Earthward Flow Research Articles

Plasma Sheet Counterparts for Auroral Beads and Vortices in Advance of Fast Flows: New Evidence for Near‐Earth Substorm Onset

AbstractThe relationship between auroral, ground, and plasma sheet signatures in the late growth phase is crucial for understanding the sequence of events during a substorm expansion phase onset. Here we show conjugate ground‐auroral‐satellite observations of a substorm that occurred on 18 September 2021, between 04:45 and 05:00 UT, where four auroral activations were detected in the all‐sky imagers. An initial activation showed the brightening of an equatorward arc within the cutoff of the 630 nm emissions, indicating activity on closed field lines well inside the open‐closed field line boundary (OCFLB). During a second activation, auroral beads were observed on a brightening arc, equatorward and within the OCFLB, followed by the transformation from small‐scale to large‐scale vortices. The tail current sheet was highly disturbed during the auroral vortex evolution, including pressure and magnetic disturbances, an apparent broadening of a previously thin current sheet, and a breakdown of the frozen‐in condition. Our observations clearly show late growth phase dynamics, including arc brightenings, the formation of auroral beads, and auroral vortex development, can occur well in advance of fast Earthward flows in the tail. Indeed, it is only during that later activity that auroral breakup and strong Earthward flows, which we associate with magnetic reconnection further down the tail, are observed together with strong magnetic bays on the ground. The sequence of events is consistent with an inside‐to‐outside model at substorm expansion phase onset, most likely via a shear‐flow ballooning instability in the transition region from dipole to tail‐like fields in the near‐Earth plasma sheet.

Reconnection Inside a Dipolarization Front of a Diverging Earthward Fast Flow

AbstractWe examine a Dipolarization Front (DF) event with an embedded electron diffusion region (EDR), observed by the Magnetospheric Multiscale (MMS) spacecraft on 08 September 2018 at 14:51:30 UT in the Earth's magnetotail by applying multi‐scale multipoint analysis methods. In order to study the large‐scale context of this DF, we use conjunction observations of the Cluster spacecraft together with MMS. A polynomial magnetic field reconstruction technique is applied to MMS data to characterize the embedded electron current sheet including its velocity and the X‐line exhaust opening angle. Our results show that the MMS and Cluster spacecraft were located in two counter‐rotating vortex flows, and such flows may distort a flux tube in a way that the local magnetic shear angle is increased and localized magnetic reconnection may be triggered. Using multi‐point data from MMS we further show that the local normalized reconnection rate is in the range of R ∼ 0.16 to 0.18. We find a highly asymmetric electron in‐ and outflow structure, consistent with previous simulations on strong guide‐field reconnection events. This study shows that magnetic reconnection may not only take place at large‐scale stable magnetopause or magnetotail current sheets but also in transient localized current sheets, produced as a consequence of the interaction between the fast Earthward flows and the Earth's dipole field.

Fast Earthward Convection in the Magnetotail and Nonzero IMF By: MMS Statistics

AbstractWe statistically investigate convective earthward fast flows using data measured by the Magnetospheric Multiscale mission in the tail plasma sheet during 2017–2021. We focus on “frozen in” fast flows and investigate the importance of different electric field components in the Sun‐Earth (V⊥x) and dusk‐dawn (V⊥y) velocity components perpendicular to the magnetic field. We find that a majority of the fast flow events (52% of 429) have the north‐south electric field component (Ez) as the most relevant or dominating component whereas 26% are so‐called conventional type fast flows with Ey and Ex as the relevant components. The rest of the flow events, 22%, fall into the two ’mixed’ categories, of which almost all these fast flows, 20% of 429, have Ey and Ez important for V⊥x and V⊥y, respectively. There is no Y‐location preference for any type of the fast flows. The conventional fast flows are detected rather close to the neutral sheet whereas the other types can be measured farther away. Typical total speeds are highest in the mixed category. Typical perpendicular speeds are comparably high in the conventional and mixed categories. The slowest fast flows are measured in the Ez category. Most of the fast flow events are measured in the substorm recovery phase. Prevailing interplanetary magnetic field By conditions influence the V⊥y direction and the influence is most efficient for the Ez‐dominated fast flows.

A Statistical Analysis of the Auroral Streamer Current System

AbstractMagnetosphere‐ionosphere coupling is an important process that involves the transport of vast quantities of mass, energy, and momentum in the Earth's near‐space environment. This transport involves extended, global sheets of electrical current that flows along the magnetic field lines. Many studies have discussed the magnetosphere‐ionosphere coupling between the fast‐moving earthward flows and the north‐south oriented streamers in the ionosphere. However, only recently, the temporal evolution of the ionospheric current pattern, from the start to the end of the streamer, has been investigated through case studies, by combining the ionospheric current pattern with the magnetotail fast‐moving earthward flows. In this study, we show the statistical spatial and temporal evolution of the ionospheric current pattern associated with north‐south oriented streamers using simultaneous THEMIS (Time History of Events and Macroscale Interaction during Substorms) observations of fast‐moving earthward flows within the magnetotail, Geostationary Operational Environmental Satellite observations, auroral all‐sky images, and ionospheric equivalent current maps derived from ground magnetometer measurements. We show that the streamer related ionospheric field‐aligned currents (FACs) are statistically almost equivalent in magnitude for the 38 events that were analyzed. Results show that the streamer related ionospheric FACs are statistically approximately similar in magnitude for 38 events. The duration of these events ranges from 2 to 21 min with a median value of 9 min.

Ion Energization by Turbulent Electric Fields in Fast Earthward Flows and Its Implications for the Dynamics of the Inner Magnetosphere

AbstractLarge‐amplitude electric fields (>50 mV/m) typical to bursty bulk flow (BBF) braking regions of the Earth's magnetotail can accelerate energetic electrons and ions to many times their initial thermal energies. We follow up on the Usanova and Ergun (2022), https://doi.org/10.1029/2022JA030336, study of electron energization and examine wave and plasma observations from the THEMIS satellites over four tail seasons to investigate the transfer of BBF energy to ions by turbulent electric fields. The results show that the large‐amplitude electric fields are accompanied by an ion temperature increase of ∼50% when compared to times when the turbulence is not observed. Electric field turbulence is also associated with a roughly ten‐fold increase in temperature fluctuations and a five‐fold increase in variations of energetic ion fluxes. We discuss the contribution of this turbulent energy transfer process to the dynamics of energetic ions in the magnetosphere.

Magnetotail Ion Acceleration in 3‐D Global Hybrid Simulations

AbstractParticle injection by magnetotail reconnection plays an important role in the magnetospheric physics, since these particles may be accelerated to high energy and constituent energetic particles in the inner magnetosphere. In this paper, we trace such ion particles from a near‐Earth reconnection region to the inner magnetosphere self‐consistently in a 3‐D global hybrid simulation using ANGIE3D, and demonstrate the important roles of the earthward fast flows in the ion acceleration process in the tail plasma sheet. Although the particles can gain several times of their initial energy from the reconnection X‐line region, a dramatic increase of the ion energy (from a few keV up to a few tens of keV) occurs in a very short period of time due to their encounter with the earthward fast flows. A large portion (≳70%) of the earthward moving particles around the focused reconnection site encounter fast flows and are significantly accelerated. Our results indicate that fast flow electric fields play major roles, as opposed to other common mechanisms such as the adiabatic Betatron and Fermi processes, in ion acceleration from the near‐tail region to the inner magnetosphere. In addition, dipolarization fronts associated with magnetic reconnection also affect the particle acceleration. On the global scale, it is found that particles can encounter the reconnection region first and then return to the fast flows multiple times, and thus the acceleration is through multiple local acceleration processes, leading to particle energy ∼50 keV by reconnection/fast flows in the tail. The global simulation shows that particle acceleration involves multiple regions during their injection into the ring current.

Statistical Survey of Magnetic Forces Associated With Earthward Bursty Bulk Flows Measured by MMS 2017–2021

AbstractWe investigate the magnetic forces (the magnetic pressure gradient force, the curvature force, and their sum the j × B‐force) associated with earthward bursty bulk flows (BBFs) using Magnetospheric Multiscale (MMS) data from five tail seasons (2017–2021). For the first time, the magnetic forces are inferred downtail of XGSM = −20 RE and in the GSM XY and YZ planes. The results suggest that BBFs tend to be accelerated earthward by the magnetic pressure gradient force tailward of ∼19 RE and decelerated closer to that distance in the 2017–2018 data. The force magnitudes increase with distance. This is in accordance with earlier Cluster results. In the 2019–2021 data, the pressure gradient force magnitudes are generally smaller and no clear distance for the acceleration reversal can be determined. The curvature forces for both 2017–2018 and 2019–2021 BBFs indicate earthward acceleration independent of distance, consistent with the Cluster measurements. The sum, the j × B‐force, suggests for the 2017–2018 BBFs earthward acceleration tailward of XGSM ∼15 RE and deceleration within that distance, also consistent with Cluster. In contrast, the 2019–2021 BBFs show general earthward acceleration by j × B independent of distance. In the GSM XY plane, the average (j × B)xy vectors are earthward, and in the premidnight and postmidnight dawnward for the 2017–2018 BBFs. For 2019–2021, the average (j × B)xy vectors have components toward the tail center. In the GSM YZ plane, the average (j × B)yz vectors are toward the neutral sheet.

Quasi-parallel Whistler Waves and Their Interaction with Resonant Electrons during High-velocity Bulk Flows in the Earth’s Magnetotail

In collisionless space, plasma waves are important channels of energy conversion, affecting the local particle velocity distribution functions through wave–particle interactions. In this paper we present a comparative statistical analysis of the characteristics of quasi-parallel narrowband whistler waves and the properties of resonant electrons interacting with these waves during the intervals of earthward and tailward high-velocity bulk flows produced by the near-Earth X-line and observed by Magnetospheric Multiscale Mission spacecraft. We found that on both sides of the X-line, the suprathermal electrons (≥1 keV) having large pitch angles make the major contribution to the maximal growth rate (γ) of these waves. The whistler waves were observed almost simultaneously with strong enhancements of perpendicular magnetic gradients localized at electron scales near dipolarization fronts associated with the earthward bulk flows, and near flux ropes/magnetic islands embedded into the tailward bulk flows. Betatron energization of electrons due to the appearance of such gradients increases the perpendicular anisotropy of electron distribution, which could be responsible for the whistler wave generation. We found that in the course of electron interactions with the whistler waves the lower-energy resonant electrons can transfer a part of their kinetic energy to the higher-energy electrons, especially in the Central Plasma Sheet. This results in formation/enhancement of energy-dependent perpendicular anisotropy and power-law tails in the high-energy range of electron velocity distribution. We conclude that despite the differences in the magnetic structure of the earthward and tailward bulk flows, the mechanisms of the quasi-parallel whistler wave generation and the properties of resonant electrons are quite similar.

Statistical Characterization of the Dynamic Near‐Earth Plasma Sheet Relative to Ultra‐Low Frequency (ULF) Wave Growth at Substorm Onset

AbstractMagnetospheric substorms are a complex phenomenon. During the initial stages of a substorm a variety of important processes occur in near‐Earth space within a span of several minutes. The relative timing and links between these processes are critical to understanding how, where and when substorms may occur. One of the first observed signatures at substorm onset is the exponential increase in ULF (Ultra‐Low Frequency) wave power in the near‐Earth magnetotail (e.g., −7.5 ≤ XGSM ≤ −12.5 RE). We use the Time History of Events and Macroscale Interactions during Substorms spacecraft to examine the conditions in the magnetotail plasma sheet before, during and after local ULF wave growth. Prior to the ULF wave growth, the magnetotail stretches with convectional flows dominating. We then find strong earthward and azimuthal flows that peak at a similar time to the peak ULF wave power. These flows are found to be faster in the mid‐tail (−10 ≤ XGSM ≤ −12.5 RE) than the near‐tail (−7.5 ≤ XGSM ≤ −10 RE). Examining the local plasma energy density (magnetic, thermal and kinetic), we find no statistical decrease that could explain the exponentially growing ULF waves, in fact the local energy density is found to increase. This suggests that there may be an injection of energy from elsewhere in the magnetotail. Following the peak ULF wave power the tail is seen to dipolarize, and the local energy density is enhanced.

First 3D hybrid-Vlasov global simulation of auroral proton precipitation and comparison with satellite observations

The precipitation of charged particles from the magnetosphere into the ionosphere is one of the crucial coupling mechanisms between these two regions of geospace and is associated with multiple space weather effects, such as global navigation satellite system signal disruption and geomagnetically induced currents at ground level. While precipitating particle fluxes have been measured by numerous spacecraft missions over the past decades, it often remains difficult to obtain global precipitation patterns with a good time resolution during a substorm. Numerical simulations can help to bridge this gap and improve the understanding of mechanisms leading to particle precipitation at high latitudes through the global view they offer on the near-Earth space system. We present the first results on auroral (0.5–50 keV) proton precipitation within a 3-dimensional simulation of the Vlasiator hybrid-Vlasov model. The run is driven by southward interplanetary magnetic field conditions with constant solar wind parameters. We find that on the dayside, cusp proton precipitation exhibits the expected energy–latitude dispersion and takes place in the form of successive bursts associated with the transit of flux transfer events formed through dayside magnetopause reconnection. On the nightside, the precipitation takes place within the expected range of geomagnetic latitudes, and it appears clearly that the precipitating particle injection is taking place within a narrow magnetic local time span, associated with fast Earthward plasma flows in the near-Earth magnetotail. Finally, the simulated precipitating fluxes are compared to observations from Defense Meteorological Satellite Program spacecraft during driving conditions similar to those in the simulation and are found to be in good agreement with the measurements.

Dynamics of Earth's bow shock under near-radial interplanetary magnetic field conditions

We investigate the dynamics of Earth's quasi-parallel terrestrial bow shock based on measurements from the Magnetospheric MultiScale (MMS) spacecraft constellation during a period of near-radial interplanetary magnetic conditions, when the interplanetary magnetic field and the solar wind (SW) velocity are nearly anti-parallel. High-speed earthward ion flows with properties that are similar to those of the pristine SW are observed to be embedded within the magnetosheath-like plasma. These flows are accompanied by Interplanetary Magnetic Field (IMF) intensity of less than about 10 nT, compared to nearby magnetosheath intensities of generally greater than 10 nT. The high-speed flow intervals are bounded at their leading and trailing edges by intense fluxes of more energetic ions and large amplitude quasi-sinusoidal magnetic oscillations, similar to ultra-low frequency waves known to steepen and pileup on approach toward Earth to form the quasi-parallel bow shock. The MMS string-of-pearls configuration is aligned with the outbound trajectory and provides inter-spacecraft separations of several hundred km along its near 103 length, allowing sequential observation of the plasma and magnetic field signatures during the event by the four spacecraft. The SW-like interval is most distinct at the outer-most MMS-2 and sequentially less distinct at each of the trailing MMS spacecraft. We discuss the interpretation of this event alternatively as MMS having observed a quasi-rigid bow shock contraction/expansion cycle, ripples or undulations propagating on the bow shock surface, or a more spatially local evolution in the context of either a deeply deformed shock surface or a porous shock surface, as in the three-dimensional patchwork concept of the quasi-parallel bow shock, under the extant near-radial IMF condition.

Magnetosphere‐Ionosphere Coupling Between North‐South Propagating Streamers and High‐Speed Earthward Flows

AbstractThe magnetosphere‐ionosphere coupling between the high‐speed earthward flows and the north‐south oriented streamers in the ionosphere has been discussed for decades, but to date, no one has examined the formation of that coupling from the start of the streamer to the end. We investigate the formation and development of the magnetosphere‐ionosphere coupling using THEMIS observations of high‐speed earthward flows within the magnetotail, simultaneous auroral all‐sky images, and ionospheric equivalent current maps derived from ground magnetometer measurements. We show the formation of a downward field‐aligned current on the dawnside of the north‐south streamer and the upward current on the duskside, as well as the vortices within the equivalent currents around the field‐align‐like currents from the poleward boundary to the equatorward edge of the auroral oval for two substorm streamer events. By removing the background Birkeland current system, we can determine the current densities uniquely associated with the streamer current wedge and demonstrate that the downward and upward currents within the streamer are approximately balanced for one event. Furthermore, we find that the longitudinal size of the streamer current wedge is more transient and localized, and does not change, whereas the substorm current wedge is larger and expands during the first part of the substorm.

Newly Formed Dawnside, Duskside, and Nightside Subauroral Flows Developed During Magnetically Active Times

AbstractWe study the newly formed (with an upper limit of ∼101 min) subauroral flows in the topside ionosphere. These include the dawnside subauroral flows observed at ∼5–6 magnetic local time (MLT), the duskside sub‐auroral polarization streams (SAPS) at ∼15 MLT, and the nightside sub‐auroral ion drifts (SAID) at ∼19–20 MLT. We employ the same types of observations for investigating the various underlying magnetospheric processes related to the development of the newly formed subauroral flows. These processes include (a) the stretching of magnetic field lines (leading to plasma density dropouts) due to the unfolding magnetotail reconnection, (b) the dawnside and nightside diversions of the magnetotail‐reconnection—related earthward plasma flows, (c) the ongoing particle injections, and (d) the trapping of ring current protons at subauroral and auroral latitudes. Based on these processes (a–d), we found strong similarities in the development of the newly formed dawnside and duskside/nightside subauroral flows investigated. At the dawnside plasmapause in the inner magnetosphere equatorial plane, the outward and duskward electric (E) field components provide observational evidence that the newly formed dawnside rapid narrow subauroral flows were SAID developing in the dawn sector. Evidence shows that the newly formed dawnside broad subauroral flow channel was associated with upward Region 2 (R2) field‐aligned currents (FACs) and equatorward subauroral polarization E field, and thus, was the dawnside equivalent of the duskside SAPS associated with downward R2 FACs and poleward subauroral polarization E field. Further evidence includes the inner magnetosphere observations of the hot zone both on the dusk/nightside and on the dawnside.

Multiple conjugate observations of magnetospheric fast flow bursts using THEMIS observations

Abstract. Magnetotail earthward fast flow bursts can transport most magnetic flux and energy into the inner magnetosphere. These fast flow bursts are generally an order of magnitude higher than the typical convection speeds that are azimuthally localised (1–3 RE) and are flanked by plasma vortices, which map to ionospheric plasma vortices of the same sense of rotation. This study uses a multipoint analysis of conjugate magnetospheric and ionospheric observations to investigate the magnetospheric and ionospheric responses to fast flow bursts that are associated with both substorms and pseudobreakups. We study in detail what properties control the differences in the magnetosphere–ionosphere responses between substorm fast flow bursts and pseudobreakup events, and how these differences lead to different ionospheric responses. The fast flow bursts and pseudobreakup events were observed by the Time History of Events and Macroscale Interaction during Substorms (THEMIS), while the primary ionospheric observations were made by all-sky cameras and magnetometer-based equivalent ionospheric currents. These events were selected when the satellites were at least 6 RE from the Earth in radial distance and a magnetic local time (MLT) region of ± 5 h from local midnight. The results show that the magnetosphere and ionosphere responses to substorm fast flow bursts are much stronger and more structured compared to pseudobreakups, which are more likely to be localised, transient and weak in the magnetosphere. The magnetic flux in the tail is much stronger for strong substorms and much weaker for pseudobreakup events. The Blobe decreases significantly for substorm fast flow bursts compared to pseudobreakup events. The curvature force density for pseudobreakups are much smaller than substorm fast flow events, indicating that the pseudobreakups may not be able to penetrate deep into the inner magnetosphere. This association can help us study the properties and activity of the magnetospheric earthward flow vortices from ground data.

New Insights Into the Substorm Initiation Sequence From the Spatio-Temporal Development of Auroral Electrojets.

In the present study we examine three substorm events, Events 1–3, focusing on the spatio‐temporal development of auroral electrojets (AEJs) before auroral breakup. In Events 1 and 2, auroral breakup was preceded by the equatorward motion of an auroral form, and the ground magnetic field changed northward and southward in the west and east of the expected equatorward flow, respectively. Provided that these magnetic disturbances were caused by local ionospheric Hall currents, this feature suggests that the equatorward flow turned both eastward and westward as it reached the equatorward part of the auroral oval. The auroral breakup took place at the eastward‐turning and westward‐turning branches in Events 1 and 2, respectively, and after the auroral breakup, the westward AEJ enhanced only on the same side of the flow demarcation meridian. The zonal flow divergence is considered as an ionospheric manifestation of the braking of an earthward flow burst in the near‐Earth plasma sheet and subsequent dawnward and duskward turning. Therefore, in Events 1 and 2, the auroral breakup presumably mapped to the dawnward and duskward flow branches, respectively. Moreover, for Event 3, we do not find any pre‐onset auroral or magnetic features that can be associated with an equatorward flow. These findings suggest that the braking of a pre‐onset earthward flow burst itself is not the direct cause of substorm onset, and therefore, the wedge current system that forms at substorm onset is distinct from the one that is considered to form as a consequence of the flow braking.

On the Magnetic Dip Ahead of the Dipolarization Fronts

AbstractDipolarization front (DF), a transient structure that is associated with the earthward plasma flow, is widely considered as a result of magnetic reconnection. It is often characterized with a sharp increase in the northward magnetic field Bz preceded by a minor decrease of it. However, the small magnetic dip ahead of the DF is not always present, the reason of which is not well known. By analyzing in site Magnetospheric MultiScale spacecraft measurements at the magnetotail, we present two events of DF with and without a dip ahead of it. It is found that the magnetic dip ahead of DF is accompanied by a guide field By. To investigate the physical mechanism that determines the appearance of the dip, we perform kinetic simulations of symmetric single X line magnetic reconnection under various guide fields. The simulation results show that the dip is generated due to the separation of the gyro‐motion between ions and electrons under the influence of the guide field. Ions are reflected at the DF and further deviated by the guide field away from the neutral sheet, while more electrons remain in the neutral sheet due to their small gyroradius. In the absence of a compensating ion current, a dawnward current thus is formed, carried by the electrons, and generates the dip. Our results bring in a new perspective on the formation of the dip in single X line reconnection.

The Influence of Localized Dynamics on Dusk‐Dawn Convection in the Earth’s Magnetotail

AbstractPrevious work has shown that earthward convective flow bursts in the magnetotail have a dusk‐dawn () component that is controlled by the historical state of the Interplanetary Magnetic Field (IMF) By component. Here, we analyze 27 years of Cluster, THEMIS and Geotail plasma and magnetic field data and identify 1,639 magnetotail fast flow “detections” that demonstrate a dusk‐dawn asymmetry. We find that ∼70% has a dusk‐dawn direction consistent with that expected from the penetration of IMF By. Superposed epoch analysis suggests that the inconsistency of the remaining ∼30% is not due to a lack of the expected IMF By penetration. Instead, we find that on average, the expected sense of IMF By penetration is associated with flows irrespective of whether those flows agree or disagree with the expected dusk‐dawn asymmetry. IMF By, and the penetrated By do, however, tend to be stronger for “agree” flows. Detections which agree (disagree) tend to be accompanied by a localized perturbation to the By component of the magnetotail magnetic field in the same sign as (opposite to) the prevailing IMF By conditions, which temporarily enhances (overrides) the penetrated field. Agree (disagree) flows also appear to be observed further away from (closer to) the neutral sheet (Bx = 0) and are associated with weaker (stronger) magnetic field dipolarization. Finally, we find that the slower “background” convective flow has an average direction which is consistent with penetration of the expected IMF By, regardless of whether the fast flow itself agrees or disagrees.

Investigation of the homogeneity of energy conversion processes at dipolarization fronts from MMS measurements

We report on six dipolarization fronts (DFs) embedded in fast earthward flows detected by the Magnetospheric Multiscale mission during a substorm event on 23 July 2017. We analyzed Ohm's law for each event and found that ions are mostly decoupled from the magnetic field by Hall fields. However, the electron pressure gradient term is also contributing to the ion decoupling and likely responsible for an electron decoupling at DF. We also analyzed the energy conversion process and found that the energy in the spacecraft frame is transferred from the electromagnetic field to the plasma (J·E>0) ahead or at the DF, whereas it is the opposite (J·E<0) behind the front. This reversal is mainly due to a local reversal of the cross-tail current indicating a substructure of the DF. In the fluid frame, we found that the energy is mostly transferred from the plasma to the electromagnetic field (J·E′<0) and should contribute to the deceleration of the fast flow. However, we show that the energy conversion process is not homogeneous at the electron scales due to electric field fluctuations likely related to lower-hybrid drift waves. Our results suggest that the role of DF in the global energy cycle of the magnetosphere still deserves more investigation. In particular, statistical studies on DF are required to be carried out with caution due to these electron scale substructures.

The Source of Auroral Omegas

AbstractThe auroral wave‐like structures called “omega bands” appear within the post‐midnight sector auroral oval with shapes resembling the Greek letter omega, and are typically associated with the recovery phase of substorms. Prior work and MHD simulations suggest both high speed earthward flows and post‐midnight flow shears are possible omega band source mechanisms. However, what produces omega bands is not well understood. It is most likely that the paucity of concurrent magnetospheric data has limited our ability to understand fully the mechanism responsible for the generation of the omega bands. We have identified about 263 auroral omegas in seven different THEMIS all sky cameras (ASCs) over a 10 year period. A fraction appear to form from north‐south streamers, but some appear to form from east‐west auroral arcs. Fifty‐one of the 263 Ω also have conjugate or near conjugate THEMIS and GOES spacecraft data. There is evidence in about 80% of these events that high speed earthward flows have occurred prior to or at about the same time as the auroral omega observation. This evidence consists of high speed earthward flows, magnetic field dipolarizations, particle injections into the inner magnetosphere, and auroral streamers. 11 events also show plasma flow data azimuthally along the inner magnetosphere, but for six of these events the Vy component is inconsistent with the auroral omega direction of motion. Of the remaining five events, four have Vy flow shear events but also show evidence of magnetic field dipolarizations, particle injections into the inner magnetosphere, and auroral streamers. Our observations suggest high speed earthward flows are more likely to be the source of auroral omega bands.

Dynamics of variable dusk–dawn flow associated with magnetotail current sheet flapping

Abstract. We present Cluster spacecraft observations from 12 October 2006 of convective plasma flows in the Earth's magnetotail. Earthward flow bursts with a dawnward v⊥y component, observed by Cluster 1 (C1), are inconsistent with the duskward flow that might be expected at the pre-midnight location of the spacecraft. Previous observations have suggested that the dusk–dawn sense of the flow can be governed by the interplanetary magnetic field (IMF) By conditions, with the related “untwisting hypothesis” of magnetotail dynamics commonly invoked to explain this dependence, in terms of a large-scale magnetospheric asymmetry. In the current study, observations of the upstream solar wind conditions from OMNI, magnetic field observations by Cluster and ionospheric convection data using SuperDARN indicate a large-scale magnetospheric morphology consistent with positive IMF By penetration into the magnetotail. At the pre-midnight location of Cluster, however, the dawnward flow observed below the neutral sheet by C1 could only be explained by the untwisting hypothesis in a negative IMF By scenario. The Cluster magnetic field data also reveal a flapping of the magnetotail current sheet, a phenomenon known to influence dusk–dawn flow. Results from the curlometer analysis technique suggest that the dusk–dawn sense of the J×B force was consistent with localised kinks in the magnetic field and the flapping associated with the transient perturbations to the dusk–dawn flow observed by C1. We therefore suggest that the flapping overcame the dusk–dawn sense of the large-scale convection which we would expect to have been net duskward in this case. We conclude that invocation of the untwisting hypothesis may be inappropriate when interpreting intervals of dynamic magnetotail behaviour such as during current sheet flapping, particularly at locations where magnetotail flaring becomes dominant.