Upward Field-aligned Currents Research Articles

Near-Earth plasma sheet azimuthal pressure gradient and associated auroral development soon before substorm onset

[1] The azimuthal plasma pressure gradient in the near-Earth plasma sheet makes crucial contributions to field-aligned current (FAC) formation. Numerical simulations and statistical observations have shown that a plasma pressure peak tends to build up in the premidnight region of the near-Earth plasma sheet during the substorm growth phase owing to enhanced magnetic drift. This leads to azimuthal pressure gradients in this region. The temporal variation of the azimuthal pressure gradient may provide an indication for the FAC variations associated with the substorm growth phase and may set up a plasma sheet precondition for the substorm onset being triggered near this region. We take advantage of two of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft separated azimuthally near the orbit apogee and investigate the azimuthal plasma pressure gradient before substorm onset in the R ∼10–12 RE region. Equatorial plasma pressure is estimated by removing the curvature force effect. Five events with the spacecraft footprints mapped very close to the aurora onset region were selected. These events show substantial duskward pressure gradient enhancement 1–5 min before onset. The onset arc, which results from enhanced energetic electron precipitation, was found to intensify simultaneously with the pressure gradient enhancement before onset breakup occurs. Since the energy and energy flux of precipitating electrons reflect the upward FAC strength, these results indicate that the duskward azimuthal pressure gradient enhancement is associated with enhanced upward FAC during the late growth phase and leads to the intensification of the onset auroral arc soon before it breaks up. It is possible that this pressure gradient enhancement may lead to ballooning mode instability and thus substorm onset along the intensifying arc.

Auroral electron distributions within and close to the Saturn kilometric radiation source region

On 17 October 2008, Cassini observed for the first time the electron populations associated with the crossing of a Saturn kilometric radiation source region and its surroundings. These observations allow for the first time the constraint and quantification of the high-latitude acceleration processes, the current systems, and the origin of the low-frequency electromagnetic waves. Enhanced fluxes of field-aligned energetic electrons were measured by the Cassini electron plasma spectrometer in conjunction with unusual intense field-aligned current systems identified using the magnetometer instrument. In the region where downward field-aligned currents were measured, electron data show evidence of two types of upward accelerated electron beams: a broadband energetic (1-100 keV) electron population that is observed throughout the region and a narrow-banded (0.1-1 keV) electron population that is observed sporadically. In the regions where the magnetic field signatures showed evidence for upward field-aligned currents, we observe electron loss cone distributions and some evidence of shell-like distributions. Such nonthermal electron populations are commonly known as a potential free energy source to drive plasma instabilities. In the downward current region, the low-energy and energetic beams are likely the source of the very low frequency emissions. In the upward current region, the shell distribution is identified as a potential source for Saturn kilometric radiation generation via the cyclotron maser instability.

Six auroral generators: A review

[1] The paper reviews generator processes and configurations of six types of auroral arcs: embedded arcs, Alfvenic arcs, onset arcs, poleward arcs during substorms, auroral streamers, and auroral spirals. The arcs and generators are elements in global current systems, which are classified as Types I and II after Bostrom. The arcs may be dominated by transient processes or be quasi-stationary. The main emphasis of the paper is on the generator forces. They are pressure gradient forces, magnetic normal or shear stresses, or inertial forces. For three cases, the arcs embedded in the oval convection, the poleward arc during substorms, and the auroral streamers, simple expressions are presented of the currents injected into the ionosphere by the generator process, allowing quantitative evaluations. The relations leading from these currents to other key auroral quantities are summarized. The apparent conflict between the widths of Alfvenic arcs and the transverse scales required for energy coupling to the topside ionospheric plasma is solved by assuming current and field filamentation by multiple reflections in the ionospheric Alfven resonator. The substorm generator is described as a high-beta plasma layer arising from collapse of the tail current sheet. An essential element in this process is the shedding of excess flux tube entropy through energy dumping in the auroral acceleration process and ionospheric dissipation. The dynamics of this process needs further investigation. The physics of the connection between flow bursts in the tail plasma sheet and the flows associated with auroral streamers in the ionosphere is discussed. Regular auroral spirals and the westward traveling surge have in common a concentration of upward field-aligned current which demands strongly enhanced dissipation. They differ by the processes creating the upward current concentration.

Nightside field-aligned current during the April 6, 2000 superstorm

Using the east-west geomagnetic disturbance fields observed from stations at mid-latitudes, we investigate the characteristics of field-aligned currents on the nightside during the April 6, 2000 superstorm. The results indicate that there is an eastward disturbance on the nightside of Northern Hemisphere during the main phase of magnetic storm, while the interplanetary magnetic field (IMF) is continued southward. The positive disturbances in the postmidnight are larger than that in the premidnight. This suggests that upward field-aligned currents develop on the nightside, when the IMF is directed southward. The peak of upward field-aligned currents is located in postmidnight, and it will be more obvious at substorm expansion. We get the conclusion that both partial ring current and region-2 field-aligned current shift to dusk sector. The upward region-2 current is decreased at substorm onset, and intensified after it. We also investigate the relationship between the upward field-aligned currents on the nightside and the auroral electrojets at high latitudes. It shows a good correspondence between region-2 current and the westward electrojet in the postmidnight. We suppose that they are both associated with the convection electric field.

The Alfvén resonator revisited

Abstract. Two models for a magnetosphere-ionosphere coupling feedback instability in the lower magnetosphere are studied. In both models the instability arises because of the generation of an Alfvén wave from growing arc-like structures in the ionospheric conductivity. The first model is based on the modulation of precipitating electrons by field-aligned currents of the upward moving Alfvén wave (Modulation Model). The second model takes into consideration the reflection of the Alfvén wave from a maximum of the Alfvén velocity at about 3000 km altitude (Reflection Model). The growth of structures in both models takes place when the ionization function associated with upward field aligned current is shifted from the edges of enhanced conductivity structures to their centers. Such a shift arises because the structures move along the ionosphere at a velocity different from the E×B drift velocity. As a result, field-aligned currents of upward propagating Alfvén wave at some altitude appear shifted with respect to the edges of the structures. Although both models may work, the growth rate for the first model, as based on the modulation of the precipitating accelerated electrons, for typical conditions, may be tens or more times larger than that for the second model based on the Alfvén wave reflection. The proposed models can provide the growth of both single and periodic structures. When applied to auroral arc generation the studied instability leads to high growth rates and narrow arcs. The physical mechanism is mostly suitable for the generation of auroral arcs with widths of the order of 1 km and less. The growth rate of the instability for such structures can be as large as 0.3 s−1. In the case of periodic structures, their motion must lead to the generation of magnetic pulsations with periods of about 1–6 s, which is close to the expected period of Alfvén resonant oscillations in the lower magnetosphere. However, these oscillations (for the first and most effective model MM) are not exactly Alfvén resonant oscillations. These oscillations are modulations in the ionospheric density, which propagate along the ionospheric currents and not along the magnetic field lines.

Nightside ionospheric electrodynamics associated with substorms: PFISR and THEMIS ASI observations

Observations from the newly available Poker Flat Incoherent Scatter Radar have been used to study the ionospheric electrodynamics during the substorm expansion phase. Substorm events selected have been divided into three categories based on onset location relative to the radar, and repeatable features have been identified. West of onset, westward flows increased in response to the passage of the westward traveling surge (WTS) poleward of them. The enhanced flows occurred within the Sub‐Auroral Polarization Streams region of downward Region 2 currents, which extend equatorward from near the equatorward boundary of electron precipitation. Strong E region ionization increase associated with the WTS occurred poleward of the enhanced westward flows, suggesting that poleward Pedersen currents fed by the downward Region 2 field‐aligned current (FAC) further equatorward contribute at least partially to closure of the upward FAC of the WTS. An enhanced eastward flow channel developed east of onset. E region ionization decreased poleward of this flow peak, implying formation of downward FAC, and further increased equatorward of the flow peak, suggesting strengthening of preexisting Region 2 upward FAC. These observations indicate that the downward FAC formed at onset at least partially closes via the upward Region 2 currents. The enhanced eastward flows decreased after passage of the auroral bulge, and the reduced E region ionization increased again. When onset occurred within the radar fov, the thin breakup arc of onset occurred at the center of the Harang reversal, while if the optical onset was poleward of the radar fov, the Harang reversal was observed after the onset. A 2‐D picture of the evolution of ionospheric electrodynamics was generated by synthesizing observations from the three categories, which provides a schematic description of the relationship between Region 2 and substorm expansion electrodynamics.

Formation of the Harang reversal and its dependence on plasma sheet conditions: Rice convection model simulations

The goal of this paper is to understand the formation of the Harang reversal and its association with the region 2 field‐aligned current (FAC) system, which couples the plasma sheet transport to the ionosphere. We have run simulations with the Rice convection model (RCM) using the Tsyganenko 96 magnetic field model and realistic plasma sheet particle boundary conditions on the basis of Geotail observations. Our results show that the existence of an overlap in magnetic local time (MLT) of the region 2 upward and downward FAC is necessary for the formation of the Harang reversal. In the overlap region the downward FAC, which is located at lower latitudes, is associated with low‐energy ions that penetrate closer to Earth toward the dawn side, while the upward FAC, which is located at higher latitudes, is associated with high‐energy ions. Under the same enhanced convection we compare the Harang reversal resulting from a hotter and more tenuous plasma sheet with the one resulting from a colder and denser plasma sheet. For the former case the shielding of the convection electric field is less efficient than for the latter case, allowing low‐energy protons to penetrate further earthward, resulting in a Harang reversal that extends to lower latitudes, expands wider in MLT, and is located further equatorward than the upward FAC peak and the conductivity peak. The return flows of the Harang reversal in the hot and tenuous case are located in a low conductivity region. This leads to an enhancement of these westward flows, resulting in subauroral polarization streams (SAPS).

Simultaneous ground-satellite optical observations of postnoon shock aurora in the Southern Hemisphere

[1] On 14 June 2005, a transient postnoon shock aurora, induced by an interplanetary (IP) shock, was observed simultaneously with the FUV imager onboard the IMAGE satellite and the all-sky imager (ASI) at the South Pole Station (−74.3° magnetic latitude (MLAT), ∼15 magnetic local time). The global evolution of the shock aurora was identified by the FUV, whereas the detailed spatial-temporal structure was identified by the ASI. Both optical emissions for the shock aurora showed a reasonable agreement in a common field of view. During the transient auroral brightenings in the dayside oval detected by the FUV, a two-step development of the shock aurora was identified by the ASI in the afternoon sector of the oval. Just after the IP shock, the ASI first observed a diffuse 557.7 nm aurora expanding duskward at the equatorward edge of the oval (−70° to −73° MLAT). About 5 min later, new discrete auroral forms (arcs) were detected in the middle of the oval (−73° to −76° MLAT) by both 557.7 nm and 630.0 nm ASI images. The discrete arcs developed with relatively brighter emissions, and had a lifetime of the order of 10 min, during the main impulse (MI) of geomagnetic sudden commencement. The spectrographic auroral imagers onboard the IMAGE satellite indicated that proton precipitation played an effective role in the first shock auroral emission observed with the ASI, while intense electron precipitation played an effective role in the second one. Mechanisms of the first and second postnoon shock auroras presented here are speculated to be associated with the wave-particle interaction process, and the field-aligned acceleration process in the region of the MI-related upward field-aligned currents, respectively.

Field-aligned currents of postnoon auroral arcs

[1] On 9 January 2008, postnoon (at about 15–16 magnetic local time [MLT]) poleward moving auroral arcs were observed over Svalbard during slightly negative interplanetary magnetic field (IMF) Bz and the IMF By gradually changing from near zero to about −3 nT. The arcs appeared from the western (dayside) horizon and developed toward the east coming to the postnoon sector. The steerable 32-m antenna of the European Incoherent Scatter (EISCAT) radar was directed at an elevation angle of 45° to the north-west at about 45° to geomagnetic west, allowing for measurement of azimuthal (along east-west) plasma flow in a range of latitudes. On the basis of this radar configuration, a new method has been developed and applied for calculating the field-aligned currents (FACs) in the vicinity of the arcs. The arcs were located in the region of the postnoon convection reversal, and the background upward FAC had a density of about 1 μA/m2. The optical arcs were of the order of 30–40 km wide and coincided with peaks of upward FAC. The return downward FAC and the southward electric field were observed poleward of the arcs. Equatorward of the arcs, secondary weaker arcs were detected so that the postnoon arcs appear to have a double structure. These features are surprisingly similar to the electric field and current patterns of morning Sun-aligned arcs. We suggest that the double arc structure can be explained by the nonlinear ionospheric feedback that results in splitting of the elongated magnetospheric plasma structures associated with the arcs.

Auroral arc and oval electrodynamics in the Harang region

Auroral arcs are typically described in terms of an upward field‐aligned current (FAC) sheet above the arc, connected by ionospheric Pedersen current to a downward FAC sheet near the arc. On the basis of data measured by the FAST spacecraft, conjugate with ground optical observations, we present first a wide and stable winter evening arc, where this standard model does not apply. The arc is observed in the Harang region during the growth phase of a modest substorm, poleward of the convection reversal (CR) boundary. Although the magnetic field data suggest the typical configuration, the two FAC sheets appear to be decoupled near the satellite footprint: the upward FAC is fed by the westward electrojet (WEJ), while the downward FAC feeds the eastward electrojet (EEJ). The examination of the arc by the newly developed ALADYN technique confirms this peculiar current topology. For comparison, we apply ALADYN also to a second evening arc, located within the Harang region equatorward from the CR. The arc is confirmed to have the standard configuration, consistent with a former study, but substantial FAC‐EJ coupling is inferred in the auroral oval both poleward and equatorward of the arc. A key element for the topology of the current closure is the westward component of the electric field, which influences the relative location of the CR with respect to the large‐scale FAC reversal (FR) boundary. As proved by tests on synthetic data, a westward component of the electric field pushes the CR toward the FR, preventing thus the standard FAC closure, while the conductance and FAC pattern shape the CR profile. Since a westward electric field is often measured in the Harang region, the FAC‐EJ coupling is expected to be an essential ingredient there. This has important implications for the current closure in the equatorial magnetosphere and for the auroral current circuit in the WEJ region, closely related to the substorm process.

Statistical study of polar distribution of mesoscale field-aligned currents

[1] Field-aligned currents (FACs) play an important role in the magnetosphere-ionosphere coupling system for transporting electromagnetic energy. Although large-scale region 1 (R1) and region 2 (R2) FACs have been statistically investigated for a few decades, there have been relatively fewer investigations of the fundamental characteristics of mesoscale (ranging from about 10 to 100 km) FACs and the relationship between them and large-scale FACs. Here we report some fundamental characteristics of mesoscale FACs on the basis of a statistical analysis using long-term Akebono satellite data from March 1989 to December 1996. We found that the current density of mesoscale FACs with a spatial scale of 10–80 km at ionospheric altitude is on average the largest in the dayside region of 75°–82° in corrected geomagnetic latitude and 0600–1500 in corrected geomagnetic local time. The current density is especially enhanced under the sunlit condition in summer. The intensities and distributions of mesoscale FACs change with interplanetary magnetic field conditions. We also found altitude dependence of mesoscale FACs for the first time. Current densities of mesoscale FACs gradually increase with the altitude. Mesoscale FACs tend to distribute in the R1 current region, and the current density in the morning to noon sector (downward R1 FAC region) is stronger than that in the noon to evening sector (upward R1 FAC region). Further, the seasonal variations of mesoscale FACs correspond to those of large-scale FACs on the dayside but areopposite to those of large-scale FACs on the nightside.

Modeling magnetospheric current response to solar wind dynamic pressure enhancements during magnetic storms: 1. Methodology and results of the 25 September 1998 peak main phase case

We present a methodology for using the modular Tsyganenko storm magnetic field model (TS04) as a tool to investigate the response of magnetospheric currents to solar wind dynamic pressure enhancements during magnetic storms. We demonstrate the technique by examining the contribution of each model current to the observed dawn‐dusk asymmetric ground H perturbation during a peak storm main phase event. We add present pressure terms to the parameterizations of several individual model currents and fit them to the observed low‐latitude and midlatitude H perturbations. We find that the asymmetric H perturbation for this case is primarily due to a net field‐aligned current (FAC) system, with upward FACs peaking in the evening and downward FACs peaking in the morning, and the equatorial portion of the partial ring current (PRC). This net FAC system includes the region 1 and region 2 FACs and the closure FAC of the PRC, with the largest contribution from the PRC closure FACs. The model results show that in this main phase case, the PRC played a more important role in causing the dawn‐dusk asymmetric H perturbation than did the other currents owing to its strength and asymmetry. The model results do not show significant contributions from the symmetric ring current and tail current, but the magnetopause current gives a significant positive perturbation at all MLTs. In the companion paper, our methodology is applied to obtain a more complete picture of the storm time current response to pressure enhancements by examining pressure increases occurring during different storm phases.

Separation of Cowling channel and local closure currents in the vicinity of a substorm breakup spiral

The substorm breakup spiral demarcates the westernmost terminator of the substorm current wedge. A long‐standing debate has been devoted to the question of to what extent the strong upward field‐aligned currents (FAC) in that spiral are either closed by downward FAC through Pedersen currents flowing radially to the center of the spiral (“local closure”) or by currents that flow westward though a Cowling channel which extends in the region eastward of the spiral (“Cowling closure”). In particular, studies using ground‐based and space‐based instrumentation came to systematically different results. In order to address this question, we reanalyze a pseudobreakup spiral event on 3 February 1999, ∼2204 UT, that has been observed with the MIRACLE network of ground‐based instruments in Fennoscandia and for which spatial results of electric fields, conductances, and currents have been calculated by Amm et al. (2001). We apply a new technique that allows, under certain geometrical assumptions, the current systems to separate in the vicinity of the breakup spiral into their local closure and Cowling closure parts. The result shows that for the event analyzed, ∼68% of the upward FAC in the spiral is closed via the local closure current system and the remaining ∼32% via the Cowling closure current system. Therefore, the local closure mechanism is the dominating provider of the upward FAC in the breakup spiral overall. However, a localized area at the western flank of the spiral, where conductance gradients are largest, exists in which the Cowling closure dominates and feeds more than 80% of the upward FAC in that area. Further studies are required in order to examine the generality of the results obtained for our specific event.

Ionospheric feedback effects on the quasi-stationary coupling between LLBL and postnoon/evening discrete auroral arcs

Abstract. We discuss a model for the quasi-stationary coupling between magnetospheric sheared flows in the dusk sector and discrete auroral arcs, previously analyzed for the case of a uniform height-integrated Pedersen conductivity (ΣP). Here we introduce an ionospheric feedback as the variation of ΣP with the energy flux of precipitating magnetospheric electrons (εem). One key-component of the model is the kinetic description of the interface between the duskward LLBL and the plasma sheet that gives the profile of Φm, the magnetospheric electrostatic potential. The velocity shear in the dusk LLBL plays the role of a generator for the auroral circuit closing through Pedersen currents in the auroral ionosphere. The field-aligned current density, j||, and the energy flux of precipitating electrons are given by analytic functions of the field-aligned potential drop, ΔΦ, derived from standard kinetic models of the adiabatic motion of particles. The ionospheric electrostatic potential, Φi (and implicitely ΔΦ) is determined from the current continuity equation in the ionosphere. We obtain values of ΔΦ of the order of kilovolt and of j|| of the order of tens of μA/m2 in thin regions of the order of several kilometers at 200 km altitude. The spatial scale is significantly smaller and the peak values of ΔΦ, j|| and εem are higher than in the case of a uniform ΣP. Effects on the postnoon/evening auroral arc electrodynamics due to variations of dusk LLBL and solar wind dynamic and kinetic pressure are discussed. In thin regions (of the order of kilometer) embedding the maximum of ΔΦ we evidence a non-linear regime of the current-voltage relationship. The model predicts also that visible arcs form when the velocity shear in LLBL is above a threshold value depending on the generator and ionospheric plasma properties. Brighter arcs are obtained for increased velocity shear in the LLBL; their spatial scale remains virtually unmodified. The field-aligned potential drop tends to decrease with increasing LLBL density. For higher values of the LLBL electron temperature the model gives negative field-aligned potential drops in regions adjacent to upward field-aligned currents.

Current systems in the Jovian magnetosphere

The plasma dynamics in the Jovian magnetosphere is characterized by rapid rotation of the planet. In this paper, a new magnetohydrodynamic (MHD) simulation scheme is developed to precisely calculate the current systems in the Jovian magnetosphere and to relate them with the plasma dynamics. A field‐aligned current (FAC) pattern that is expected for the ionosphere to drive the corotation is reproduced in the present simulation result with current away from the ionosphere at lower latitudes and current toward the ionosphere at higher latitudes. In the magnetospheric region, a thin equatorial current sheet with eastward current flowing around Jupiter (ring current) dominates in the middle magnetosphere. By tracing current lines, it is found that the upward FAC on the low‐latitude side in the ionosphere is first connected to the ring current and then to the Chapman‐Ferraro current through the plasma sheet current. Downward FAC in the high‐latitude ionosphere is spirally connected to the tail lobe. From the distribution of J·E, the places of dynamo and energy consumption region are investigated to understand the energy balance associated with the corotation by magnetosphere‐ionosphere (M‐I) current system. From these analyses, the FAC, ring current, the Chapman‐Ferraro current and plasma distribution in the Jovian magnetosphere are understood as a self‐consistently organized structure through corotation and confinement. In addition, some interesting field‐line structures and plasmoid formation are found by tracing the magnetic field line.

High-latitude ionosphere convection and Birkeland current response for the 15 May 2005 magnetic storm recovery phase

The high‐latitude response of sunward E × B flow and Birkeland field‐aligned currents (FAC) is analyzed for the 15 May 2005 magnetic cloud that generated a great magnetic storm (SYM‐H = −305 nT at 0820 UT). The interplanetary magnetic field (IMF) clock angle, θ = arctan(By/Bz), gradually rotated from 65° to −80° during the 10‐h long northward IMF period and the recovery of this storm. DMSP observations confirm a dawnward migration of a Northern Hemisphere sunward E × B flow channel (FC) between a downward and upward FAC pair. This FAC system developed during southward IMF (θ = 109°) at the poleward edge of the duskside auroral oval as part of a four‐sheet FAC system 23 min before the IMF became northward. TIMED/GUVI observations show that the dawnward migration of the upward FAC coincides with a drifting transpolar auroral arc (TPA). IMAGE/WIC did not observe a TPA in the southern (winter) hemisphere. DMSP and Iridium observations are in good agreement with MHD simulation predictions of a northward IMF reorientation of high‐latitude FACs. The northern FC migration was likely due to summer hemisphere conductances, a strong average IMF Bx = −35 nT and the sunward dipole tilt angle that favor a northern high‐latitude reconnection mechanism for a well‐organized sunward FC and FAC system migration. The storm recovery rate appeared to be related with the region 2 FAC. A fast 11.4 nT/h rate was observed for a weak or nonexistent region 2 system during the high‐latitude FAC redistribution. The SYM‐H recovery slowed significantly to 0.9 nT/h following the 1800 UT region 2 system recovery.

Field-aligned currents and parallel electric field potential drops at Mars. Scaling from the Earth’ aurora

The observations of electron inverted ‘V’ structures by the MGS and MEX spacecraft, their resemblance to similar events in the auroral regions of the Earth, and the discovery of strong localized magnetic field sources of the crustal origin on Mars, raised hypotheses on the existence of Martian aurora produced by electron acceleration in parallel electric fields. Following the theory of this type of structures on Earth we perform a scaling analysis to the Martian conditions. Similar to the Earth, upward field-aligned currents necessary for the generation of parallel potential drops and peaked electron distributions can arise, for example, on the boundary between ‘closed’ and ‘open’ crustal field lines due to shears of the flow velocity of the magnetosheath or magnetospheric plasmas. A steady-state configuration assumes a closure of these currents in the Martian ionosphere. Due to much smaller magnetic fields as compared to the Earth case, the ionospheric Pedersen conductivity is much higher on Mars and auroral field tubes with parallel potential drops and relatively small cross scales to be adjusted to the scales of the localized crustal patches may appear only if the magnetosphere and ionosphere are decoupled by a zone with a strong E ∥ . Another scenario suggests a periodic short-circuit of the magnetospheric electric fields by a coupling with the conducting ionosphere.

A MHD mechanism for the generation of the meridional current system during substorm expansion phase

[1] In this study we propose a MHD mechanism for the generation of the meridional current system (MCS) during the substorm expansion phase (EP). The upward field-aligned current (FAC) intensifies as the dawn-dusk plasma pressure gradient enhances in the Harang region during the substorm EP. As a consequence of this intensification, the plasma convection pattern changes, with eastward flow accelerations observed in both the ionosphere and midmagnetotail during early substorm EP. The increasing eastward flow is associated with an inertial current in the midtail whose divergence give rise to a downward FAC poleward of the upward FAC region, constituting a MCS configuration. Theoretical calculations based upon the mechanism shows consistency with many observed features of the MCS. We further discuss how the present work relates to the large-scale FAC system and the interactions among its various components, with a particular emphasis on the MCS contribution to the overall substorm current closure.

Auroral post-secondary ions from the nightside ionosphere in the inner magnetosphere

During magnetically disturbed conditions, the auroral zone is occupied by widespread aurora mostly created by precipitating primary auroral electrons associated with upward field-aligned currents (FACs) that are highly variable in space and time. The energy flux carried by these electrons is substantial, with each kilo-Rayleigh (kR) of optical emission being associated with a flux of about 1 erg cm −2 s −1. The production of secondary ions and electrons is large, because only about 35 eV of precipitating electron energy is needed to produce a secondary pair. The auroral ionosphere, as well as the region above it, is very dynamic. Parallel electric fields can occur there, as inferred from observations of beam-like secondary ion signatures. Ions also undergo transverse acceleration of ions (TAI), likely due to wave-particle interactions. As a result, these suprathermal “auroral post-secondary (APS) ions” acquire a markedly different energy spectrum from the secondary electrons, with APS ion energies in the range between thermal (1 eV) and auroral (1 keV). The APS ions acquire enough energy to escape gravity and then execute bounce motion along the magnetic field, while simultaneously undergoing drift motion dominated by convective E ⇒ × B ⇒ drift, rather than the curvature-gradient drift of the higher energy auroral and ring-current ions. As a result, the ion drift trajectories follow magnetospheric equipotential surfaces that project to the ionosphere as flow streamlines. On the basis of their convection paths subsequent to their production and acceleration, they can be subdivided into three categories, namely Dusk APS (APS/D) and Subauroral APS (APS/S) ions in the evening convection cell, and Morning APS (APS/M) ions in the morning convection cell. Satellite observations are presented to show the unique energy spectra of the APS/D, APS/S, and APS/M ions, and their multi-energy band structure, with each band having a dispersive signature characterized by lower energy at lower latitude. Two possible effects of these ions are discussed. The first is the role of APS/S ions in the production of subauroral radar echo regions in the plasmatrough. The second is a decrease in the morning-side negative shielding charge because of the arrival of the positive APS/M ions that have bounced and convectively drifted from an auroral injection region near midnight.

Sheared magnetospheric plasma flows and discrete auroral arcs: a quasi-static coupling model

Abstract. We consider sheared flows in magnetospheric boundary layers of tangential discontinuity type, forming a structure that is embedded in a large-scale convergent perpendicular electric field. We construct a kinetic model that couples the magnetospheric structure with the topside ionosphere. The contribution of magnetospheric electrons and ionospheric electrons and ions is taken into account into the current-voltage relationship derived for an electric potential monotonically decreasing with the altitude. The solution of the current continuity equation gives the distribution of the ionospheric potential consistent with the given magnetospheric electric potential. The model shows that a sheared magnetospheric flow generates current sheets corresponding to upward field-aligned currents, field-aligned potential drops and narrow bands of precipitating energy, as in discrete auroral arcs. Higher velocity magnetospheric sheared flows have the tendency to produce brighter and slightly broader arcs. An increase in arc luminosity is also associated with enhancements of magnetospheric plasma density, in which case the structures are narrower. Finally, the model predicts that an increase of the electron temperature of the magnetospheric flowing plasma corresponds to slightly wider arcs but does not modify their luminosity.