Upward Field-aligned Currents Research Articles

Location of Geomagnetic Disturbances in Relation to the Field Aligned Current Boundary

AbstractGeomagnetic disturbances (GMDs) are rapid fluctuations in the strength and direction of the magnetic field near the surface of the Earth which can cause electric currents to be induced in the ground. The geomagnetically induced currents (GICs) can cause damage to pipelines and power grids. A detection algorithm has been developed to identify rapid changes in 10 s averaged magnetometer data. This higher resolution data is important in capturing the most rapid changes associated with extreme GIC events. The algorithm has been used on an array of ground‐based magnetometers from SuperMAG data from 2010 to 2022, creating a new list of global GMDs. Data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) is used to place the observed GMDs in the context of the global pattern of magnetosphere‐ionosphere field‐aligned currents (FACs). A dawn sector population of GMDs is found to lie near the boundary between the region 1 and region 2 FACs, while a pre‐midnight sector population is found to occur poleward of the FAC boundary on region 1 upward FACs. It is also shown that the latitude of the GMDs expands with the FAC boundary and their occurrence peaks around 77° magnetic latitude.

The Relationship Between Large dB/dt and Field‐Aligned Currents During Five Geomagnetic Storms

AbstractDuring periods of increased geomagnetic activity, perturbations within the terrestrial magnetosphere are known to induce currents within conducting materials, at the surface of Earth through rapid changes in the local magnetic field over time (dB/dt). These currents are known as geomagnetically induced currents and have potentially detrimental effects on ground based infrastructure. In this study we undertake case studies of five geomagnetic storms, analyzing a total of 19 days of 1‐s SuperMAG data in order to better understand the magnetic local time (MLT) distribution, size, and occurrence of “spikes” in dB/dt, with 131,447 spikes in dB/dt exceeding 5 nT/s identified during these intervals. These spikes were concentrated in clusters over three MLT sectors: two previously identified pre‐midnight and dawn region hot‐spots, and a third, lower‐density population centered around 12 MLT (noon). The noon spike cluster was observed to be associated with pressure pulse impacts, however, due to incomplete magnetometer station coverage, this population is not observed for all investigated storms. The magnitude of spikes in dB/dt are determined to be greatest within these three “hot‐spot” locations. These spike occurrences were then compared with field‐aligned current (FAC) data, provided by the Active Magnetospheric Planetary Electrodynamic Response Experiment. Spikes are most likely to be co‐located with upward FACs (56%) rather than downward FACs (30%) or no FACs (14%).

Magnetic Longitudinal and Local Time Variations of Polar Electrojet and Field‐Aligned Currents

AbstractIn this work, prominent longitudinal variation of polar electrojet (PEJ) and field‐aligned currents (FACs) are reported for the first time in different local times and hemispheres. Larger longitudinal differences exist mostly in the south than in the north with nearly opposite phase except for the nighttime. The most prominent longitudinal variation occurs around noontime. In the nighttime, the longitudinal changes of PEJ and upward FACs in both hemispheres are similar. The larger northern summer FACs and PEJ at noon occur more frequently around 0°–60° and 300°–360° magnetic longitude (MLon), while the larger southern summer currents occur around 120°–300° MLon. The summer‐winter hemispheric differences in PEJ and upward FACs disappear in the midnight sector. The northern FACs dominance exists around 0°–120° and 270°–360° MLon, except for winter when the northern dominance of upward FACs at night exist in nearly all longitudes. In local summer, PEJ shows northern dominance around 0°–60° and 270°–360° MLon, and southern dominance around 120°–240° MLon. In local winter, PEJ shows northern dominance around 180° MLon near midnight, while in most other local times, PEJ shows northern dominance around 270° MLon. These longitudinal, seasonal and hemispheric differences are discussed in terms of mixing effects of ionospheric conductance and merging electric field in different local time sectors.

Radial Transport of Io Plasma From the Inner Magnetosphere to the Tail

AbstractThe solar wind‐Jovian magnetosphere‐ionosphere interaction is studied from the global magnetohydrodynamic simulation. The calculation considers the high‐speed solar wind, Io plasma emission, high‐speed rotation, ionospheric ions, and precession of magnetic field, and consequently reproduces the confinement of Jovian magnetic field, distributions of O+ and H+, supply of H+ by the polar wind, the interchange instability, and the current system that maintains co‐rotation. The radial transport process of plasma generated from Io is traceable from this solution, such that the transport mechanism gradually changes from the Io torus to the distant tail. In the transport of Io plasma, the precession plus interchange instability is effective near 10–15 Rj, interchange instability is predominant around 15–20 Rj, and the centrifugal force is predominant beyond 25 Rj. The current system supplies torque from the planet to co‐rotating plasma beyond 25 Rj to compensate the rotation delay. The associated upward field‐aligned current (FAC) is connected to the main emission (ME) in the ionosphere. The polar emission (PE) position coincides with that of downward feedback current of upward FAC causing the ME. High‐speed polar wind develops in the ME and in the polar cap, while slow polar wind develops in lower latitudes. In the middle of transport, Io plasma is mixed around 15 Rj with H+ supplied from the ionosphere by the low‐speed polar wind. Afterward, mixed plasma diffuses outward. The equatorial diffuse emission occurs in the projected position of the plasma mixing process.

Auroral, Ionospheric and Ground Magnetic Signatures of Magnetopause Surface Modes

AbstractSurface waves on Earth's magnetopause have a controlling effect upon global magnetospheric dynamics. Since spacecraft provide sparse in situ observation points, remote sensing these modes using ground‐based instruments in the polar regions is desirable. However, many open conceptual questions on the expected signatures remain. Therefore, we provide predictions of key qualitative features expected in auroral, ionospheric, and ground magnetic observations through both magnetohydrodynamic theory and a global coupled magnetosphere‐ionosphere simulation of a magnetopause surface eigenmode. These show monochromatic oscillatory field‐aligned currents (FACs), due to both the surface mode and its non‐resonant Alfvén coupling, are present throughout the magnetosphere. The currents peak in amplitude at the equatorward edge of the magnetopause boundary layer, not the open‐closed boundary as previously thought. They also exhibit slow poleward phase motion rather than being purely evanescent. We suggest the upward FAC perturbations may result in periodic auroral brightenings. In the ionosphere, convection vortices circulate the poleward moving FAC structures. Finally, surface mode signals are predicted in the ground magnetic field, with ionospheric Hall currents rotating perturbations by approximately (but not exactly) 90° compared to the magnetosphere. Thus typical dayside magnetopause surface modes should be strongest in the East‐West ground magnetic field component. Overall, all ground‐based signatures of the magnetopause surface mode are predicted to have the same frequency across L‐shells, amplitudes that maximize near the magnetopause's equatorward edge, and larger latitudinal scales than for field line resonance. Implications in terms of ionospheric Joule heating and geomagnetically induced currents are discussed.

Kelvin-Helmholtz Vortices as an Interplay of Magnetosphere-Ionosphere Coupling

The solar wind-magnetosphere interaction drives diverse physical processes on the flanks of Earth’s magnetopause, and in turn these processes couple to the ionosphere. We investigate simultaneous multipoint in-situ spacecraft and ground-based measurements to determine the role of Kelvin-Helmholtz waves at the Earth’s magnetopause and the low-latitude boundary layer in the magnetosphere-ionosphere coupling process. Nonlinear Kelvin-Helmholtz waves develop into flow vortices that twist and/or shear flux tube magnetic fields, thereby generating localized field-aligned currents. Kelvin-Helmholtz vortices on the dusk (dawn) flanks of the magnetosphere generate clockwise (counter-clockwise) rotations and upward (downward) field-aligned currents inside the flux tubes, consistent with the region-1 field-aligned current. We present in-situ MMS and Cluster spacecraft observations of Kelvin-Helmholtz vortices at the magnetopause that map to the poleward edge of the auroral regions. The FAST spacecraft and the ground-based magnetometers from which spherical elementary currents (acting as a proxy for vertical currents) can be calculated observe corresponding field-aligned current signatures. This study demonstrates the role played by the Kelvin-Helmholtz waves in linking magnetopause boundary fluctuations to ionospheric phenomena.

Response of the Jovian Magnetosphere‐Ionosphere System to the Interplanetary Magnetic Field Discontinuity: A Simulation Study

AbstractThe response of the magnetosphere‐ionosphere (MI) system at Jupiter to the interplanetary magnetic field (IMF) tangential discontinuity is studied via global Magnetohydrodynamics (MHD) simulations. Our results show that the IMF discontinuity, which has a transition front perpendicular to the Sun‐Jupiter line, has strong effects on the Jovian MI system depending on local times and radial distances to Jupiter. The magnetospheric significant response to the discontinuity is delayed by ∼2–6 hr after the encounter of the discontinuity front with the bow shock, while the delay time for the ionospheric field‐aligned currents (FACs) is ∼3–8 hr. The outer magnetosphere, particularly at dayside, tends to respond slightly quicker than the inner regions. At the ionosphere, the response of downward FACs is found to occur earlier than that of the upward ones by a time ranging from less than 1 hr to several hours, which is also more prominent at dayside sectors. Using a new MI mapping method with Jupiter's JRM09 magnetic field model, we reproduce for the first time the ionospheric FACs consistent with the morphology of the observed ultraviolet (UV) main auroral emissions. This indicates a potential application of the simulated upward FACs on the morphology of the UV main auroral ovals.

COMPASS: A New COnductance Model Based on PFISR And SWARM Satellite Observations

AbstractIonospheric conductance plays a crucial and active role in magnetosphere‐ionosphere‐thermosphere coupling processes. Despite its importance, direct global observations of conductance are unavailable. This limitation inspires the development of empirical models that are widely used to specify global distributions of conductance indirectly. In this work, a new model, COnductance Model based on PFISR And SWARM Satellite observations, describing the statistical relationships between conductance and field‐aligned currents (FACs) is presented. The conductance was calculated using the electron densities measured by Poker Flat Incoherent Scattering Radar (PFISR), and the FACs were determined by the magnetic perturbations measured by SWARM at Low‐Earth Orbit. Between 2014 and 2020, there were ∼3,900 conjunction events between PFISR and SWARM, providing a large data set for investigating the relationship between conductance and FACs. It is found that both Hall and Pedersen conductances vary as a power of , and the power index a depends on magnetic local time and the direction of FACs, ranging from 0.0 to 0.6. Properties of this power index a are founded as follows: (a) the largest power index is obtained on the dawn side, and the minimum is at noon; and (b) the power indices are positive for both upward and downward FACs and are larger for upward FACs than downward FACs. The underlying physical mechanisms of the observed variations of the model parameters are also discussed. Despite the complicated relationship between FACs and conductance, this model provides a convenient way to specify global distributions of the auroral zone conductance.

Roles of the M‐I Coupling and Plasma Sheet Dissipation on the Growth‐Phase Thinning and Subsequent Transition to the Onset

AbstractThe plasma‐sheet dynamics leading to the substorm onset is analyzed from a global simulation. We calculated three cases, the normal (case 1), reduced magnetosphere‐ionosphere (M‐I) coupling (case 2) and reduced tail dissipation (case 3). The normal solution reproduces the thinning, reconnection, tail flow, and the dipolarization in the magnetosphere, and the AU/AL, the initial brightening, the beads, the pre‐onset arc, and the poleward expansion in the ionosphere. Case 2 shows that the M‐I coupling promotes the thinning by evacuating magnetic field from the plasma sheet. Case 3 shows that it is implausible to reproduce the AU/AL of the expansion phase only with numerical dissipation. The projection area of the growth phase plasma sheet is so narrow that the quiet arcs on the high‐latitude side are projected to the lobe. The key point to understand such narrow projection is to consider the origin of the field‐aligned current (FAC). The FAC occurs to transmit motion, and particle precipitation is the consequence. The upward FACs for the quiet arc and initial brightening, which produce the substorm sequence, are generated to transmit enhanced convection from the magnetosphere to the ionosphere. The substorm onset is the transmission of the convection transient. The onset FAC is generated by the near‐earth dynamo. Before the onset, a two‐stage reconnection occurs at the medium‐tail (MTNL) and the near‐earth (NENL) neutral lines through retreating nulls. The accompanying flow explains the model that assumes two causal areas for the onset in the near‐earth and middle tails.

An Examination of Magnetosphere‐Ionosphere Influences During a SAPS Event

AbstractThe sub‐auroral polarization stream (SAPS) is a region of westward high velocity plasma convection equatorward of the auroral oval that plays an important role in mid‐latitude space weather dynamics. In this study, we present observations of SAPS flows extending across the North American sector observed during the recovery phase of a minor geomagnetic storm. A resurgence in substorm activity drove a new set of field‐aligned currents (FACs) into the ionosphere, initiating the SAPS. An upward FAC system is the most prominent feature spreading across most SAPS local times, except near dusk, where a downward current system is pronounced. The location of SAPS flows remained relatively constant, firmly inside the trough, independent of the variability in the location and intensity of the FACs. The SAPS flows were sustained even after the FACs weakened and retreated polewards with a decline in geomagnetic activity. The observations indicate that the mid‐latitude trough plays a crucial role in determining the location of the SAPS and that SAPS flows can be sustained even after the magnetospheric driver has weakened.

Two-Dimensional Structure of Flow Channels and Associated Upward Field-Aligned Currents: Model and Observations

Flow bursts are a major component of transport within the plasma sheet and auroral oval (where they are referred to as flow channels), and lead to a variety of geomagnetic disturbances as they approach the inner plasma sheet (equatorward portion of the auroral oval). However, their two-dimensional structure as they approach the inner plasma sheet has received only limited attention. We have examined this structure using both the Rice Convection Model (RCM) and ground-based radar and all sky imager observations. As a result of the energy dependent magnetic drift, the low entropy plasma of a flow burst spreads azimuthally within the inner plasma sheet yielding specific predictions of subauroral polarization stream (SAPS) and dawnside auroral polarization stream (DAPS) enhancements that are related to the field-aligned currents associated with the flow channel. Flow channels approximately centered between the dawn and dusk large-scale convection cells are predicted to give significant enhancements of both SAPS and DAPS, whereas flow channel further toward the dusk (dawn) convection cell show a far more significant enhancement of SAPS (DAPS) than for DAPS (SAPS). We present observations for cases having good coverage of flow channels as they approach the equatorward portion of the auroral oval and find very good qualitative agreement with the above RCM predictions, including the predicted differences with respect to flow burst location. Despite there being an infinite variety of flow channels’ plasma parameters and of background plasma sheet and auroral oval conditions, the observations show the general trends predicted by the RCM simulations with the idealized parameters. This supports that RCM predictions of the azimuthal spread of a low-entropy plasma sheet plasma and its associated FAC and flow responses give a realistic physical description of the structure of plasma sheet flow bursts (auroral oval flow channels) as they reach the inner plasma sheet (near the equatorward edge of the auroral oval).

The extreme space weather events in October 1788

Abstract Solar eruptions launch interplanetary coronal mass ejections (ICMEs) and cause geomagnetic storms and equatorial extension of the auroral oval. Their rare and unique nature has made analyses of historical events extremely important to increase their data availability. In this study, we analyzed the space weather event of 1788 October, which was characterized with simultaneous auroral observations. We extended archival surveys and confirmed the auroral visibilities down to Barcelona (46.0° MLAT) on October 21/22 as well as Mizuhara (27.5° MLAT) and Rome (44.8° MLAT) on October 22/23. The end of auroral reports overlapped with a reported declination disturbance at Mannheim, indicating a ΔD amplitude of ≥1.15°. Two positive excursions of ΔD were recorded, lasting for several tens of minutes. Upward field-aligned currents could have flowed poleward of Mannheim associated with substorms. We identified the equatorial boundary of the auroral oval down to 46.5° ILAT in the European sector and approximately ≤41.6° ILAT in the Japanese sector. This is compared with the reported equatorial auroral boundaries during extreme storms. The long storm sequence indicates the arrival of multiple ICMEs, thereby enhancing solar activity at that time. This sequence is indeed contextualized immediately after the maximum of Solar Cycle 4. Because sunspot observations are extremely scarce around 1788, it is challenging to identify the source active region. This in turn makes these auroral records valuable for the analyses of long-term solar activity before the onset of the Dalton Minimum.

The Universal Time Variations of the Intensity of Afternoon Aurora in Equinoctial Seasons

AbstractThe afternoon auroral emissions are investigated in the equinoxes for geomagnetically quiet conditions (Kp = 1) using auroral images from ultraviolet imager (UVI) aboard the Polar satellite. They are compared with solar illumination effects (the solar zenith angle [SZA] and the consequent ionospheric conductivity) and the dipole tilt angle, as well as the observational region 1 upward field‐aligned currents (FACs) from AMPERE data. The averaged afternoon auroral emissions have pronounced universal time (UT) variations with valley (2.8 photons/cm2/s) at around 01:00–03:00 UT and peak (4.7 photons/cm2/s) at around 17:00–19:00 UT. They generally vary with the solar illumination, the dipole tilt angle and the observed region 1 upward FACs as a function of UT. The afternoon auroral intensity is anticorrelated with the SZA and positively proportional to the solar EUV‐produced Pedersen conductivity, region 1 upward FACs and dipole tilt angle. Additionally, they depend weakly on solar flux under geomagnetically quiet conditions. These results suggest that in the afternoon auroral region, the peak auroral emissions are closely associated with the peak conductivity and the maximum upward FACs. Other mechanisms, such as the dipole tilt angle, may also contribute. Further comparison between the northern afternoon aurora and the FACs in the two conjugate hemispheres suggests little contributions on the auroral UT variations from the interhemispheric FACs in the equinoxes.

Effects of Solar Illumination and Substorms on Auroral Electrojets Based on CHAMP Observations

AbstractWe investigate the effects of solar illumination and the substorm process on the auroral electrojets using CHAMP high‐resolution magnetic field data obtained over a period of 10 years. The eastward electrojet was found to be proportional to the ionospheric conductance induced by solar irradiation over the whole day. More notably, a stronger westward current was detected in times of lower fluxtube‐integrated conductance in both hemispheres. The relative spatial distribution and evolution of the westward electrojet and field‐aligned current (FAC) during the substorm periods were also investigated. The position of the westward current coincided with that of the upward FAC in 2000–2200 magnetic local time (MLT), in keeping with the structure of a westward traveling surge. Furthermore, the upward FAC in the local time sector of the poleward of the westward electrojet was supplied mostly by the remote closure current. The downward FACs in the poleward and upward FACs in the equatorward of the westward electrojet existed in the MLT sectors after 2200 h. Finally, the ratio of the peak intensity of the westward electrojet to that of the meridional Pedersen current, in the 2000–2200 MLT sector increased by about 250%, and the value in the 2200–0200 MLT increased by about 10%–20%. These results provide observational evidence of the increased efficiency of the Cowling channel in the night‐time during substorm periods and contribute to our understanding of the formation mechanism of the auroral electrojet.

Formation and Release of the Harang Reversal Relating With the Substorm Onset Process

AbstractWith the interplanetary magnetic field (IMF) that turns from northward to southward, the global simulation successively reproduces the growth phase, the onset, and the expansion phase of the substorm. The calculated ionospheric convection for the growth phase reproduces the development of the Harang reversal (HR), the upward field‐aligned current (FAC), and the upcoming onset point as observed. Magnetic field lines traced from the center of the nightside upward FAC are open, while the magnetic field line traced from the onset point is closed. These open magnetic field lines map to flow shear just outside the O/C boundary. Seen from the magnetic configuration, the growth phase proceeds as the replacement process of nulls. Two new nulls appear on the dayside under the southward IMF, while two old nulls under the northward IMF retreat tailward forming two bifurcation regions on the dawn and dusk flanks. Flow shear around the O/C boundary forms by magnetospheric convection that returns to the dayside via bifurcation regions. The expansion phase proceeds through a topological change by the near‐earth neutral line (NENL). The NENL occurs inside the thinned structure of the northward IMF remnant, on the low‐latitude side of the flow shear, and projects down to the low‐latitude edge of the upward FAC. Associated with the NENL, the convection return path changes to the center of the plasma sheet and reveals in the ionosphere as the release of the HR. By the shrinkage of projecting magnetic field line, the O/C boundary migrates poleward in the ionosphere.

The Quasipersistent Feature of Highly Structured Field‐Aligned Currents in the Duskside Auroral Oval: Conjugate Observation Via Swarm Satellites and a Ground All‐Sky Imager

AbstractDuring northward interplanetary magnetic field (IMF), irregular magnetic perturbations were observed in the duskside aurora oval via Swarm satellites instead of large‐scale Region 1/2 magnetic perturbations. By taking advantage of Swarm constellation measurements, and their conjugate observations with an all‐sky imager on the ground, the features of the irregular magnetic perturbations were examined. Detailed analysis of the data from Swarm A and Swarm C for two events demonstrated that the irregular magnetic perturbations are a result of highly structured quasistatic field‐aligned currents (FACs), not dynamic Alfvén waves. The typical latitudinal size of the upward FACs is 20–30 km. In each region of the upward FACs, 630‐nm aurora emissions are relatively strong, indicating that the energy flux of precipitating electrons having energies of a few hundred electron volts is high in each of the upward FAC regions. The enhanced mesoscale auroras continued to exist for at least approximately 30 min. These indicate that the mesoscale FAC structures also have quasipersistent features. The precipitating particle data from the Defense Meteorological Satellite Program satellite, which passed through the field of view of the all‐sky imager, indicate that the source of the precipitating particles is the duskside low‐latitude boundary layer (LLBL). We suggest that the highly structured quasipersistent FACs flow along the magnetic field lines connected to the duskside LLBL where cold dense ions exist. The highly structured FACs in the duskside aurora oval are the phenomena that are pertinent to the magnetosphere for a northward IMF condition, not a simple remnant of the typical Region 1.

Spatial and Temporal Evolution of Different‐Scale Ionospheric Irregularities in Central and East Siberia During the 27–28 May 2017 Geomagnetic Storm

AbstractWe present a multi‐instrumental study of ionospheric irregularities of different scales (from tens of centimeters to few kilometers) observed over the Central and East Siberia, Russia, during a moderate‐to‐strong geomagnetic storm on 27–28 May 2017. From high‐frequency (HF) and ultrahigh‐frequency (UHF) radar data, we observed an intense auroral backscatter developed right after the initial phase of the geomagnetic storm. Additionally, we examined variations of Global Positioning System (GPS)‐based ROT (rate of TEC changes, where TEC is total electron content) for available GPS receivers in the region. Ionosondes, HF, and UHF radar data exhibited a presence of intense multi‐scale ionospheric irregularities. We revealed a correlation between different‐scale Auroral/Farley‐Buneman ionospheric irregularities of the E layer during the geomagnetic storm. The combined analysis showed that an area of intense irregularities is well connected and located slightly equatorward to field‐aligned currents (FACs) and auroral oval at different stages of the geomagnetic storm. An increase and equatorward displacement of Region 1 (R1)/Region 2 (R2) FACs leads to appearance and equatorward expansion of ionospheric irregularities. During downward (upward) R1 FAC and upward (downward) R2 FAC, the eastward and upward (westward and downward) E × B drift of ionospheric irregularities occurred. Simultaneous disappearance of UHF/HF auroral backscatter and GPS ROT decrease occurred during a prolonged near noon reversal of R1 and R2 FAC directions that accompanied by R1/R2 FAC degradation and disappearance of high‐energy auroral precipitation.

Extreme Field-Aligned Currents during Magnetic Storms of the 24th Solar Cycle: March 2015 and September 2017

Characteristics of field-aligned currents (FACs) obtained from the observations of SWARM satellites during two strong magnetic storms in March 16–19, 2015 and September 6–9, 2017 are presented. The storms were accompanied by high-intensity substorms. Satellites crossed the dusk, noon, pre- and after midnight MLT sectors. It was shown that variations in FAC density and the latitudinal position of the equatorial FAC boundaries are controlled by both the development of the storm and, to a considerable degree, substorm activity. The average densities of downward and upward FACs reach 3–4 μA/m2 at a substorm’s peak, while the undisturbed level is about 0.2 μA/m2. The minimum latitude of equatorial FAC boundaries is limited to 50° MLat. Large-scale FACs consist of small-scale filamentary structures with high current density, which are always present in SWARM observations. Small-scale peak currents with a density of 50–100 µA/m2 appear during periods of a general increase in density during activation of substorms. Local increases in concentration and temperature of electrons show that the current structure can be associated with a mesoscale discrete auroral arc.

Estimation of the contributions of conductance and electric field to the intensity of field-aligned currents in the night polar ionosphere during substorm expansion phase

Based on the output data of the magnetogram inversion technique, we analyze the dynamics of the Pedersen ionospheric currents and field-aligned currents (FACs) in the night mesoscale cells of the three main large-scale Iijima and Potemra Regions (R1, R2 and R0) during the expansion phase of the summer and winter substorms. FACs play a key role in the ionosphere-magnetosphere interaction. The continuity equation for the electric current density contains two types of possible feedback within the electric circuit parts that connect Pedersen currents and FACs in each cell. Pedersen ionospheric conductivity is dominant to Type 1, and the electric field dominates in Type 2 feedback. This paper studies both types of physical feedback in mesoscale cells of downward and upward FACs on the night side of the polar ionosphere from the data of two selected substorms of the summer and winter seasons in the northern hemisphere.

E‐POP and Red Line Optical Observations of Alfvénic Auroras

AbstractSuprathermal electron bursts (STEBs), characterized by a board energy spectrum and a field‐aligned pitch angle distribution, have been well recognized to be associated with electron acceleration by inertial Alfvén waves and are thus conventionally termed as “Alfvénic aurora.” In this study, we report joint Enhanced‐Polar‐Outflow‐Probe (e‐POP) and ground‐based optical observations of Alfvénic auroras. In particular, we highlight the prominence of 630‐nm red line emissions under low‐energy Alfvénic auroral precipitation. During the event interval, e‐POP traverses two arcs. One bright arc dominated by green line emissions is clearly seen by all optical instruments; it is embedded in upward field‐aligned currents (FACs) yet leaves little imprint on the e‐POP suprathermal electron imager (SEI), likely due to that the precipitation is well above the upper energy limit of SEI. On the other hand, there is a red line arc that is pronounced only in 630‐nm images. Such a red‐line‐only arc is located in a transition from large‐scale upward FACs to downward FACs and is associated with a prominent STEB structure detected by e‐POP SEI. The STEB features an inverse energy time dispersion, namely, that lower‐energy electrons are seen earlier while higher‐energy electrons appear later. The red‐line‐only arc and its separation from the green line arc evolve in a repeatable fashion, each stemming from a poleward auroral intensification (PAI) propagated from higher latitudes. Following each poleward auroral intensification the green line arc progressively moved southward, while the red‐line‐only arc is quasi‐stationary and stayed relatively stable in latitude. We propose tentative interpretations of the above features based upon stationary inertial Alfvén waves.