Mapping transition region flows to the ionosphere in a global hybrid-Vlasov simulation

<strong class="journal-contentHeaderColor">Abstract.</strong> The dynamics of the inner magnetosphere and magnetotail are determined by a number of factors such as magnetic reconnection, plasma instabilities, and large-scale plasma motion. We use the global hybrid-Vlasov simulation Vlasiator to study these dynamics as well as their signatures in the ionosphere. We observe magnetic reconnection, fast flows, and vorticity in the transition region between the Earth's dipolar field and the magnetotail. In our simulation, reconnection is first triggered at the dawn and dusk sides of the magnetotail current sheet. It then spreads across the current sheet. Concurrently, an azimuthally periodic, wave-like density structure develops in the transition region along with fast Earthward flows and enhanced vorticity patterns. The Earthward flows and vorticity induce field-aligned currents, which map onto the ionospheric simulation domain, creating a patchy current distribution. We find that the event is driven by the combination of reconnection-induced fast flows and the ballooning/interchange instability.

<strong class="journal-contentHeaderColor">Abstract.</strong> The dynamics of the inner magnetosphere and magnetotail are determined by a number of factors such as magnetic reconnection, plasma instabilities, and large-scale plasma motion. We use the global hybrid-Vlasov simulation Vlasiator to study these dynamics as well as their signatures in the ionosphere. We observe magnetic reconnection, fast flows, and vorticity in the transition region between the Earth's dipolar field and the magnetotail. In our simulation, reconnection is first triggered at the dawn and dusk sides of the magnetotail current sheet. It then spreads across the current sheet. Concurrently, an azimuthally periodic, wave-like density structure develops in the transition region along with fast Earthward flows and enhanced vorticity patterns. The Earthward flows and vorticity induce field-aligned currents, which map onto the ionospheric simulation domain, creating a patchy current distribution. We find that the event is driven by the combination of reconnection-induced fast flows and the ballooning/interchange instability.

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)

The transition region between the Earth's dipole field and the stretched magnetotail is a highly dynamic region of space where a variety of complex physical phenomena occur. The aim of this study is to understand the formation and evolution of large-scale plasma flow vortices in the transition region. The vorticity is found in the global magnetospheric hybrid-Vlasov model Vlasiator, which now features an ionospheric solver, enabling the study of magnetosphere-ionosphere coupling. We compare the results of the simulation to other magnetospheric simulations and observations of similar phenomena.The vortices are highly structured and spread over the nightside with an azimuthal wavelength of about 3.5 RE (Earth radii). The vorticity in the magnetotail is induced by reconnection resulting in Earthward bulk flow with properties similar to bursty bulk flows (BBFs). In addition to BBF-like signatures, we observe that the features of the event are consistent with it originating from the ballooning/interchange instability, in combination with the fast Earthward flow. The fast flows and vorticity in the magnetosphere map onto the ionospheric grid of the simulation, and it can be seen that the Earthward flows create field-aligned currents. Our study investigates the formation of the vortices in the magnetotail, and the resulting ionospheric effects.

&amp;lt;p&amp;gt;&amp;lt;span&amp;gt;In July 2017, the MMS constellation was evolving in the magnetotail with an apogee of 25 Earth radii and an average inter-satellite distance of 10 km (i.e. at electron scales). On 23&amp;lt;/span&amp;gt;&amp;lt;sup&amp;gt;&amp;lt;span&amp;gt;rd&amp;lt;/span&amp;gt;&amp;lt;/sup&amp;gt;&amp;lt;span&amp;gt; of July around 16:19 UT, MMS was located at the edge of the current sheet which was in a quasi-static state. Then, MMS suddenly entered in the central plasma sheet and detected the local onset of a small substorm as indicated by the AE index (~400 nT). Fast plasma flows towards the Earth were measured for about 1 hour starting with a period of quasi-steady flow and followed by a series of saw-tooth plasma jets (&amp;amp;#8220;bursty bulk flows&amp;amp;#8221;). In the present study, we focus on a short sequence related to the crossing of an ion scale current sheet embedded in a fast earthward flow. The current sheet appears to be corrugated and with a significant guide field (BL/BM~0.5). Tailward propagating electrostatic solitary waves are detected just after the magnetic equator crossing and at the edge of the current sheet. We also analyze in detail an electron vortex magnetic hole also detected at the edge of this current sheet and discuss the Ohm&amp;amp;#8217;s law and energy conversion processes. We find that the energy dissipation associated with the electron vortex is three times greater (0.15nW/m3) than at the current sheet crossing (0.05nW/m3). Based on estimated statistical weight of these vortices we discuss possible consequences for the energy dissipation associated with fast earthward plasma flows.&amp;lt;/span&amp;gt;&amp;lt;/p&amp;gt;

Magnetic reconnection in Earth's magnetotail produces fast earthward flows in the plasma sheet. Tailward flows are often observed associated with the earthward flows. Both return flow vortices at the flanks of an earthward flow channel and rebound of the earthward flow from the intense dipolar magnetic field of the inner magnetosphere have been shown to explain tailward flows observed near Earth. We combine plasma sheet measurements from Cluster with conjugate ground‐based magnetic and auroral data to examine the development of earthward and tailward flow signatures during a substorm onset. We show for the first time observations of ionospheric signatures that appear to be associated with rebound flows. Because of the highly dynamic magnetotail configuration, special care is taken with the satellite footprint mapping. The ionospheric footprints produced by the event oriented AM02 model drift equatorward and poleward in response to tail magnetic field stretching and dipolarization, respectively. The footprint motion matches that of the ambient ionospheric structures, and the plasma flow measured by Cluster agrees with that inferred from the conjugate ionospheric observations, confirming the validity of the AM02 mapping. The ionospheric signatures of fast earthward flows during a substorm onset are shown to resemble the known signatures of quiet‐time flows, including equatorward propagating auroral streamers inside a channel of enhanced poleward equivalent current. However, the large‐scale dipolarization results in additional poleward expansion of the signatures, as has been predicted by simulations.

&amp;lt;p&amp;gt;In July 2017, the MMS constellation was evolving in the magnetotail with an apogee of 25 Earth radii and an average inter-satellite distance of 10 km (i.e. at electron scales). On 23 rd of July around 16:19 UT, MMS was located at the edge of the current sheet which was in a quasi-static state. Then, MMS&amp;lt;br&amp;gt;suddenly entered in the central plasma sheet and detected the local onset of a small substorm as indicated by the AE index (~400 nT). Fast earthward plasma flows were measured for about 1 hour starting with a period of quasi-steady flow and followed by a saw-tooth like series of plasma jets (&amp;amp;#8220;bursty bulk flows&amp;amp;#8221;). In the present study, we focus on a short sequence related to an ion scale current sheet crossing embedded in a fast earthward flow. We analyse in detail two other kinetic structures in the vicinity of this current sheet: an ion-scale flux rope and an electron vortex magnetic hole and discuss the Ohm&amp;amp;#8217;s law and conversion energy processes.&amp;lt;/p&amp;gt;

Particle 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.

The present study examines the temporal structure of the fast flow in the plasma sheet using both observations and simulations. The data analysis part adopts the strictest criterion ever for the satellite location so that selected flows are mostly convective. From Geotail measurements at X &gt; −31 RE, 818 earthward‐flow and 290 tailward‐flow events are selected. Superposed epoch analyses are conducted with two different reference times: the start of the fast flow and the time of a sharp change in the Bz component. The results are summarized as follows: (1) The magnetic field becomes dipolar in the course of the fast earthward flow; (2) Sharp dipolarization tends to be preceded by a transient decrease in BZ, which starts along with the fast flow and is accompanied by an increase in the plasma density; (3) The corresponding signatures, albeit less clear, can also be found for the tailward flow; (4) Whereas the plasma density decreases in association with the fast flow irrespective of the flow direction (though, more gradually for the tailward flow), the ion temperature increases for the earthward flow and decreases for the tailward flow; (5) The plasma and total pressures decrease in the course of the fast flow, suggesting the reduction of the lobe field strength; (6) In general, magnetic field and plasma parameters change more gradually in time for the tailward flow than for the earthward flow. Those characteristics of the fast flow can be found irrespective of the X distance, even though the ambient magnetic field and plasma vary significantly between X = −5 and −31 RE. The near‐Earth reconnection is inferred to be the responsible mechanism for most, if not all, flow events, and the difference between the earthward and tailward flows presumably reflects difference in downstream conditions. On the earthward side of the reconnection site, the flow needs to proceed against the rigid terrestrial magnetic field, whereas on the tailward side the flow does not have any obstruction once reconnection reaches the lobe magnetic field. This idea is consistent with the change of the magnetic inclination, which suggests that the plasma sheet becomes thicker and thinner in the course of the earthward and tailward flows, respectively. These observational results are compared with fast plasma flows modeled by two‐fluid simulations of magnetic reconnection. A focus is placed on the reduction of BZ prior to dipolarization for the earthward flow (the precursory BZ increase for the tailward flow) since this is the new finding owing to our strict condition for the convective flow. It is found that the fragmentation of the current sheet and the formation of multiple neutral lines create signatures similar to the satellite observations. After multiple X lines form, one of them dominates and establishes the overall flow pattern associated with reconnection. Magnetic islands formed between the X lines are swept downstream by the reconnection process. The signature of this earthward convection of a magnetic island past a satellite at rest in the magnetotail is a strongly bipolar signature in Bz with a sudden enhancement in the density: Bz spikes negative and then positive in rapid succession, with a maximum in the density between these two spikes. It is therefore suggested that the temporal structure of the observed fast plasma flows contains information directly linked to their genesis.

Measurements of the magnetic field and low energy plasma by the GEOTAIL spacecraft have been used to study fast earthward plasma flows in the distant (130–200 RE) and middle (40–80 RE) tail and their relationship to substorms. Results of the analysis show that high‐speed earthward ion flows are often observed in the mid/distant tail under conditions of low magnetic activity (AE ∼ 100 nT), when the IMF BZ component is northward or oscillates to about zero. A considerable portion of the fast earthward flows (29% in the distant tail and 53% in the middle tail) seems to be a precursor to substorm expansion onsets on the Earth. These flows occur at closed magnetic field lines. The cause and effect relationship between fast earthward plasma flows in the distant tail and substorm activity suggests that a neutral line can form at distances far beyond 100 RE during quiet conditions.

Simultaneous measurements from THEMIS spacecraft, GOES-11 and ground stations (Canadian Array for Realtime Investigations of Magnetic Activity or CARISMA, and 210° magnetic meridian or MM) on March 18, 2009 allow the study of dynamic processes in the near-Earth magnetotail and corresponding Pi2 pulsations on the ground in great detail. Fast earthward flows along with traveling Alfven waves and fast mode waves in the Pi2 band were observed by three Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes (P3, P4 and P5) in the near-Earth plasmasheet. At the mid- to high-latitude nightside, the CARISMA stations located near the foot points of the three probes recorded Pi2s with two periods, about 80 s after the earthward fast flows observed by the P4 probe. The long-period Pi2 (140–150 s) belongs to the transient response Pi2 (TR Pi2), since the travel time of the Alfven waves between the plasma sheet and CARISMA stations is very close to half the period of the long-period Pi2. The short-period Pi2 (60–80 s) has the same period band as the perpendicular velocity of the fast flows, which indicates that it may relate to the inertial current caused by periodic braking of the earthward fast flows. The 210° MM stations located at the low-latitude duskside also observed Pi2s with the same start time, waveform and frequency, about ∼120 s after the earthward fast flows. Strong poloidal oscillations are shown by GOES-11 (∼23 MLT) and the compressional component (Bb) is highly correlated with H components of the 210° MM stations, whereas the other two components (Br and Be) are not. These results confirm that the low-latitude Pi2s are generated by cavity mode resonance, which is driven by an impulsive broadband source in the near-Earth magnetotail.

Difference between Earthward and Tailward Flows in Their Dependences on Geomagnetic and IMF Conditions

In this study we first identify earthward plasma sheet fast flows from Geotail plasma and magnetic fields with a criterion of V⊥x &gt; 300 km/s, where V⊥x is the X component of the plasma flow perpendicular to the ambient magnetic field. We then estimate rates of change of the nightside auroral power over the courses of the fast flows using Polar Ultraviolet Imager auroral images. It is found that 68 earthward fast flows observed at ∣Y∣ &lt; 6 RE during 1997–1998 can be classified into two classes. One class of the earthward fast flows (Class I) was often observed near X = ∼−10 RE and the other class (Class II) was found at X &lt; −15 RE. The auroral power rates of change of the fast flows in Class I in terms of time are found to be high, indicating that the total auroral power on the nightside was significantly increasing during these fast flows. The corresponding auroral images show an apparent substorm bulge developed on the nightside, i.e., a significant global auroral development. The auroral power rates for most of the earthward fast flows in Class II are low. The auroral features, such as poleward boundary intensifications and pseudobreakups, are found to be associated with these fast flows. Some of the earthward fast flows in Class II can propagate earthward and provide a favorable condition for a substorm onset, leading to an auroral bulge developed on the nightside. We have also tested another criterion of VxBz &gt; 2 mV/m for an identification of fast flows, where Vx is the X component of the plasma flow and Bz is the Z component of the geomagnetic field. It is found that V⊥x is a better parameter than VxBz to differentiate the two classes of earthward fast flows in the plasma sheet.

Statistical characteristics of impulsive electric field (IEFD) associated with fast earthward ion flows in the inner central plasma sheet were studied by using electric field data obtained from the double‐probe instrument on board the Geotail satellite. It is shown that the strongest electric field is produced in the region between XAGSM = −15 RE and −9 RE where the fast earthward flow is decelerated. The IEFDs are short‐lived with a timescale of ∼2 min on the average, compatible with the timescale of other signatures of substorm fine structures. This strong cross‐tail electric field, coincident with the braking of the fast flow and correlated with the magnetic field dipolarization, has important implications for the particle acceleration in the near‐Earth plasma sheet.

Study of three dimensional structure of the fast convection flow in the plasma sheet by MHD simulations on the basis of spontaneous fast reconnection model