Subauroral Ion Drift Research Articles

Investigating the Development of Abnormal Subauroral Ion Drift (ASAID) and Abnormal Subauroral Polarization Stream (ASAPS) During the Magnetically Active Times of September 2003

AbstractThis study investigates two recently reported subauroral phenomena: the abnormal subauroral ion drift (ASAID) appearing as an inverted SAID and the shielding‐E—SAID structure depicting a SAID feature on the poleward side of a small eastward or antisunward flow channel that is the shielding electric (E) field's signature. We have analyzed polar cross sections, constructed with multi‐instrument Defense Meteorological Satellite Program data, for the development of these subauroral phenomena. New results show the features of abnormal subauroral polarization stream (ASAPS) and ASAID‐ASAPS comprised by a narrow ASAID embedded in a wider ASAPS. We have identified undershielding, perfect shielding, and overshielding events. Our observational results demonstrate SAPS development during undershielding, the absence of subauroral flow channel during perfect shielding, and ASAID/ASAPS and shielding‐E—SAID/SAPS development during overshielding. The appearance of an ASAID‐ASAPS structure together with a pair of dayside‐nightside eastward auroral flow channels implies the intensification of region 2 field‐aligned currents via the westward traveling surge and thus the strengthening of overshielding conditions. From the observational results presented we conclude for the magnetically active time period studied that (i) the shielding E field drove the wider ASAPS flow channel, (ii) the ASAID‐ASAPS structure's narrow antisunward flow channel developed due to the injections of hot ring current ions in a short‐circuited system wherein the hot ring current plasma was closer to the Earth than the cold plasmaspheric plasma, and (iii) overshielding created this hot‐cold plasma configuration via the development of a plasmaspheric shoulder.

Distinguishing Subauroral Ion Drifts From Birkeland Current Boundary Flows

AbstractThe Swarm satellites regularly observe latitudinally narrow ion velocity jets exceeding 1 km/s relative to the surrounding plasma at high latitude. Two distinct categories of duskward directed flow channels are observed. The most common are situated between the Region 1 and Region 2 currents, are observed primarily during quiet geomagnetic conditions, and are called Birkeland current boundary flows. During active conditions, a second type of duskward directed ion velocity jet is observed at the equatorward edge of the Region 2 currents, colocated with a sharp poleward directed plasma density gradient, consistent with subauroral ion drifts. Pedersen current and electric field estimates from the Swarm satellites show both types of flow channel to be associated with low height‐integrated Pedersen conductivity relative to the surrounding plasma. In this study we show two cases in which the Swarm satellites passed through both types of event on the same orbit, as well as the average ion velocity, field‐aligned current, and plasma density surrounding 57 of these events. Some field‐aligned current morphologies previously associated with subauroral ion drifts suggest that they may have instead been Birkeland current boundary flows. In this study we provide a comparative description of both phenomena to facilitate their distinction in future studies.

SAPS/SAID revisited: A causal relation to the substorm current wedge

AbstractWe present multispacecraft observations of enhanced flow/electric field channels in the inner magnetosphere and conjugate subauroral ionosphere, i.e., subauroral polarization streams (SAPS) near dusk and subauroral ion drifts (SAID) near midnight. The channels collocate with ring current (RC) injections lagging the onset of substorms by a few to ∼20 min, i.e., significantly shorter than the gradient‐curvature drift time of tens of keV ions. The time lag is of the order of the propagation time of reconnection‐injected hot plasma jets to the premidnight plasmasphere and the substorm current wedge (SCW) to dusk. The observations confirm and expand on the previous results on the SAID features that negate the paradigm of voltage and current generators. Fast‐time duskside SAPS/RC injections appear intimately related to a two‐loop circuit of the substorm current wedge (SCW2L). We suggest that the poleward electric field inherent in the SCW2L circuit, which demands closure of the Region 1 and Region 2 sense field‐aligned currents via meridional currents, is the ultimate cause of fast RC injections and SAPS on the duskside.

Investigating the development of double‐peak subauroral ion drift (DSAID)

AbstractThis study focuses on the newly described ionospheric feature, called double‐peak subauroral ion drift (DSAID), which is a subclass of the well‐known single‐peak SAID. Double‐layer Region 2 (R2) field aligned currents (FACs) could be the main driver of DSAID. Our aim is to gain new insights into the development of DSAID during its two‐stage progression. Observational results are provided by five scenarios, each demonstrating a certain progression sequence of DSAID. Results show that SAID/DSAID occurred during flux transfer events and was accompanied by flow channels (FCs) associated with dayside magnetopause (FC‐2) and nightside magnetotail (FC‐3) reconnections, with westward electrojet (eastward FC), and with auroral streamers (FC‐4). In the premidnight magnetic local time (MLT) sector of stage 2, DSAID development was due to the short‐circuiting of the reconnection‐injected plasma jets during substorms or pseudobreakups. Thus, the related ring current pressure buildup enhanced the downward R2 FACs leading to double/multiple circuits forming double‐layer R2 FACs. During the midnight MLT hours of stage 2, DSAID development was closely related to the westward traveling surge (WTS)/substorm current wedge (SCW). WTS/SCW‐related strong upward R1 FACs closed with meriodional currents producing eastward and downward (i.e., downward R2 FAC‐style) return currents enhancing the downward R2 FACs and thus leading to double/multiple circuits forming double‐layer R2 FACs. Auroral streamers/FC‐4 represent a substorm substructure and their occurrence with DSAID after stage 2 demonstrates that this substructure occasionally includes DSAID. Our results demonstrate also that the short‐circuited system underlying SAID/DSAID acted sometimes as a current generator and sometimes as a voltage generator.

Postmidnight ionospheric troughs in summer at high latitudes

AbstractIn this article we identify possible mechanisms for the formation of postmidnight ionospheric troughs during summer, in sunlit plasma. Four events were identified in measurements of European Incoherent Scatter and ESR radars during CP3 experiments, when the ionosphere was scanned in a meridional plan. The spatial and temporal variation of plasma density, ion, and electron temperatures were analyzed for each of the four events. Super Dual Auroral Radar Network plasma velocity measurements were added, when these were available. For all high‐latitude troughs the ion temperatures are high at density minima (within the trough), at places where the convection plasma velocity is eastward and high. There is no significant change in electron temperature inside the trough, regardless of its temporal evolution. We find that troughs in sunlit plasma form in two steps: the trough starts to form when energetic electron precipitation leads to faster recombination in the F region, and it deepens when entering a region with high eastward flow, producing frictional heating and further depleting the plasma. The high‐latitude plasma convection plays an important role in formation and evolution of troughs in the postmidnight sector in sunlit plasma. During one event a second trough is identified at midlatitudes, with different characteristics, which is most likely produced by a rapid subauroral ion drift in the premidnight sector.

Double‐peak subauroral ion drifts (DSAIDs)

AbstractThis paper reports double‐peak subauroral ion drifts (DSAIDs), which is unique subset of subauroral ion drifts (SAIDs). A statistical analysis has been carried out for the first time with a database of 454 DSAID events identified from Defense Meteorological Satellite Program observations from 1987 to 2012. Both case studies and statistical analyses show that the two velocity peaks of DSAIDs are associated with two ion temperature peaks and two region‐2 field‐aligned currents (R2‐FACs) peaks in the midlatitude ionospheric trough located in the low‐conductance subauroral region. DSAIDs are regional and vary significantly with magnetic local time. DSAIDs can evolve from/to SAIDs during their lifetimes, which are from several minutes to tens of minutes. Comparisons between the ionospheric parameters of DSAIDs and SAIDs indicate that double‐layer region‐2 field‐aligned currents (R2‐FACs) may be the main driver of DSAIDs. It is also found that DSAIDs happen during more disturbed conditions compared with SAIDs.

Hemispheric asymmetry of subauroral ion drifts: Statistical results

AbstractA large database of more than 18,000 subauroral ion drift (SAID) events from DMSP observations from 1987 to 2012 is used to systematically investigate the features of SAID. SAID occurs mostly at ~62°/−60° magnetic latitude (MLAT) and ~22:15/22:45 magnetic local time (MLT) for geomagnetically quiet conditions and at ~58°/−56° MLAT and ~22:15/22:45 MLT for geomagnetically disturbed conditions in the North Hemisphere (NH)/South Hemisphere (SH), respectively. Significant north‐south asymmetries in SAID occurrence, shape, and geomagnetic activity variations are found in this statistical study. The latitudinal width of a SAID is larger in the NH than in the SH. An interesting finding of this work is that the SAID occurrence probability peaks have an ~180° difference in longitude between the two hemispheres in the geographic coordinates for both geomagnetically quiet and disturbed conditions. The SAID width peaks in almost the same geomagnetic meridian zone with a geomagnetic longitude of ~80°–120° in both hemispheres. Significant hemispheric asymmetries and spike signatures with sharp dips are found in all the latitudinal profiles of the horizontal velocities of SAIDs. The SAID is highly correlated to geomagnetic activity, indicating that the location and evolution of the SAID might be influenced by global geomagnetic activity, auroral dynamics, and the dynamics of ring currents.

Counter equatorial electrojet and overshielding after substorm onset: Global MHD simulation study

AbstractBy performing a global magnetohydrodynamic (MHD) simulation, we have demonstrated for the first time that an electrojet at the dayside magnetic equator can be reversed and an overshielding condition can be established in the inner magnetosphere after substorm onset without northward turning of the interplanetary magnetic field. Near the substorm onset, the plasma pressure is highly enhanced in the inner magnetosphere on the nightside. The Region 2 field‐aligned current diverges from the diamagnetic current on the surface of the dayside extension of the high‐pressure region, which is connected to the ionosphere in the relatively low‐conductivity region a few degrees equatorward of the main auroral oval that is formed as the projection of the plasma sheet. The separation of the equatorward boundary of the auroral region and the equatorward boundary of the Region 2 current results in dusk‐dawn electric fields that generate a counter electrojet (CEJ) at the dayside magnetic equator. Poleward electric fields in a narrow latitudinal width, which may be regarded as subauroral ion drift and subauroral polarization stream, are simultaneously intensified. The dusk‐dawn electric fields may propagate to the inner magnetosphere along a field line as shear Alfvén waves. Then, the inner magnetosphere is completely constrained by the overshielding condition. The intensity and polarity of the CEJ depend largely on at least the ionospheric conductivity that is related to the plasma pressure (probably associated with diffuse aurora). This may explain the observational fact that overshielding does not always occur after onset.

Solar cycle, seasonal, and diurnal variations of subauroral ion drifts: Statistical results

AbstractThe solar cycle, seasonal, and diurnal variations of the subauroral ion drifts (SAIDs) are investigated for the first time to use such a large database of 18,226 SAID events observed by the DMSP satellites during 1987–2012. Statistical results show that SAIDs occur mostly at 60.1° invariant latitude and 2230 magnetic local time with a typical half width of 0.57°, move equatorward during high solar activities with large widths, and have two occurrence peaks in spring and fall equinoxes and two valleys in summer and winter solstices. The seasonal variation of SAID latitude has two valleys in spring and fall, and SAID width has a valley distribution with a minimum in summer. SAIDs exhibit a clear day‐to‐night difference in latitude. The diurnal variation of SAID width has a morning valley and an afternoon peak. The generation mechanism of SAID associated with the electron precipitation and the downward field‐aligned current is also supported in this study.

Fine structure of subauroral electric field and electron content

AbstractSmall‐scale structure of the plasma convection and electron content within the subauroral polarization stream (SAPS) is investigated. We present ionospheric observations during the main phase of the geomagnetic storm on 17 March 2013, during which a sequence of intense, highly localized, and fast‐moving electric field (EF) structures within SAPS was observed by the Super Dual Auroral Radar Network Christmas Valley West (CVW) radar. The CVW EF measurements at 60 s resolution are analyzed in context of coincident GPS measurements of the total electron content (TEC) at 30 s resolution. The strong and narrow feature of the subauroral ion drift (SAID) was observed poleward of the TEC trough, with a TEC enhancement (peak) seen in the SAPS (SAID) region. The SAPS wave activity commenced ~2 h (15 min) after first appearance of SAPS (SAID). The SAPS structures appeared near the poleward edge of the trough, propagated westward, and merged with SAID near TEC peak. The propagation velocity was comparable with convection velocity within each EF structure. The SAPS TEC exhibited a general decrease toward the end of the period. On a smaller time scale, TEC exhibited a small but appreciable decrease within EF structures. The wavelet spectra of EF and TEC showed similar variations, with wave period of ~5 min period near onset and increasing to 8–10 min toward the end of the period with significant wave activity. A scenario is discussed, in which the SAPS wave activity may modify the ionospheric conductance and TEC at small scales, with large‐scale magnetosphere‐ionosphere feedback acting to continuously deplete TEC where/when such activity does not occur.

Interaction of substorm injections with the subauroral geospace: 1. Multispacecraft observations of SAID

Subauroral ion drifts (SAID) are the prominent feature of the active subauroral geospace, just earthward of the electron plasma sheet (PS) boundary in the premidnight sector. We explore magnetically conjugate satellite observations of substorm SAID near the magnetic equator and in the topside ionosphere confirming and expanding on our previous results. The SAID channels reside between the hot electron (PS/auroral) boundary at the plasmapause and the drop in the hot ion flux inside the plasmasphere. The overall features are inconsistent with the paradigm of voltage and current generators. Rather, they are explained in terms of a short circuiting of substorm‒injected hot plasma jets over the plasmapause and formation of a turbulent plasmaspheric boundary layer. The short circuiting occurs when the cold plasma density exceeds a critical value of 5–10 cm−3. As the polarization field at the front of the hot plasma jet is shorted out, the hot electrons are arrested, while the hot ions yet move inward. This provides a natural explanation of the long‒known dispersionless auroral precipitation boundary. Enhanced plasma turbulence within the SAID channel provides anomalous circuit resistivity and magnetic diffusion, as with the well‒documented plasmoid‒magnetic barrier problem.

Plasmaspheric trough evolution under different conditions of subauroral ion drift

The statistical characteristics of the subauroral ion drift (SAID) in the ionosphere and the plasmaspheric trough evolution under different conditions of SAID were investigated in this paper, based on 566 SAID events observed by Akebono, Astrid-2, DE-2, and Freja satellites. The relationships between the latitudinal location of SAID and the Kp, AL, and Dst indices for these events were also discussed. It was found that the SAID events happened mainly at invariant latitude (ILAT) of 60.4° and magnetic local time (MLT) of 21.6 MLT and that 92.4% of the events happened when the Kp index was below 5.0, indicating a medium geomagnetic activity. The latitudinal half-width of SAID varied from 0.5° to 3.0° with a typical half-width of 1.0°. The SAID would happen at low latitudes if the geomagnetic activity was high. The effects of SAID on equatorial outer plasmasphere trough evolutions were studied with the dynamic global core plasma model (DGCPM) driven by the statistical results of SAID signatures. It was noted that locations, shapes and density of troughs vary with ILAT, MLT, latitudinal width, cross polar cap potential and lifetime of SAID events. The evolution of a trough is determined by the extent of SAID electric field penetrating into plasmasphere and not all SAID events can result in trough formations.

Dynamic subauroral ionospheric electric fields observed by the Falkland Islands radar during the course of a geomagnetic storm

[1] We present an analysis of ionospheric electric field data observed during a geomagnetic storm by the recently deployed HF radar located on the Falkland Islands. On 3 August 2010 at ∼1800 UT evidence of the onset of a geomagnetic storm was observed in ground magnetometer data in the form of a decrease in the Sym-H index of ∼100 nT. The main phase of the storm was observed to last ∼24 hours before a gradual recovery lasting ∼3 days. On 4 August, during the peak magnetic disturbance of the storm, a high velocity (>1000 m s−1) channel of ionospheric plasma flow, which we interpret as a subauroral ion drift (SAID), located between 53° and 58° magnetic south and lasting ∼6.5 hours, was observed by the Falkland Islands radar in the pre-midnight sector. Coincident flow data from the DMSP satellites and the magnetically near-conjugate northern hemisphere Blackstone HF radar reveal that the SAID was embedded within the broader subauroral polarization streams (SAPS). DMSP particle data indicate that the SAID location closely followed the equatorward edge of the auroral electron precipitation boundary, while remaining generally poleward of the equatorward boundary of the ion precipitation. The latitude of the SAID varied throughout the interval on similar timescales to variations in the interplanetary magnetic field and auroral activity, while variations in its velocity were more closely related to ring current dynamics. These results are consistent with SAID electric fields being generated by localized charge separation in the partial ring current, but suggest that their location is more strongly governed by solar wind driving and associated large-scale magnetospheric dynamics.

Large electric field at the nightside plasmapause observed by the Polar spacecraft

We report an example of large electric field with a peak amplitude of ∼60 mV/m observed at the plasmapause by the Polar spacecraft on April 25, 1998. This electric field pointed radially outward and predominantly perpendicular to the ambient magnetic field. As comparing the plasmapause crossings in the previous and following Polar passes, we found that the large electric field is associated with forming a sharp plasmapause structure. Comparing with previous observations of subauroral electric fields in the ionosphere and magnetosphere, we suggest that the large electric field observed at Polar is associated with the subauroral ion drift phenomenon. In addition, we observed a negative‐then‐positive magnetic field perturbation at the plasmapause. That is, the magnetic field is reduced outside the plasmapause and enhanced inside the plasmapause. The field perturbation is dominant in the meridional plane. We suggest that the magnetic field perturbation is due to the dawnward plasmapause current (Jpp), which is associated with the balance of forces (∇P ∼ Jpp × B) at the plasmapause, perpendicular to the background magnetic field.

Dynamics of ring current and electric fields in the inner magnetosphere during disturbed periods: CRCM–BATS‐R‐US coupled model

We present simulation results from a one‐way coupled global MHD model (Block‐Adaptive‐Tree Solar‐Wind Roe‐Type Upwind Scheme, BATS‐R‐US) and kinetic ring current models (Comprehensive Ring Current Model, CRCM, and Fok Ring Current, FokRC). The BATS‐R‐US provides the CRCM/FokRC with magnetic field information and plasma density/temperature at the polar CRCM/FokRC boundary. The CRCM uses an electric potential from the BATS‐R‐US ionospheric solver at the polar CRCM boundary in order to calculate the electric field pattern consistent with the CRCM pressure distribution. The FokRC electric field potential is taken from BATS‐R‐US ionospheric solver everywhere in the modeled region, and the effect of Region II currents is neglected. We show that for an idealized case with southward‐northward‐southward Bz IMF turning, CRCM–BATS‐R‐US reproduces well known features of inner magnetosphere electrodynamics: strong/weak convection under the southward/northward Bz; electric field shielding/overshielding/penetration effects; an injection during the substorm development; Subauroral Ion Drift or Polarization Jet (SAID/PJ) signature in the dusk sector. Furthermore, we find for the idealized case that SAID/PJ forms during the substorm growth phase, and that substorm injection has its own structure of field‐aligned currents which resembles a substorm current wedge. For an actual event (12 August 2000 storm), we calculate ENA emissions and compare with Imager for Magnetopause‐to‐Aurora Global Exploration/High Energy Neutral Atom data. The CRCM–BATS‐R‐US reproduces both the global morphology of ring current and the fine structure of ring current injection. The FokRC‐BATS‐R‐US shows the effect of a realistic description of Region II currents in ring current–MHD coupled models.

SAID: A turbulent plasmaspheric boundary layer

This paper presents novel features of subauroral ion drifts (SAID) observed from a unique conjunction of the Cluster, DMSP, and Polar satellites, including the discovery of SAID‐related plasma waves. These observations confirm and expand on our proposed concept of the SAID channel being a turbulent boundary layer, formed via a short circuit of the substorm‐injected plasmoid by the plasmasphere. We show that SAID formation is related to enhanced lower hybrid/fast magnetosonic waves. Their excitation leads to anomalous circuit resistivity and magnetic diffusion, similar to the well‐documented plasmoid‐magnetic barrier problem, including impulsive penetration at the magnetopause.

Auroral and sub-auroral phenomena: an electrostatic picture

Abstract. Many auroral and sub-auroral phenomena are manifestations of an underlying magnetosphere-ionosphere coupling. In the electrostatic perspective the associated auroral current circuit describes how the generator (often in the magnetosphere) is connected to the load (often in the ionosphere) through field-aligned currents. The present paper examines the generic properties of the current continuity equation that characterizes the auroral circuit. The physical role of the various elements of the current circuit is illustrated by considering a number of magnetospheric configurations, various auroral current-voltage relations, and different types of behaviour of the ionospheric conductivity. Based on realistic assumptions concerning the current-voltage relation and the ionospheric conductivity, a comprehensive picture of auroral and sub-auroral phenomena is presented, including diffuse aurora, discrete auroral arcs, black aurora, and subauroral ion drift. The electrostatic picture of field-aligned potential differences, field-aligned currents, ionospheric electric fields and plasma drift, and spatial scales for all these phenomena is in qualitative agreement with observations.

Statistical study on the occurrence of ASAID electric fields

Abstract. The first statistical results on the occurrence of abnormal subauroral ion drifts (ASAID) are presented based on electric and magnetic field measurements from the low-altitude Astrid-2 satellite. ASAID are narrow regions of rapid eastward ion drifts observed in the subauroral ionosphere. They correspond to equatorward-directed electric fields with peak amplitudes seen to vary between 45 mV/m and 185 mV/m, and with latitudinal extensions between 0.2° and 1.2° Corrected Geomagnetic Latitude (CGLat), reaching in some cases up to 3.0° CGLat. Opposite to subauroral ion drifts (SAID) that are known to be substorm-related, ASAID are seen to occur predominantly during extended periods of low substorm activity. Our results show that ASAID are located in the vicinity of the equatorward edge of the auroral oval, mainly in the postmidnight sector between 23:00 and 03:00 magnetic local time. They are associated with a local current system with the same scale-size as the corresponding ASAID, composed by a region of downward field-aligned currents (FACs) flowing in the ASAID poleward side, and a region of upward flowing FACs in the equatorward side. The FACs have densities between 0.5 and 2.0 μA/m2. The data suggest that ASAID do not contribute significantly to the reduction of the ionospheric conductivity. ASAID are seen to have life times of at least 3.5 h. A discussion on possible mechanisms for the generation of ASAID is presented. We speculate that the proximity of the electron to the ion plasma sheet inner boundaries and of the plasmapause to the ring current outer edge, during extended quiet times, is an important key for the understanding of the generation of ASAID electric fields.

Electric Fields and Magnetic Fields in the Plasmasphere: A Perspective From CLUSTER and IMAGE

The electric field and magnetic field are basic quantities in the plasmasphere measured since the 1960s. In this review, we first recall conventional wisdom and remaining problems from ground-based whistler measurements. Then we show scientific results from Cluster and Image, which are specifically made possible by newly introduced features on these spacecraft, as follows. 1. In situ electric field measurements using artificial electron beams are successfully used to identify electric fields originating from various sources. 2. Global electric fields are derived from sequences of plasmaspheric images, revealing how the inner magnetospheric electric field responds to the southward interplanetary magnetic fields and storms/substorms. 3. Understanding of sub-auroral polarization stream (SAPS) or sub-auroral ion drifts (SAID) are advanced through analysis of a combination of magnetospheric and ionospheric measurements from Cluster, Image, and DMSP. 4. Data from multiple spacecraft have been used to estimate magnetic gradients for the first time.

Eastward sub-auroral ion drifts or ASAID

Abstract. From satellite data sampling the top ionosphere in the Northern Hemisphere we have identified strong eastward ion drifts, with speeds larger than 1 km/s, widths of 1°–2°, occurring at similar temporal and spatial locations as rapid westward ion drifts known as sub-auroral ion drifts (SAID). We have called these events "abnormal sub-auroral ion drifts" (ASAID). Two events observed in the 20:00–22:00 MLT interval are discussed: the first occurring on 21 September 2003 and the other on 12 October 2003. Tomographic reconstructions of the electron density in the F-region, based on satellite data, provided by the Scandinavian tomography chain, were also available. We have observed that ASAID are accompanied by upward flows with a speed of the same order as that of the zonal ion drift. They coincide with deep, narrow troughs in the total ion density, both at the altitude of the F15 DMSP satellite (850 km) and in the F-region of the ionosphere, but do not seem to be a feature of the convective transport. During the entire duration of ASAID the electron temperature is very high while, contrary to SAID, the ion temperature has no clear variation. Both events described in this paper end up turning into classical SAID. Satellite data indicate that the generator of ASAID could be located inside the plasmasphere close to the plasmapause and we suggest a possible mechanism for their formation.