electrons from the ionosphere. With the observed wave-borne large electric field excited by the interplanetary shock impact, the kinetic energy can increase to a maximum of 23 percent in one bouncing cycle for plasmaspheric electrons satisfying the drift-bounce resonance condition by taking account of both the corotating drift and bounce motion of the local plasmaspheric electron.

Anomalous variations in the structure of the ionospheric F 2 region at geomagnetic midlatitudes of the Southern and Northern hemispheres in going from summer to winter conditions at high solar activity

The structure and dynamics of the ionosphere and plasmasphere at high solar activity under quiet geomagnetic conditions of June 2–3, 1979, and January 5–6, 1980, over Millstone Hill station and Argentine Islands ionosonde, the locations of which are approximately magnetically conjugate, have been theoretically calculated. The plasma drift velocity, determined by comparing the calculated and measured heights of the F2 layer maximum (hmF2), and the correction of [N2] and [O2], found in the NRLMSISE-00 model, make it possible to coordinate the electron densities (NmF2) calculated at the hmF2 height and the measured anomalous variations in NmF2 over the Argentine Islands ionosonde as well as the calculated and measured NmF2 and electron temperature at the hmF2 height over Millstone Hill station. It has been shown that, if the interference of the diffusion velocities of O+(4S) and H+ ions is taken into account, the additional heating of plasmaspheric electrons leads to an increase in the flux of O+(4S) ions from the topside ionosphere to lower F2 layer altitudes, as a result of which an anomalous nighttime increase in NmF2 6, observed on January 6, 1980, over Millstone Hill station, is mainly produced. The second component of the formation of anomalous night-time NmF2 is the plasma drift along the magnetic field caused by the neutral wind, which shifts O+(4S) ions to higher altitudes where the recombination rate of O+(4S) with N2 and O2 is lower and slows down a decrease in NmF2 in the course of time. It has been shown that the influence of electronically excited O+ ions and vibrationally excited N2 and O2 molecules on electron density (Ne) considerably differs under winter and summer conditions. This difference forms significant part of the winter anomaly in Ne at heights of the F2 region and topside ionosphere over Millstone Hill station.

Reply to comment by R. M. Thorne and R. B. Horne on Khazanov et al. [2002] and Khazanov et al. [2006

[1] It is well-known that the effects of electromagnetic ion cyclotron (EMIC) waves on ring current (RC) ion and radiation belt (RB) electron dynamics strongly depend on such particle/wave characteristics as the phase-space distribution function, frequency, wave-normal angle, wave energy, and the form of wave spectral energy density. The consequence is that accurate modeling of EMIC wave spatialtemporal-spectral distributions and RC particles requires robust inclusion of the interdependent dynamics of wave growth/damping and wave propagation/refraction/reflection, along with wave tunneling and mode conversion and particles. Such a self-consistent RC-EMIC wave model is being progressively developed by Khazanov et al. [2002, 2003a, 2006, 2007]. This model is based on a system of coupled kinetic equations for the RC and EMIC wave power spectral density with explicit inclusion of the ray tracing equations. [2] The theoretical formalism for RC ions and RB electrons is well established and is based on gyroaveraged and bounce-averaged kinetic equations that have been developed over the years and systemized in the book by Khazanov [1979]. The application of this formalism to RC ions was continued by Khazanov and Kozyra at the University of Michigan during 1991–1994 and formed a large part of the Ph.D. dissertation work of Fok [1993] and of Jordanova [1995]. Khazanov et al. also applied this formalism to the study of ionosphere-plasmasphere transport of suprathermal electrons [Khazanov et al., 1992, 1994], and global photo [Khazanov et al., 1996; Khazanov and Liemohn, 2002] and plasma sheet [Khazanov et al., 1998] electron transport. This formalism was also generalized to study relativistic electron transport [Khazanov et al., 1999, 2000], as well as different aspects of RC and RB electron formation using various magnetospheric electric and magnetic field topologies [Khazanov et al., 2003b, 2004b, 2004c]. All these above-mentioned studies are the heritage of our RC-EMIC wave model that was presented by Khazanov et al. [2006]. [3] Thorne and Horne [2007, hereinafter referred to as TH2007] call the Khazanov et al. [2002, 2006] results into question in their Comment. The points in contention can be summarized as follows. TH2007 claim that (1) important damping of waves by thermal heavy ions is treated incorrectly in our model, and Landau damping during resonant interaction with thermal electrons is not included; (2) EMIC wave damping due to RC O is not included in our simulation of the 2–7 May 1998 storm; (3) nonlinear processes limiting EMIC wave amplitude are not included in our model; (4) growth of the background electromagnetic fluctuations to a physically significant amplitude must ‘‘occur during a single transit of the unstable region’’ with subsequent damping in the vicinity of the bi-ion latitude, and consequently the bounce-averagedwave kinetic equation employed in the code is not valid. Our reply will address each of these points as well as other criticisms mentioned in the Comment. [4] TH2007 is focused on two of our papers that are separated by 4 years. Significant progress in the selfconsistent treatment of the RC-EMIC wave system has been achieved during those years. The paper by Khazanov et al. [2006] presents the latest version of our model, and in this Reply we refer mostly to this paper.

Self‐consistent model of magnetospheric ring current and propagating electromagnetic ion cyclotron waves: 2. Wave‐induced ring current precipitation and thermal electron heating

This paper continues presentation and discussion of the results from our new global self‐consistent theoretical model of interacting ring current ions and propagating electromagnetic ion cyclotron waves (Khazanov et al., 2006) currently developing in NASA Marshall Space Flight Center. To study the effects of electromagnetic ion cyclotron wave propagation and refraction on the wave induced ring current precipitation and heating of the thermal plasmaspheric electrons, we simulate the May 1998 storm. The main findings after the simulation can be summarized as follows. First, the wave induced ring current precipitation exhibits quite a lot of fine structure and is highly organized by location of the plasmapause gradient. The strongest fluxes of about 4 × 106 (cm2 s sr)−1 are observed during the main and early recovery phases of the storm. The very interesting and probably more important finding is that in a number of cases the most intense precipitating fluxes are not connected to the most intense waves in simple manner. The characteristics of the wave power spectral density distribution over the wave normal angle are extremely crucial for the effectiveness of the ring current ion scattering. Second, comparison of the global proton precipitating patterns with the results from RAM (Kozyra et al., 1997a) reveals that although we observe a qualitative agreement between the localizations of the wave induced precipitations in the models, there is no quantitative agreement between the magnitudes of the fluxes. The quantitative differences are mainly due to a qualitative difference between the characteristics of the wave power spectral density distributions over the wave normal angle in RAM and in our model. Third, the heat fluxes to plasmaspheric electrons caused by Landau resonate energy absorption from electromagnetic ion cyclotron waves are observed in the postnoon‐premidnight MLT sector and can reach the magnitude of 1011 eV/(cm2 s). The Coulomb energy degradation of the RC H+ and O+ ions maximizes at about 1011 eV/(cm2 s) and typically leads to electron energy deposition rates of about 2 × 1010 eV/(cm2 s) which are observed during two periods, 32–48 hours and 76–86 hours after 1 May, 0000 UT. The theoretically derived spatial structure of the thermal electron heating caused by interaction of the ring current with the plasmasphere is strongly supported by concurrent and conjugate plasma measurements from the plasmasphere, ring current, and topside ionosphere (Gurgiolo et al., 2005). Finally, the wave induced intense electron heating has a structure of the spot‐like patches along the most enhanced density gradients in the plasmasphere boundary layer and can be a possible driver of the observed but still not explained small‐scale structures of enhanced emissions in the stable auroral red arcs.

Self‐consistent model of magnetospheric ring current and propagating electromagnetic ion cyclotron waves: Waves in multi‐ion magnetosphere

The further development of a self‐consistent theoretical model of interacting ring current ions and electromagnetic ion cyclotron waves (Khazanov et al., 2003) is presented. In order to adequately take into account wave propagation and refraction in a multi‐ion magnetosphere, we explicitly include the ray tracing equations in our previous self‐consistent model and use the general form of the wave kinetic equation. This is a major new feature of the present model and, to the best of our knowledge, the ray tracing equations for the first time are explicitly employed on a global magnetospheric scale in order to self‐consistently simulate the spatial, temporal, and spectral evolution of the ring current and of electromagnetic ion cyclotron waves. To demonstrate the effects of EMIC wave propagation and refraction on the wave energy distribution and evolution, we simulate the May 1998 storm. The main findings of our simulation can be summarized as follows. First, owing to the density gradient at the plasmapause, the net wave refraction is suppressed, and He+‐mode grows preferably at the plasmapause. This result is in total agreement with previous ray tracing studies and is very clearly found in presented B field spectrograms. Second, comparison of global wave distributions with the results from another ring current model (Kozyra et al., 1997) reveals that this new model provides more intense and more highly plasmapause‐organized wave distributions during the May 1998 storm period. Finally, it is found that He+‐mode energy distributions are not Gaussian distributions and most important that wave energy can occupy not only the region of generation, i.e., the region of small wave normal angles, but all wave normal angles, including those to near 90°. The latter is extremely crucial for energy transfer to thermal plasmaspheric electrons by resonant Landau damping and subsequent downward heat transport and excitation of stable auroral red arcs.

Overlap of the plasmasphere and ring current: Relation to subauroral ionospheric heating

The overlap of the ring current with the outer plasmasphere is thought to play a major role in storm‐time related increases in the subauroral ambient topside electron temperature. Instabilities generated within the overlap region, Coulomb collisions of plasmaspheric electrons with the ring current ions, and charge exchange are all thought to work either individually or together to generate a downward heat flux into the ionosphere to produce the increase in temperature. Analysis of IMAGE two‐dimensional ring current and plasmasphere density maps together with in situ DMSP ambient electron temperature data shows that the heating generally occurs over a small radial extent within the plasmasphere/ring current overlap region and may be significantly earthward of the plasmapause. This argues against collisional heat conduction as the source of the heat flux and for an instability‐based process. The finding that the heating occurs within well‐defined ratios of the cold plasmasphere to hot ring current density strengthens this supposition.

Simulating plasmaspheric field‐aligned density profiles measured with IMAGE/RPI: Effects of plasmasphere refilling and ion heating

The radio plasma imager (RPI) on board the IMAGE satellite measures electron density profiles along magnetic lines through radio sounding techniques. We simulate three density profiles measured by the RPI on 8 June 2001 using the field line interhemispheric plasma (FLIP) model and compare the modeled density profiles with observed profiles. While careful consideration of depletion of flux tubes followed by refilling over several days allows fair agreement between simulated and measured densities around the equator, the simulated densities in the off‐equator regions remain significantly lower than those measured for all simulated flux tubes. It is shown that direct heating of ions in the plasmasphere during the plasmasphere refilling, in addition to heating through Coulomb collisions with hotter plasmaspheric electrons, can improve the agreement in the off‐equator regions. The enhanced ion pressure in the high altitude part of magnetic field line due to the ion heating impedes the ionospheric inflow from both hemispheres toward the equator, giving rise to decreased transport of ionospheric ions into the equatorial region and thus increased density gradient along the magnetic field line.

Ring current activity during the early Bz < 0 phase of the January 1997 magnetic cloud

The passage at Earth of the January 10–11, 1997, magnetic cloud induced a storm of moderate geomagnetic activity with Dst index reaching minimum values of about −83 nT. We study ring current formation during the early Bz negative phase of this magnetic cloud, using energetic particle data from three instruments on the Polar spacecraft and geosynchronous plasma data from the LANL spacecraft. We use our kinetic drift‐loss model to simulate the evolution of ring current H+, He+, and O+ ion distributions and associated aeronomical effects during this period. The results from two Volland‐Stern type magnetospheric electric field model formulations are compared: (1) Kp‐dependent and (2) interplanetary magnetic field (IMF) dependent. We demonstrate that while both electric field models reproduce well the main trends of ring current formation and decay during the storm, the IMF‐dependent model reproduces the rapidity of the main storm growth phase and its strength better. Comparing model results during the main phase of the storm with HYDRA, TIMAS, and CAMMICE data we find that the model reproduces very well the ring current distributions near dawn. The formation of the nose event, i.e., the rise of the 10–30 keV energy particles near dusk due to abruptly increased convection is, however, overestimated by the model. We compute plasmaspheric heating through Coulomb collisions as the storm evolves and find that maximum heating occurs initially on the nightside near L∼3.5 and subsequently moves earthward to L∼2.75, in agreement with Millstone Hill radar observations of midlatitude electron temperature enhancement on January 10. However, the magnitude of the energy transferred to plasmaspheric electrons through Coulomb collisions appears to be not sufficient to yield the observed elevated electron temperature at ∼0830 UT, suggesting that additional energy sources should be considered during this event.

Correlations between SAR arc intensity and solar and geomagnetic activity

Abstract. We present a study of statistical relationships between SAR arc intensities acquired by the Pacific Northwest Laboratory Photometer Network during 1978-1988 and solar and geomagnetic activity indices Dst, F10.7, and Kp by use of the method of multiple regression analysis. We found significant correlations between intensity and all of the indices involved. In the present work we show for the first time that the partial correlation coefficients depend on the time offset, t, between the time of SAR arc intensity observations and the onset of the geomagnetic storm recovery phase, with the largest correlations being observed when 8 h < t < 16 h. It is also shown that there are significant differences between partial correlation coefficients calculated for SAR arcs associated with strong (Dstmin< -100 nT) and weak (Dstmin>-100 nT) geomagnetic storms. We observe also that the multiple correlation coefficients for strong storms are much larger than for weak ones. We found that the variations in the electron temperature, Te, in the SAR arc region are not mainly produced by variations in the electron density of the ionosphere but are strongly driven by the additional heating of the electron gas due to an interaction of the ring current ions and the plasmaspheric electrons. As a result, variations of Te in the SAR arc region with characteristic time scales from several minutes to several hours are stipulated by time variations of ring current parameters.Key words. Atmospheric composition and structure (airglow and aurora) · Ionosphere (ionosphere · atmosphere interactions; ionosphere · magnetosphere interactions)

Angular distributions of suprathermal electrons observed at geosynchronous orbit

Six months of low‐energy electron plasma data collected using the Los Alamos National Laboratory magnetospheric plasma analyzer (MPA) onboard the geosynchronous satellite 1989–046 have been surveyed. The MPA instrument measures the three‐dimensional energy per unit charge distributions of cold ions and electrons, allowing for the simultaneous determination of the angular distribution and ambient plasma regime. Suprathermal electrons in the energy range 15 to 200 eV were characterized by local time of occurrence, angular distribution, and ambient plasma regime. Results indicate a local time dependence in angular distributions, with trapped distributions, i.e., enhanced fluxes of particles with pitch angles near 90° and 270°, primarily being observed in the morning, coincident trapped and field‐aligned angular distributions (enhanced fluxes of particles with pitch angles near 90° and 270° coincident with enhanced fluxes of particles with pitch angles near 0° and 180°) occurring around noon, a lack of detectable low‐energy electron fluxes near dusk, and a complex combination of angular distributions on the nightside. When both trapped and field‐aligned angular distributions are present, the field‐aligned component generally has lower energy than the trapped component. Dayside field‐aligned angular distributions are interpreted as being photoelectrons from the ionosphere, while trapped angular distributions are from the low‐energy tail of the plasma sheet distribution. A plasma regime dependence in angular distributions was also observed. Plasma sheet angular distributions are generally isotropic or trapped. The plasma trough angular distributions are trapped or coincident trapped and field‐aligned. Plasmaspheric electrons commonly have energies below our 15 eV threshold.

Correlations between SAR arc intensity and solar and geomagnetic activity

We present a study of statistical relationships between SAR arc intensities acquired by the Pacific Northwest Laboratory Photometer Network during 1978-1988 and solar and geomagnetic activity indices Dst, F10.7, and Kp by use of the method of multiple regression analysis. We found significant correlations between intensity and all of the indices involved. In the present work we show for the first time that the partial correlation coefficients depend on the time offset, t, between the time of SAR arc intensity observations and the onset of the geomagnetic storm recovery phase, with the largest correlations being observed when 8 h < t < 16 h. It is also shown that there are significant differences between partial correlation coefficients calculated for SAR arcs associated with strong (Dstmin< -100 nT) and weak (Dstmin>-100 nT) geomagnetic storms. We observe also that the multiple correlation coefficients for strong storms are much larger than for weak ones. We found that the variations in the electron temperature, Te, in the SAR arc region are not mainly produced by variations in the electron density of the ionosphere but are strongly driven by the additional heating of the electron gas due to an interaction of the ring current ions and the plasmaspheric electrons. As a result, variations of Te in the SAR arc region with characteristic time scales from several minutes to several hours are stipulated by time variations of ring current parameters.Key words. Atmospheric composition and structure (airglow and aurora) · Ionosphere (ionosphere · atmosphere interactions; ionosphere · magnetosphere interactions)

Decay of equatorial ring current ions and associated aeronomical consequences

The decay of the major ion species which constitute the ring current is studied by solving the time evolution of their distribution functions during the recovery phase of a moderate geomagnetic storm. In this work, only equatorially mirroring particles are considered. Particles are assumed to move subject to E×B and gradient drifts. They also experience losses along their drift paths. Two loss mechanisms are considered: charge exchange with neutral hydrogen atoms and Coulomb collisions with thermal plasma in the plasmasphere. Thermal plasma densities are calculated with a plasmaspheric model employing a time‐dependent convection electric field model. The drift‐loss model successfully reproduces a number of important and observable features in the distribution function. Charge exchange is found to be the major loss mechanism for the ring current ions; however the important effects of Coulomb collisions on both the ring current and thermal populations are also presented. The model predicts the formation of a low‐energy (&lt; 500 eV) ion population as a result of energy degradation caused by Coulomb collisions of the ring current ions with the plasmaspheric electrons; this population may be one source of the low‐energy ions observed during active and quiet periods in the inner magnetosphere. The energy transferred to plasmaspheric electrons through Coulomb collisions with ring current ions is believed to be the energy source for the electron temperature enhancement and the associated 6300 Å (stable auroral red [SAR] arc) emission in the subauroral region. The calculated energy‐deposition rate is sufficient to produce a subauroral electron temperature enhancement and SAR arc emissions that are consistent with observations of these quantities during moderate magnetic activity levels.

The role of ring current nose events in producing stable auroral red arc intensifications during the main phase: Observations during the September 19‐24, 1984, Equinox Transition Study

An examination of stable auroral red (SAR) arc emissions over the last solar cycle (Slater and Kleckner, 1989) indicates that the strongest emissions, during the lifetime of a particular SAR arc, often occur in association with the main phase of the magnetic storm. Previous observations of thermal and energetic particle populations at high and low altitudes on SAR arc field lines by Kozyra et al. (1987a) indicate that the energy source for these emissions is the O+ in the ring current. The O+ content of the ring current increases with increasing magnetic activity reaching its maximum percentage contribution near minimum Dst for a particular storm. This variation in the O+ content of the ring current is inconsistent with an early main phase enhancement of SAR arc emissions. To investigate the source of main phase enhancements in SAR arc emissions, a study of the September 19‐24, 1984, magnetic storm period during which SAR arc emissions were observed by the ground‐based mobile automatic scanning photometer network in both the main and recovery phases is presented. The emissions associated with the main phase (∼ 400 R) were an order of magnitude greater than those associated with the recovery phase (tens of R). Energetic particle measurements from the DE 1 and AMPTH spacecraft, on field lines that map to the SAR arc position at low altitude, were examined to determine if differences in the energy sources during these time periods were evident. In agreement with previous work, ring current O+ supplied the bulk of the electron heating during storm recovery phase as a result of Coulomb collisions of O+ with the plasmaspheric electrons; contributions by ring current H+ were negligible. A new result of the present work is that an enhancement of the 15‐25 keV H+ component of the ring current during the main phase of the September 19 magnetic storm was responsible for an approximately one order of magnitude increase in the electron heating rate and SAR arc emissions during the main phase compared to the recovery phase. The increase in the H+ flux occurred in association with a ring current “nose event”, a front of ions injected into the inner magnetosphere in response to a discontinuous change in the cross‐tail electric field. The association between nose events and intensifications of SAR arc emissions in the main phase has not previously been explored but is a natural consequence of the injection of significant fluxes of relatively low‐energy ring current ions earthward of the plasmapause during early storm time ring current formation.

Spectrometric and photometric observations of low‐latitude aurorae

The properties of a low‐latitude aurora of September 22, 1982, observed with photometers and a spectrograph from McDonald Observatory, Texas, and an all‐sky mapping photometer from a site near Boulder, Colorado, are discussed. The dominant emission was [OI] 630 nm with a 630/558 nm emission ratio of 3.8. Other weaker emissions were present that are characteristic of heavy particle aurorae and can be attributed to energetic (keV) ions and neutral atoms from the ring current. The properties of this and other low‐latitude aurorae are discussed in the context of stable auroral red (SAR) arcs and type d aurorae, and of energetic ion and neutral aurorae. The emissions associated with heavy particle precipitation are characteristic of main phase conditions. The observed equatorial movement of main phase low‐latitude red aurorae is consistent with the inward convection of ring current particles and the inward movement of the plasmapause. Previous work has hypothesized that coulomb collisions of plasmaspheric electrons with ring current O+ cause the energization of the electrons exciting SAR arcs. Calculations of [OI] 630 and 558 nm emissions show that for red aurorae with 630/558 nm ratios ∼4, additional energization processes producing electrons in the energy range 10–10,000 eV are required. We propose some clarification of nomenclature for several types of aurorae occurring at low latitudes.