Earthward Flow Research Articles

Ion Convection as a Function of Distance to the Neutral Sheet in Earth's Magnetotail

AbstractWe utilized 33 years of data obtained by the Geotail, THEMIS, Cluster and MMS missions to investigate the slow (<200 km/s) ion flows perpendicular to the magnetic field in Earth's magnetotail plasma sheet. By using plasma β as a proxy of distance to the neutral sheet, we find that the ion flow patterns vary systematically within the plasma sheet. Particularly, in regions farther from the neutral sheet, earthward (tailward) flows exhibit a strong tendency to diverge (converge) quasi‐symmetrically, with respect to the midnight meridional plane. As the distance becomes closer toward the neutral sheet, this tendency to diverge and converge gradually weakens. Moreover, duskward flows become the dominant components in both the earthward and tailward flows. These variations in ion flow patterns with distance to neutral sheet are hemispherically independent. We suggest that the spatial profiles of the electric and diamagnetic drift vary with distance to the neutral sheet and are therefore responsible for the varying ion flow patterns.

The Role of Diffuse Electron Precipitation in the Formation of Subauroral Polarization Streams

AbstractThe role of diffuse electron precipitation in the formation of subauroral polarization streams (SAPS) is investigated with the Multiscale Atmosphere‐Geospace Environment (MAGE) model. Diffuse precipitation is derived from the distribution of drifting electrons. SAPS manifest themselves as a separate mesoscale flow channel in the duskside ionosphere, which gradually merges with the primary auroral convection toward dayside as the equatorward auroral boundary approaches the poleward Region‐2 field‐aligned currents (FACs) boundary. SAPS expand to lower latitudes and toward the nightside during the main phase of a geomagnetic storm, associated with magnetotail earthward plasma flows building up the ring current and intensifying Region‐2 FACs and electron precipitation. SAPS shrink poleward and sunward as the interplanetary magnetic field turns northward. When diffuse precipitation is turned off in a controlled MAGE simulation, ring current and duskside Region‐2 FACs become weaker, but subauroral zonal ion drifts are still comparable to auroral convection. However, subauroral and auroral convection manifest as a single broad flow channel without showing any mesoscale structure. SAPS overlap with the downward Region‐2 FACs equatorward of diffuse precipitation, where poleward electric fields are strong due to a low conductance in the subauroral ionosphere. The Region‐2 FACs extend to latitudes lower than the diffuse precipitation because the ring current protons penetrate closer to the Earth than the electrons do. This study reproduces the key physics of SAPS formation and their evolution in the coupled magnetosphere‐ionosphere during a geomagnetic storm. Diffuse electron precipitation is demonstrated to play a critical role in determining SAPS location and structure.

Relevance of the North‐South Electric Field Component in the Propagation of Fast Convective Earthward Flows in the Magnetotail: An Event Study

AbstractFast earthward plasma flows are commonly observed in the magnetotail plasma sheet. These flows are often termed as bursty bulk flows because of their bursty nature, and they are considered to be generated by magnetic reconnection. Close to the neutral sheet (Bx ∼ 0), the fast flows are considered to be associated with an enhanced dawn‐to‐dusk electric field (Ey > 0), which together with the northward magnetic field component (Bz > 0) protrude the plasma earthward via enhanced E × B‐drift. Sometimes, reversals in the dawn‐dusk velocity component perpendicular to the magnetic field (V⊥y) are measured in association with Bx sign changes in the flows. This suggests that the electric field component in the north‐south direction (Ez) can play a role in determining the dawn‐dusk direction of the enhanced drift. We present data measured by the Magnetospheric Multiscale, which demonstrate that Ez can have a dictating role for V⊥y of fast flows. Furthermore, it is shown that the critical contribution of Ez is not limited only to V⊥y, but it can also dominantly determine the enhanced drift of the fast flows in the X direction (V⊥x). The latter can occur also near and at the neutral sheet, which adds an alternative configuration to the conventional picture of Ey and Bz being the main players in driving the earthward fast flows. The domination of Ez in the studied events appears with potential signatures of an influence of a nonzero dawn‐dusk component of the interplanetary magnetic field (IMF By) on the magnetotail.

Tailward Flows in the Vicinity of Fast Earthward Flows

AbstractThe occurrence of tailward flows in the magnetotail plasma sheet is closely linked to the dynamics of earthward bursty bulk flows (BBFs). Tailward flows that are observed in the vicinity of these BBFs (or TWABs – Tailward flows around BBFs) may hold unique information on its origin. In this study, we conduct a statistical survey on TWABs by using data from the Cluster mission. We find that TWABs are observed in the vicinity of ∼75% of the BBFs and their occurrence does not depend on BBF velocity magnitude. TWABs have a flow convection pattern consistent with the general tailward flows (GTWs) in the plasma sheet and they do not resemble vortical‐like flows. However, TWABs have a flow velocity magnitude twice larger than the GTWs. The plasma density and temperature of TWABs are comparable with BBFs. It is more common to observe a TWAB succeeding than preceding a BBF. However, there is no distinctive difference (in flow pattern, plasma density and temperature) between preceding and succeeding TWABs. We suggest that TWABs are likely the “freshly” rebounded BBFs from the near‐Earth region where the magnetic field is stronger. TWABs may represent the early stage of the evolution of tailward flows in the plasma sheet. We also discuss and argue that other mechanisms such as shear‐induced vortical flows and tailward slipping of depleted flux tubes cannot be the principal causes of TWABs.

Magnetotail Flux Accumulation Leads to Substorm Current Wedge Formation: A Case Study

AbstractReconnection‐generated earthward flows, magnetic field dipolarizations, and auroral expansions are related to substorm current wedge (SCW) development. It has been suggested that field‐aligned currents (FACs) within the SCW can be generated by flow vortices, pressure gradients, or both. Observations related to these generation mechanisms differ from one event to another, due to their different locations relative to SCW's central meridian and timing relative to the SCW's evolutionary state. A pattern of in situ observations consistent with these generation mechanisms has yet to emerge. Obtaining such a pattern of in situ observations relies on the satellite locations relative to the FAC driver regions, which are hard to determine because coincident magnetotail observations are sparse. To solve this problem, an SCW inversion technique was used to model the FAC locations and determine the connections between magnetospheric and ionospheric phenomena. Using this technique, the magnetic flux, a parameter that is relatively insensitive to FAC locations, was analyzed during an isolated substorm on February 13, 2008. We compared the temporal variations of the accumulated flux that caused magnetic dipolarization in the SCW and the flux within the auroral poleward boundary. We found them to be in good agreement with the flux transported by earthward flows. This agreement suggests that the accumulation of the magnetic flux leads to the generation of the SCW, causing magnetic dipolarization and auroral poleward expansion. The amount of accumulated flux was found to be positively correlated with the amplitudes of these substorm‐related phenomena.

Complex Sub‐Auroral Flow Channel Structure Formed by Double‐Peak Sub‐Auroral Ion Drifts (DSAID) and Abnormal Sub‐Auroral Ion Drifts (ASAID)

AbstractBy utilizing multi‐instrument and multi‐satellite/spacecraft observations, we report the repeated development of a temporary and complex sub‐auroral flow channel (FC) structure during the intensive storms investigated. Appearing as combined double‐peak sub‐auroral ion drifts (DSAID) and abnormal SAID (ASAID), it is called DSAID‐ASAID FC structure. Revealing its development, various other FCs and features appeared in the auroral and polar regions simultaneously. These include the auroral Harang Region (HR) that is the Substorm Current Wedge's ionospheric signature and the Eastward FC (E‐FC) indicating the arrival of the Westward Travelling Surge. Results show that in the HR and E‐FC, the locally enhanced duskside upward Region 1 Field Aligned Currents (↑R1 FACs) connected in the magnetosphere with the meridional partial ring current characterized by a double plasma density/pressure peak structure, of which signature was seen in the 225 keV proton density data, leading to the development of two sets of ↓R2 FACs and DSAID. Meanwhile, large auroral undulations appeared and were driven, via magnetosphere‐ionosphere coupling, by Kelvin‐Helmholtz instability mechanisms that also drove plasma sheet rippling leading to ASAID development. Plasma sheet rippling occurred during magnetotail reconnection, as demonstrated with its ionospheric polar cap signature, known as FC‐3, in a stretched tail region according to the b2i boundary located at ∼60° magnetic latitude. Then, the earthward flows' braking/slowing down provided the energy for driving the R1‐R2 FACs. From the observational evidence presented we conclude that the DSAID‐ASAID features investigated developed due to the maintenance of DSAID during plasma sheet rippling triggering ASAID development.

Multiple transpolar auroral arcs reveal insight about coupling processes in the Earth’s magnetotail

A distinct class of aurora, called transpolar auroral arc (TPA) (in some cases called "theta" aurora), appears in the extremely high-latitude ionosphere of the Earth when interplanetary magnetic field (IMF) is northward. The formation and evolution of TPA offers clues about processes transferring energy and momentum from the solar wind to the magnetosphere and ionosphere during a northward IMF. However, their formation mechanisms remain poorly understood and controversial. We report a mechanism identified from multiple-instrument observations of unusually bright, multiple TPAs and simulations from a high-resolution three-dimensional (3D) global MagnetoHydroDynamics (MHD) model. The observations and simulations show an excellent agreement and reveal that these multiple TPAs are generated by precipitating energetic magnetospheric electrons within field-aligned current (FAC) sheets. These FAC sheets are generated by multiple-flow shear sheets in both the magnetospheric boundary produced by Kelvin-Helmholtz instability between supersonic solar wind flow and magnetosphere plasma, and the plasma sheet generated by the interactions between the enhanced earthward plasma flows from the distant tail (less than -100 RE) and the enhanced tailward flows from the near tail (about -20 RE). The study offers insight into the complex solar wind-magnetosphere-ionosphere coupling processes under a northward IMF condition, and it challenges existing paradigms of the dynamics of the Earth's magnetosphere.

An Event Study of Simultaneous Earthward and Tailward Reconnection Exhaust Flows in the Earth's Midtail

AbstractWe report an event of two‐satellite measurements of simultaneous earthward and tailward fast flows of ~500 km/s in the midtail at X ~ −63 RE and evaluate magnetic reconnection as a responsible mechanism by comparing the observations with a particle‐in‐cell (PIC) simulation. The two satellites were near midnight separated mainly along the X direction by ~5 RE. As they moved across the current sheet from the northern to southern lobes, the one closer to the Earth crossed the x line with fast flows changing from tailward to earthward, while the other one simultaneously observed tailward flows. The observed plasma and fields showed several key reconnection signatures, including the Walén relation, the fast reconnection rate of ~0.1, the Hall magnetic and electric fields, and counterstreaming electrons in the separatrix, indicating the fast flow was the reconnection exhaust. The observed temporal variations of flow speeds and magnetic fields suggested that the x line was moving tailward to a location between the two satellites and the exhaust was moving up and down. Within the exhaust, plasma pressure was highly anisotropic, and the current sheet can be unstable to the mirror, ion cyclotron, and firehose instabilities. Current sheet flapping and enhanced compressional waves near proton's local gyro frequencies were observed around the current sheet. Comparing with the PIC simulation suggests that the waves were mainly a result of oblique firehose instability.

A Statistical Study of Near‐Earth Magnetotail Evolution During Pseudosubstorms and Substorms With THEMIS Data

AbstractPseudosubstorms (pseudobreakups) and substorms are similar phenomena. In terms of auroral morphology, however, the former are not accompanied by poleward expansion, while the latter are. To understand what causes this difference, we studied temporal and spatial development of the near‐Earth magnetotail at to around pseudosubstorm and substorm onsets, based on superposed epoch analysis of Time History of Events and Macroscale Interactions during Substorms (THEMIS) data. We find that the earthward flow begins to increase at to just before onset and dipolarization for both pseudosubstorms and substorms, possibly due to near‐Earth magnetic reconnection or the preceding relaxation of the thin current sheet in a tailward region, but the earthward flow is slower for pseudosubstorms than for substorms. Dipolarization, together with magnetic field fluctuation, is nearly the same at for both cases, but it is weaker at other distances for pseudosubstorms than for substorms. This result suggests that the current disruption related to dipolarization does not expand tailward and hence auroral poleward expansion does not occur for pseudosubstorms. Furthermore, the total pressure is larger at to for several minutes before onset for substorms than for pseudosubstorms. The total pressure gradient increases more largely after onset for substorms than for pseudosubstorms. We suggest that these differences are important factors for determining whether ballooning instability causing current disruption grows in a wide area, that is, whether the initial action develops into a substorm or subsides as a pseudosubstorm.

IMF By Influence on Magnetospheric Convection in Earth's Magnetotail Plasma Sheet

AbstractWe use Geotail, Cluster, and Time History of Events and Macroscale Interactions during Substorms data over 15 years (1995–2009) to statistically investigate convective ion flows (V⊥xy<200 km/s) in the magnetotail plasma sheet under the influence of a clearly nonzero dawn‐dusk interplanetary magnetic field (IMF By). We find that IMF By causes an interhemispheric asymmetry in the flows, which depends on the direction of IMF By. On the average, one magnetic hemisphere is dominated by a dawn‐dusk flow component, which is oppositely directed compared to that in the other hemisphere. This asymmetry is observed for both earthward and tailward flows. A comparison to tail By reveals that the region where the asymmetry in the average flows appears agrees with the appearance of the tail By direction collinear to IMF By. The results imply that IMF By has a major influence on the direction of the magnetic flux transport in the magnetotail.

Storm-time convection dynamics viewed from optical auroras

A series of statistical and event studies have demonstrated that the motion of patches in regions of Patchy Pulsating Aurora (PPA) is very close to, if not exactly, E×B convection. Therefore, 2D maps of PPA motion provide us the opportunity to remotely sense magnetospheric convection with relatively high space and time resolution, subject to uncertainties associated with the mapping between the ionosphere and magnetosphere. In this study, we use THEMIS ASI (All Sky Imager) aurora observations combined with RBSP electric field and magnetic field measurements to explore convection dynamics during storm time. From 0500 UT to 0600 UT on March 19 2015, auroral observations across ~4 h of magnetic local time (MLT) show that increases in the westward velocities of patches are closely related to earthward flow bursts in the inner plasma sheet. Together with the meridian scanning photometer (MSP) data, this suggests that the increase in the westward velocities of PPA patches is caused by earthward-moving ion injection structures carried by the fast earthward flows.

Ionospheric Footprints of Detached Magnetotail Interchange Heads

AbstractPritchett and Coroniti (2011, https://doi.org/10.1029/2011GL047527, 2013, https://doi.org/10.1029/2012JA018143) have predicted that the kinetic ballooning/interchange instability (BICI) can provoke reconnection onsets that lead to detached azimuthally thin earthward intrusions (heads) of depleted plasma tubes when 100. Such detached BICI heads would be seen as localized earthward‐propagating dipolarization fronts. Using Time History of Events and Macroscale Interactions during Substorms observations in the plasma sheet at XGSM≈−11RE and conjugate All‐Sky Imager and magnetometer networks observations on the ground, we show four examples when prominent dipolarization fronts with moderate earthward flows were observed amidst azimuthally drifting interchange heads and concurrently with the ionospheric current intensifications near Time History of Events and Macroscale Interactions during Substorms footprints and auroral bright spots originating from dimmer azimuthal beads/rays. These events support the idea that some of the BICI heads detach from the region with reversed radial gradient of BZ due to local reconnection. The detached BICI heads propagate earthward‐driving ionospheric pseudo‐breakups.

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.

Utilizing the Heliophysics/Geospace System Observatory to Understand Particle Injections: Their Scale Sizes and Propagation Directions

AbstractThe injection region's formation, scale size, and propagation direction have been debated throughout the years, with new questions arising with increased plasma sheet observations by missions like Cluster and THEMIS. How do temporally and spatially small‐scale injections relate to the larger injections historically observed at geosynchronous orbit? How to account for opposing propagation directions—earthward, tailward, and azimuthal—observed by different studies? To address these questions, we used a combination of multisatellite and ground‐based observations to knit together a cohesive story explaining injection formation, propagation, and differing spatial scales and timescales. We used a case study to put statistics into context. First, fast earthward flows with embedded small‐scale dipolarizing flux bundles transport both magnetic flux and energetic particles earthward, resulting in minutes‐long injection signatures. Next, a large‐scale injection propagates azimuthally and poleward/tailward, observed in situ as enhanced flux and on the ground in the riometer signal. The large‐scale dipolarization propagates in a similar direction and speed as the large‐scale electron injection. We suggest small‐scale injections result from earthward‐propagating, small‐scale dipolarizing flux bundles, which rapidly contribute to the large‐scale dipolarization. We suggest the large‐scale dipolarization is the source of the large‐scale electron injection region, such that as dipolarization expands, so does the injection. The >90‐keV ion flux increased and decreased with the plasma flow, which died at the satellites as global dipolarization engulfed them. We suggest the ion injection region at these energies in the plasma sheet is better organized by the plasma flow.

Continent‐Wide R1/R2 Current System and Ohmic Losses by Broad Dipolarization‐Injection Fronts

AbstractWe employ Magnetospheric Multiscale, Geostationary Operational Environmental and Los Alamos National Laboratory satellites, as well as the ground magnetometer networks over Greenland and North America to study a substorm on 9 August 2016 between 9 and 10 UT. We found that during the substorm two earthward flows, whose dipolarization‐injection fronts exceeded 6.5 and 4 Earth's radii (RE) in YGSM, impinged and rebounded from Earth's dipolar field lines at L=6–7 downtail, where L is the McIlwain number. The impingements and rebounds ended with a substorm current system of downward R1 and upward R2 currents which grew to azimuthally cover the whole North American continent. At the fronts, regions of enhanced negative j·E′ were formed and peaked toward the end of the impingements. These regions appeared to be conjugate with eastward moving aurora (along the growth phase arc and together with eastward drifting energetic electrons at geosynchronous equatorial orbit), which manifests ionospheric Ohmic losses.

Contribution of patchy reconnection to the ion-to-electron temperature ratio in the Earth's magnetotail

The ion-to-electron temperature ratio is a good indicator of the processes involved in the plasma sheet. Observations have suggested that patchy reconnection and the resulting earthward bursty bulk flows (BBFs) transport may be involved in causing the lower temperature ratios at smaller radial distances during southward IMF periods. In this paper, we estimate theoretically how a patchy magnetic reconnection electric field can accelerate ions and electrons differently. If both ions and electrons are non-adiabatically accelerated only once within each reconnection, the temperature ratio would be preserved. However, when reconnection occurs closer to the Earth where magnetic field lines are shorter, particles mirrored back from the ionosphere can cross the reconnection region more than once within one reconnection; and electrons, moving faster than ions, can have more crossings than do ions, leading to electrons being accelerated more than ions. Thus as particles are transported from tail to the near-Earth by BBFs through multiple reconnection, electrons should be accelerated by the reconnection electric field more times than are ions, which can explain the lower temperature ratios observed closer to the Earth.

Revisiting substorm events with preonset aurora

Abstract. Nishimura et al. (2010) proposed a new plasma intrusion or preonset aurora scenario of substorm triggering. In this scenario, a substorm is triggered by a fast earthward flow generated at the distant neutral line which corresponds to a preonset auroral streamer or arc in the ionosphere propagating from the auroral poleward boundary to the initial auroral brightening site, i.e., “preonset aurora”. In the present paper, we revisited three substorm events reported as being triggered by such a mechanism related to preonset auroras, based on THEMIS ground-based all-sky imager data. Unlike previous studies, we examined the arrival timing of the preonset aurora relative to the three steps of auroral onset arc development (initial brightening, enhancement of the wave-like structure, and poleward expansion) to make the role of the preonset aurora in the auroral steps clearer. Our detailed timing analysis found that preonset auroral streamers reached the auroral onset arc but away from the initial brightening site after initial brightening for two events, while no preonset aurora reaching the initial brightening site could be identified for the other event. This result suggests that the processes associated with auroral streamers are unlikely to affect at least initial brightening, even if we consider not only the presence and arrival timing and location of the auroral streamers but also the scale of the corresponding flow and flow vortices. We list a series of open questions for testing the preonset aurora scenario further in future studies. Keywords. Magnetospheric physics (storms and substorms; auroral phenomena; magnetotail)

Fast plasma sheet flows and X line motion in the Earth's magnetotail: results from a global hybrid-Vlasov simulation

Abstract. Fast plasma flows produced as outflow jets from reconnection sites or X lines are a key feature of the dynamics in the Earth's magnetosphere. We have used a polar plane simulation of the hybrid-Vlasov model Vlasiator, driven by steady southward interplanetary magnetic field and fast solar wind, to study fast plasma sheet ion flows and related magnetic field structures in the Earth's magnetotail. In the simulation, lobe reconnection starts to produce fast flows after the increasing pressure in the lobes has caused the plasma sheet to thin sufficiently. The characteristics of the earthward and tailward fast flows and embedded magnetic field structures produced by multi-point tail reconnection are in general agreement with spacecraft measurements reported in the literature. The structuring of the flows is caused by internal processes: interactions between major X points determine the earthward or tailward direction of the flow, while interactions between minor X points, associated with leading edges of magnetic islands carried by the flow, induce local minima and maxima in the flow speed. Earthward moving flows are stopped and diverted duskward in an oscillatory (bouncing) manner at the transition region between tail-like and dipolar magnetic fields. Increasing and decreasing dynamic pressure of the flows causes the transition region to shift earthward and tailward, respectively. The leading edge of the train of earthward flow bursts is associated with an earthward propagating dipolarization front, while the leading edge of the train of tailward flow bursts is associated with a tailward propagating plasmoid. The impact of the dipolarization front with the dipole field causes magnetic field variations in the Pi2 range. Major X points can move either earthward or tailward, although tailward motion is more common. They are generally not advected by the ambient flow. Instead, their velocity is better described by local parameters, such that an X point moves in the direction of increasing reconnection electric field strength. Our results indicate that ion kinetics might be sufficient to describe the behavior of plasma sheet bulk ion flows produced by tail reconnection in global near-Earth simulations. Keywords. Magnetospheric physics (magnetospheric configuration and dynamics; plasma sheet) – space plasma physics (numerical simulation studies)

Convection Electric Field and Plasma Convection in a Twisted Magnetotail: A THEMIS Case Study 1–2 January 2009

AbstractWe investigate THEMIS satellite measurements made in a tail‐aligned constellation during a time interval on 1–2 January 2009, which has previously been attributed to an interval of an interplanetary magnetic field By‐driven magnetotail twisting. We find evidence for that the orientation of the convection electric field in the tail is twist‐mode dependent. For earthward flow and a negative twist (induced tail By< 0), the electric field is found to have northward Ez and tailward Ex components. During a positive twist (induced tail By> 0), the directions of Ez and Ex are reversed. The Ey component shows the expected dawn‐to‐dusk direction for earthward flow. The electric field components preserve their orientation across the neutral sheet, and a quasi‐collinear field is observed irrespective to the tail distance. The electric field associated with the tailward flow has an opposite direction compared to the earthward flow for the negative twist. For the positive twist, the results are less clear. The corresponding plasma convection and thus the magnetic flux transport have an opposite dawn‐dusk direction above and below the neutral sheet. The directions depend on the tail twist mode. The hemispherically asymmetric earthward plasma flows are suggested to be a manifestation of an asymmetric Dungey cycle in a twisted magnetotail. The role of tailward flows deserve further investigation.

Characterizing Ion Flows Across a Magnetotail Dipolarization Jet

AbstractThe structure of dipolarization jets with finite width in the dawn‐dusk direction relevant to magnetic reconnection in the Earth's magnetotail is explored with particle‐in‐cell simulations. We carry out Riemann simulations of the evolution of the jet in the dawn‐dusk, north‐south plane to investigate the dependence of the jet structure on the jet width in the dawn‐dusk direction. We find that the magnetic field and Earth‐directed ion flow structure depend on the dawn‐dusk width. A reversal in the usual Hall magnetic field near the center of the current sheet on the duskside of larger jets is observed. For small widths, the maximum velocity of the earthward flow is significantly reduced below the theoretical limit of the upstream Alfvén speed. However, the ion flow speed approaches this limit once the width exceeds the ion Larmor radius based on the normal magnetic field, Bz.