Subauroral Ion Drift Research Articles

Rainbow of the Night: First Direct Observation of a SAR Arc Evolving Into STEVE

AbstractDuring the 17 March 2015 geomagnetic storm, citizen scientist observations from Dunedin (45.95°S, 170.32°E), New Zealand, revealed a bright wide red arc known as stable auroral red (SAR) arc evolving into a thin white‐mauve arc, known as Strong Thermal Emission Velocity Enhancement (STEVE). An all‐sky imager at the Mount John Observatory (43.99°S, 170.46°E), 200 km north of Dunedin, detected an extremely bright arc in 630.0 nm, with a peak of ∼6 kR, colocated with the arc measured at Dunedin at an assumed height of 425 km. Swarm satellite data measured plasma parameters that showed strong subauroral ion drift signatures when the SAR arc was observed. These conditions intensified to extremely high values in a thinner channel when STEVE was present. Our results highlight the fast evolution of plasma properties and their effects on optical emissions. Current theories and models are unable to reproduce or explain these observations.

Statistical Study of Magnetospheric Conditions for SAPS and SAID

AbstractInner‐magnetospheric conditions for subauroral polarization streams (SAPS) and subauroral ion drifts (SAID) have been investigated statistically using Time History of Events and Macroscale Interactions during Substorms and RBSP observations. We found that plasma sheet electron fluxes at its earthward edge are larger for SAID than SAPS. The ring current ion flux for SAID formed a local maximum near SAID, but the ion flux for SAID was not necessarily larger than for SAPS. The median potential drop across SAID and SAPS is nearly the same, but the potential drop for intense SAID is substantially larger than that for SAPS. The plasmapause is sharper and electromagnetic waves were more intense for SAID. The SAID velocity peak does not strongly correlate with solar wind or geomagnetic indices. These results indicate that local plasma structures are more important for SAPS/SAID velocity characteristics as compared to global magnetospheric conditions.

A Time‐Dependent Two‐Dimensional Model Simulation of Lower Ionospheric Variations Under Intense SAID

AbstractThe subauroral ion drift (SAID) denotes a latitudinally narrow channel of fast westward ion drift in the subauroral region, often observed during geomagnetically disturbed intervals. The recently recognized subauroral optical phenomena, the Strong Thermal Emission Velocity Enhancement (STEVE) and the Picket Fence, are both related to intense SAIDs. In this study, we present a 2D time‐dependent model simulation of the self‐consistent variations of the electron/ion temperature, density, and FAC, under strong SAID, with more focus in the lower ionosphere. The anomalous electron heating in the E‐region is evaluated using an empirical model. Our simulation reproduces many key features of SAID, such as the strong electron temperature enhancement in the upper F‐region, the intense ion frictional heating, and the plasma density depletion. Most importantly, the ion Pedersen drifts is found to play a crucial role in the density variations and FAC dynamics in the lower ionosphere. The transport effect of ion Pedersen drifts leads to strong density depletion in the lower ionosphere in a large portion of SAID. The FAC inside SAID is mainly downward with magnitude ≤ ∼1 μA/m2. At the poleward edge of SAID, the ion Pedersen drift leads to a pileup of the plasma density and an upward FAC. Our simulation results also corroborate the presence of strong gradients of plasma density, temperature, and flows, at the edge of SAID, which may be conducive to certain plasma instabilities. Our model provides a useful tool for the future exploration of the generation mechanisms of STEVE and Picket Fence.

First Simultaneous Observation of STEVE and SAR Arc Combining Data From Citizen Scientists, 630.0 nm All‐Sky Images, and Satellites

AbstractOn September 28, 2017 citizen scientist observations at Alberta, Canada (51°N, 113° W) detected aurora and a thin east‐west purplish arc, known as strong thermal emission velocity enhancement (STEVE) that lasted less than 20 min. All‐sky imagers at subauroral latitudes measured stable auroral red (SAR) arcs during the entire night. The imager at Bridger, MT (45.3°N, 108.9°W) also measured a STEVE. The overlapping geometry allowed to determine that the height of STEVE was 225–275 km. STEVE is brighter in the 630.0 nm images in the West and almost merges with the SAR arc in the East. A DMSP satellite pass in the southern hemisphere was at the conjugate location of the Bridger imager during the STEVE observation. When mapped into the northern hemisphere intense subauroral ion drift and subauroral polarization streams were detected associated with the two optical signatures measured in 630.0 nm.

Neutral Wind Dynamics Preceding the STEVE Occurrence and Their Possible Preconditioning Role in STEVE Formation

AbstractStrong Thermal Emission Velocity Enhancement (STEVE) is a nightsky optical phenomenon of great research interest in recent years. Recent findings indicated that STEVE likely represents certain extremely intensified chemiluminescence airglow instead of traditional aurora. In this study, we investigate the patterns and variations of the neutral wind and temperature before the STEVE emergence using joint scanning Doppler imager (SDI 630 nm) and optical all‐sky imager (ASI) observations, and make an initial effort to explore the potential preconditioning role of neutral winds in the STEVE production. Neutral winds enhance in westward and southward directions following substorm auroral intensification, and show an equatorward propagating trend from auroral latitudes. However, in STEVE events the enhanced equatorward winds feature a steep stop/reversal at certain subauroral latitude, and strong wind convergence is developed there. This pattern sustains for ∼15–20 min, and then STEVE arises at about this stop latitude. The strength of the southward wind intensification and wind convergence is in general weaker or absent in nonSTEVE substorm events. We propose that enhanced equatorward winds may transport relevant neutrals species that are key to the STEVE airglow production to subauroral latitudes, and pile up at the stop latitude of the equatorward winds due to the strong convergence there. Such a transport/pileup effect led by the neutral winds may prepare a reservoir of neutral constituent which, when further aided by subauroral ion drift, leads to a dramatic increase of the airglow production and the STEVE occurrence.

Investigating the Development of Distinctive Subauroral Flow Channels During the November 7–8, 2004 Superstorm

AbstractWe investigate the development of large (∼3,000 m/s) subauroral flows occurring during the November 7–8, 2004 Superstorm by utilizing multisatellite and multiinstrument measurements from the Magnetosphere (M), topside Ionosphere (I), and Thermosphere (T). We present eight scenarios (Sc‐1–Sc‐8) depicting the development of large SubAuroral Polarization Streams (SAPS) in the initial phase (Sc‐1–Sc‐3) and recovery phase (Sc‐5), and structured SAPS (Sc‐4, Sc‐6–Sc‐7) and Double‐peak SubAuroral Ion Drifts (DSAID) in the recovery phase (Sc‐8). Magnetospheric observations reveal dayside magnetopause reconnection increasing both the magnetospheric convection electric field and the large‐scale Field‐Aligned Currents (FACs), long‐lasting (∼5 h) secondary dipolarizations during the initial phase, and a series of dipolarization events during the recovery phase. Ionospheric observations reveal the development of these large subauroral flows (i) during forward and reverse plasma convections—when the topside ionosphere received earthward energy from the magnetosphere and when the diversion of cross‐tail currents in the stretched magnetotail activated the Substorm Current Wedge (SCW)—and (ii) in a short‐circuited system that acted as a voltage generator. Thermospheric observations reveal earthward energy deposition from the ionosphere that drove wind surges—implying the presence of Atmospheric Gravity Waves (AGWs) and Traveling Ionospheric Disturbances (TIDs)—propagating equatorward/poleward and passing through the trough region, and further enhancing the subauroral flows by deepening the trough. From these results we conclude that the strong M‐I‐T coupling occurring under superstorm conditions played a significant role in the enhancement of the subauroral flows investigated.

Dynamics of Auroral Precipitation Boundaries Associated With STEVE and SAID

AbstractUsing Defense Meteorological Satellite Program (DMSP) and National Oceanic and Atmospheric Administration (NOAA) satellite observations and ground‐based observations by the THEMIS all‐sky imagers (ASIs) and SuperDARN radars, we determine how the equatorward boundary locations of ring current ions and plasma sheet electrons at pre‐midnight relate to occurrence of strong thermal emission velocity enhancement (STEVE) and intense subauroral ion drifts (SAID) during substorms. We found that the STEVE events are associated with a sharper gradient of electron precipitating flux, lower precipitating ion flux, and a narrower (<1°) latitudinal gap between the equatorward boundaries of trapped ring current ions and precipitating plasma sheet electrons and narrower region‐2 field‐aligned currents (FACs) than for the non‐STEVE events. The narrow gap of the particle boundaries contains intense SAID, higher upflow velocity, lower trough density, and slightly higher electron temperature than those for the non‐STEVE events. The non‐STEVE substorms have much wider gaps between the trapped ions and precipitating electrons, and subauroral polarization streams (SAPS) do not show intense SAID. These results indicate that subauroral flows and downward FACs for the STEVE events can only flow within the latitudinally narrow subauroral low‐conductance region between the ion and electron boundaries, resulting in intense SAID and heating. During the non‐STEVE events, the SAPS flows can flow in the latitudinally wide region without forming intense SAID.

Magnetospheric Conditions for STEVE and SAID: Particle Injection, Substorm Surge, and Field‐Aligned Currents

AbstractTo understand magnetosphere‐ionosphere conditions that result in thermal emission velocity enhancement (STEVE) and subauroral ion drifts (SAID) during the substorm recovery phase, we present substorm aurora, particle injection, and current systems during two STEVE events. Those events are compared to substorm events with similar strength but without STEVE. We found that the substorm surge and intense upward currents for the events with STEVE reach the dusk, while those for the non‐STEVE substorms are localized around midnight. The Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite observations show that location of particle injection and fast plasma sheet flows for the STEVE events also shifts duskward. Electron injection is stronger and ion injection is weaker for the STEVE events compared to the non‐STEVE events. SAID are measured by Super Dual Auroral Radar Network during the STEVE events, but the non‐STEVE events only showed latitudinally wide subauroral polarization streams without SAID. To interpret the observations, Rice Convection Model (RCM) simulations with injection at premidnight and midnight have been conducted. The simulations successfully explain the stronger electron injection, weaker ion injection, and formation of SAID for injection at premidnight, because injected electrons reach the premidnight inner magnetosphere and form a narrower separation between the ion and electron inner boundaries. We suggest that substorms and particle injections extending far duskward away from midnight offer a condition for creating STEVE and SAID due to stronger electron injection to premidnight. The THEMIS all‐sky imager network identified the east‐west length of the STEVE arc to be ~1900 km (~2.5 h magnetic local time) and the duration to be 1–1.5 h.

A Mechanism for the STEVE Continuum Emission

AbstractWe describe a mechanism to explain the subauroral emission feature called STEVE (Strong Thermal Emission Velocity Enhancement), with a focus on its continuum spectrum. Spacecraft observations show that emissions co‐occur with typically invisible plasma flows known as subauroral ion drifts. If these flows are fast enough, nitrogen molecules are vibrationally excited by collisions with ions, overcoming the activation energy of the N2+O→NO+N reaction. The resulting NO combines with ambient O, producing NO2 and spectrally broad light. Importantly, this mechanism also produces N, which reduces the lifetime of NO from hours to seconds and thus explains why the emission is confined to a discrete arc. The predicted emission altitude ( 130 km) and occurrence conditions ( 4‐km/s flows) match well with observations. We simulate this mechanism using a simple photochemical model to demonstrate its validity. This mechanism is initiated by fast ion flows and is thus distinct from auroral and airglow processes.

Early Ground‐Based Work by Auroral Pioneer Carl Størmer on the High‐Altitude Detached Subauroral Arcs Now Known as “STEVE”

AbstractSTEVE (Strong Thermal Emission Velocity Enhancement) phenomenon related structures have received much attention from space weather audiences in recent years. MacDonald et al. (2018, https://doi.org/10.1126/sciadv.aaq0030), reports on a link between these ionospheric structures and the subauroral ion drift for the first time. This commentary summarizes previously overlooked observations of a distinct type of detached subauroral arc by the aurora pioneer Carl Størmer in Norway between 1911 and 1940, including coordinated simultaneous multistation‐based altitude measurements. This commentary points out that his phenomenological descriptions and height measurements show a striking resemblance to modern descriptions and measurements of STEVE.

НАБЛЮДЕНИЕ БЫСТРЫХ СУБАВРОРАЛЬНЫХ ДРЕЙФОВ ИОНОСФЕРНОЙ ПЛАЗМЫ ПО ДАННЫМ ЯКУТСКОЙ МЕРИДИОНАЛЬНОЙ ЦЕПОЧКИ СТАНЦИЙ

Using long-term data from Yakut meridional chain of Yakutsk — Zhigansk — Batagay — Tixie ionospheric stations, we study ionospheric signatures of fast subauroral ion drift. Sharp drops or “falls” of critical frequencies (FCF) of the ionospheric F layer are shown to be one of the main signatures of the development of fast subauroral ion drifts near or at the zenith of the observation station. Comparison between long-term ground-based and satellite measurements indicates that there is good agreement between seasonal variation in the probability of occurrence of FCF derived from ground-based data and subauroral ion drifts derived from DMSP satellite data. Such a coincidence implies that both satellite and ground-based measurement methods register the same phenomenon in the boundary layers of the plasmasphere, namely, the appearance and development of electric fields of magnetospheric origin. The local time for recording of falls of the critical frequency derived from the ground-based data is shown to closely coincide with the appearance time of subauroral polarization streams of plasma according to satellite data. We can therefore conclude that most of the observed FCFs derived from ground-based data refer to intense storms.

FAST SUBAURORAL DRIFTS OF IONOSPHERE PLASMA ACCORDING TO DATA FROM YAKUT MERIDIONAL CHAIN OF STATIONS

Using long-term data from Yakut meridional chain of Yakutsk — Zhigansk — Batagay — Tixie ionospheric stations, we study ionospheric signatures of fast subauroral ion drift. Sharp drops or “falls” of critical frequencies (FCF) of the ionospheric F layer are shown to be one of the main signatures of the development of fast subauroral ion drifts near or at the zenith of the observation station. Comparison between long-term ground-based and satellite measurements indicates that there is good agreement between seasonal variation in the probability of occurrence of FCF derived from ground-based data and subauroral ion drifts derived from DMSP satellite data. Such a coincidence implies that both satellite and ground-based measurement methods register the same phenomenon in the boundary layers of the plasmasphere, namely, the appearance and development of electric fields of magnetospheric origin. The local time for recording of falls of the critical frequency derived from the ground-based data is shown to closely coincide with the appearance time of subauroral polarization streams of plasma according to satellite data. We can therefore conclude that most of the observed FCFs derived from ground-based data refer to intense storms.

Identifying STEVE's Magnetospheric Driver Using Conjugate Observations in the Magnetosphere and on the Ground

AbstractThe magnetospheric driver of strong thermal emission velocity enhancement (STEVE) is investigated using conjugate observations when Van Allen Probes' footprint directly crossed both STEVE and stable red aurora (SAR) arc. In the ionosphere, STEVE is associated with subauroral ion drift features, including electron temperature peak, density gradient, and westward ion flow. The SAR arc at lower latitudes corresponds to regions inside the plasmapause with isotropic plasma heating, which causes redline‐only SAR emission via heat conduction. STEVE corresponds to the sharp plasmapause boundary containing quasi‐static subauroral ion drift electric field and parallel‐accelerated electrons by kinetic Alfvén waves. These parallel electrons could precipitate and be accelerated via auroral acceleration processes powered by Alfvén waves propagating along the magnetic field with the plasmapause as a waveguide. The electron precipitation, superimposed on the heat conduction, could explain multiwavelength continuous STEVE emission. The green picket‐fence emissions are likely optical manifestations of electron precipitation associated with wave structures traveling along the plasmapause.

The Magnetospheric Driving Source of Double‐Peak Subauroral Ion Drifts: Double Ring Current Pressure Peaks

AbstractDouble‐peak subauroral ion drifts (DSAIDs), characterized by two high‐speed flow channels, is a newly identified flow structure in the subauroral ionosphere. He et al. (2016, https://doi.org/10.1002/2016GL069133) proposed that two region 2 field‐aligned currents (R2 FACs) might cause the DSAIDs. However, the underlying physical process that drives the double R2 FACs is unknown. This study reports a DSAIDs event and reveals its magnetospheric drivers. Defense Meteorological Satellite Program F18 satellite observed DSAIDs in the duskside subauroral region, which corresponded well to two low‐density troughs and two R2 FACs. The Van Allen Probe B demonstrated that intense substorm ion injections recurrently occurred prior to the formation of DSAIDs, suggesting a potential magnetospheric driver of DSAIDs. Simulation confirms that recurrent ion injections intensify the partial ring current and create double pressure peaks in the near‐Earth dusk‐to‐midnight region, leading two R2 FACs to flow into the ionosphere. The two R2 FACs are thus responsible for the DSAIDs formation. This study unveils the generation mechanism of DSAIDs and deepens the knowledge of the complex magnetosphere‐ionosphere system.

Abnormal Subauroral Ion Drifts (ASAID) and Pi2s During Cross‐Tail Current Disruptions Observed by Polar on the Magnetically Quiet Days of October 2003

AbstractIn this study we further investigate the development of abnormal subauroral ion drifts/polarization streams (ASAID/ASAPS) during the magnetically quiet days of 9–10 October 2003. Our main aim is to provide direct observational evidence verifying that the Defense Meteorological Satellite Program F15 detected quiet‐time ASAID/ASAPS features developed due to the formation of a magnetospheric cold plasma ripple‐hot ring current interface. By utilizing multisatellite and multi‐instrument Cluster_C2, Polar, and Defense Meteorological Satellite Program observations, we investigate plasma rippling and Pi2 pulsation events. In good agreement with the associated empirical scenario of plasma sheet rippling, we demonstrate with various C2 and Polar events the following observational results. (1) The rippling of the plasma sheet's inner edge was related to the process of cross‐tail current disruption and its earthward propagation. (2) The Pi2 pulsation of the cross‐tail EY field reversed the local convection flow when EY was dawnward directed and thus caused plasma ripples while (3) energy was provided by the earthward flows' braking/deceleration. (4) The ASAID/ASAPS feature's underlying EY is antisunward (or dawnward) and EZ is inward (or earthward). By tracking the cross‐tail current disruptions' tailward (by Cluster_C2) and earthward (by Polar) propagations, almost simultaneously, our results demonstrate also that the convection flows' local reversal by the dawnward EY is a common feature of both the tailward and the earthward flows. This study demonstrates also, with multiple plasmapause and EY detections made by Polar, the multiple and mixed occurrences of ASAID and SAID in the plasmasphere's rippled outer edge.

Steve: The Optical Signature of Intense Subauroral Ion Drifts

AbstractLittle is currently known about the optical phenomenon known as Steve. The first scientific publication on the subject suggests that Steve is associated with an intense subauroral ion drift (SAID). However, additional inquiry is warranted as this suggested relationship as it is based on a single case study. Here we present eight occurrences of Steve with coincident or near‐coincident measurements from the European Space Agency's Swarm satellites and show that Steve is consistently associated with SAID. When satellite observations coincident with Steve are compared to that of typical SAID, we find the SAID associated with Steve to have above average peak ion velocities and electron temperatures, as well as extremely low plasma densities.

Magnetospheric Signatures of STEVE: Implications for the Magnetospheric Energy Source and Interhemispheric Conjugacy

AbstractWe present three STEVE (strong thermal emission velocity enhancement) events in conjunction with Time History of Events and Macroscale Interactions (THEMIS) in the magnetosphere and Defense Meteorological Satellite Program (DMSP) and Swarm in the ionosphere, for determining equatorial and interhemispheric signatures of the STEVE purple/mauve arc and picket fence. Both types of STEVE emissions are associated with subauroral ion drifts (SAID), electron heating, and plasma waves. The magnetosphere observations show structured electrons and flows and waves (likely kinetic Alfven, magnetosonic, or lower‐hybrid waves) just outside the plasmasphere. Interestingly, the event with the picket fence had a >~1 keV electron structure detached from the electron plasma sheet, upward field‐aligned currents (FACs), and ultraviolet emissions in the conjugate hemisphere, while the event with only the mauve arc did not have precipitation or ultraviolet emission. We suggest that the electron precipitation drives the picket fence, and heating drives the mauve as thermal emission.

Multi-point observations and modeling of subauroral polarization streams (SAPS) and double-peak subauroral ion drifts (DSAIDs): A case study

While subauroral polarization streams (SAPS) are well recognized as representatively one of the most important features of magnetosphere-ionosphere (M-I) coupling processes in the subauroral region, the double-peak subauroral ion drifts (DSAIDs) is a newly recognized ionospheric phenomenon, categorized as a subset of subauroral ion drifts (SAIDs). In this study, we investigate both SAPS and DSAIDs that appear during the storm main phase of the 17 March 2015 event through a combination of multi-point observations and numerical simulations. We find that when SAPS/DSAIDs are observed by the DMSP spacecraft near the dusk subauroral region, strong electric fields are detected minutes later by the Van Allen Probes almost in the same conjugate region near the equatorial plane. Numerical simulations are carried out not only to reveal the global context and dynamic evolution of the SAPS in both the magnetospheric and ionospheric systems, but also to aid the understanding of the effect of conductance on the DSAIDs. Our results confirm that SAPS are indeed associated with Region 2 field-aligned currents (FACs) flowing into the low conductance region. On the other hand, the DSAIDs may be related to the double-conductance-trough in the subauroral region.

Investigating the Development of Abnormal Subauroral Ion Drifts (ASAID) During the Magnetically Quiet Times of October 2003

AbstractAppearing as inverted subauroral ion drifts (SAID) during magnetically disturbed and quiet times, the abnormal SAID (ASAID) is a recently reported phenomenon. Disturbed‐time ASAID development can be explained with a magnetospheric cold‐plasma‐shoulder‐hot‐ring‐current interface layer that generates an earthward (or inward) magnetospheric electric (E) field, which maps down to the ionospheric cold‐plasma‐trough‐hot‐ring‐current interface layer as a southward (or equatorward) ionospheric E field. However, this plasmaspheric shoulder cannot develop under magnetically quiet conditions. Therefore, the quiet time ASAID development is still not clear and our main aim is to unravel its development. We analyze multisatellite and multiinstrument observations and show four scenarios demonstrating ASAID events during the magnetically quiet period of 9–11 October 2003. Results obtained for the longer 9–12 October 2003 time period provide observational evidence of the continuous presence of solar wind Alfven waves (i) driving dayside magnetopause and nightside magnetotail reconnections, (ii) bouncing back and forth in the coupled magnetosphere and ionosphere, and thus (iii) creating bouncing Alfven waves that ripple the plasma sheet's inner edge during the ASAID events investigated. These observational results suggest that the bouncing Alfven waves led to the development of a magnetospheric cold‐plasma‐ripple‐hot‐ring‐current interface layer during these quiet times leading to quiet time ASAID development. Confirming this suggestion, we demonstrate and analyze 10 February and 5 March 1991 scenarios, depicting ASAID events, and invoke published magnetospheric cross‐tail EY observations evidencing bouncing Alfven waves and plasma sheet rippling during the underlying energetic particle injection events.

A Statistical Analysis of STEVE

AbstractThere has been an exciting recent development in auroral research associated with the discovery of a new subauroral phenomenon called STEVE (Strong Thermal Emission Velocity Enhancement). Although STEVE has been documented by amateur night sky watchers for decades, it is as yet an unidentified upper atmosphere phenomenon. Observed first by amateur auroral photographers, STEVE appears as a narrow luminous structure across the night sky over thousands of kilometers in the east‐west direction. In this paper, we present the first statistical analysis of the properties of 28 STEVE events identified using Time History of Events and Macroscale Interactions during Substorms (THEMIS) all‐sky imager and the Redline Emission Geospace Observatory (REGO) database. We find that STEVE occurs about 1 hr after substorm onset at the end of a prolonged expansion phase. On average, the AL index magnitude is larger and the expansion phase has a longer duration for STEVE events compared to subauroral ion drifts or substorms. The average duration for STEVE is about 1 hr, and its latitudinal width is ~20 km, which corresponds to ~¼ of the width of narrow auroral structures like streamers. STEVE typically has an equatorward displacement from its initial location of about 50 km and a longitudinal extent of 2,145 km.