Strong Energy Conversion by a Magnetic Hole Behind a Dipolarization Front

Using the high‐resolution measurements from magnetospheric multiscale mission, here we present the observational evidence of kinetic interchange instabilities (ICIs) at dipolarization fronts (DFs). Specifically, we present the observation of two DFs caused by ICI, and discover the sub‐ion scale magnetic depression caused by kinetic‐scale ICI at the second DF. By using multi‐spacecraft observation, the typical process inside the magnetic hole (MH)—by electron trapping process—is clearly shown in the magnetic depression, evidencing that the depression is a sub‐ion MH generated at DF by kinetic ICI. Based on observations and Minimum Variance Analysis (MVA) analysis, we present a two‐dimensional image of the ICI in the nonlinear stage. This observational study demonstrates the kinetic ICI at DFs, confirming the existence of MH generated by ICI at DF, and improves the understanding of dynamics at DFs.

We carried out a statistical study of equatorial dipolarization fronts (DFs) detected by the Magnetospheric Multiscale mission during the full 2017 Earth's magnetotail season. We found that two DF classes are distinguished: class I (74.4%) corresponds to the standard DF properties and energy dissipation and a new class II (25.6%). This new class includes the six DF discussed in Alqeeq et al. (2022, https://doi.org/10.1063/5.0069432) and corresponds to a bump of the magnetic field associated with a minimum in the ion and electron pressures and a reversal of the energy conversion process. The possible origin of this second class is discussed. Both DF classes show that the energy conversion process in the spacecraft frame is driven by the diamagnetic current dominated by the ion pressure gradient. In the fluid frame, it is driven by the electron pressure gradient. In addition, we have shown that the energy conversion processes are not homogeneous at the electron scale mostly due to the variations of the electric fields for both DF classes.

We report in situ observation of magnetic reconnection between magnetic flux rope (MFR) and magnetic hole (MH) in the magnetosheath by the Magnetospheric Multiscale mission. The MFR was rooted in the magnetopause and could be generated by magnetopause reconnection therein. A thin current sheet was generated due to the interaction between MFR and MH. The sub-Alfvénic ion bulk flow and the Hall field were detected inside this thin current sheet, indicating an ongoing reconnection. An elongated electron diffusion region characterized by non-frozen-in electrons, magnetic-to-particle energy conversion, and crescent-shaped electron distribution was detected in the reconnection exhaust. The observation provides a mechanism for the dissipation of MFRs and thus opens a new perspective on the evolution of MFRs at the magnetopause. Our work also reveals one potential fate of the MHs in the magnetosheath which could reconnect with the MFRs and further merge into the magnetopause. 

Dipolarization front (DF), a common magnetic structure at the leading edge of fast plasma jets in the Earth's magnetotail, plays an important role in magnetic energy release and may even modify the magnetotail energy budgets. To date, the physical process within the structure has been well studied. However, the effect of the structure on the background plasma sheet remains elusive. Using recent high-quality data from the Magnetospheric Multiscale (MMS) mission, we statistically investigate this issue. Ahead of the DF, we find a bipolar By structure, which is consistent with the local Hall current system. The main carriers of the current system are also studied. Our research advances the understanding of the interaction between the DF and ambient undisturbed plasma environment.

In-situ and optical observations of sub-ion magnetic holes

Magnetic holes (MHs), characterized by depressions in the magnetic field magnitude, are transient magnetic structures ubiquitous in space plasmas. The electron pitch-angle distribution inside the MHs is key to diagnosing the MH properties and has been suggested to mainly exhibit a pancake-type distribution showing pitch angles near 90°. Here, we present the first observation of electron rolling-pin distribution—showing electron pitch angles mainly at 0°, 90°, and 180°—within an electron-scale MH, by using Magnetospheric Multiscale mission high-resolution measurements. With a second-order Taylor expansion method, the magnetic field topology of the MH is reconstructed, and the characteristics of the rolling-pin distribution inside the MH are investigated. We find that the rolling-pin distribution primarily appears near the MH center and at energies ranging from 110 to 1200 eV. We interpret the rolling-pin formation as a consequence of the combination of local-scale electron trapping and global-scale Fermi acceleration. These results can improve current understanding of electron dynamics in the MHs.

The coronal heating region is able to generate mirror mode structures by ion mirror instabilities. Linear magnetic holes are believed to be the remnants of mirror mode structures, thus they are believed to be messengers from the coronal heating region. They can be convected to ∼9 au with the solar wind flow, indicating that a stabilizing mechanism is necessary to make the magnetic holes survive for such a long time. Here, we investigate a magnetic hole with a size of ∼6.7 ρ i in the solar wind based on observations by the Magnetospheric Multiscale mission. The unprecedented high-resolution data enable us to reveal the existence of the ion vortex inside the structure for the first time. Such an ion vortex forms a ring-like current, which is consistent with the magnetic field depression. The self-consistent structure of the magnetic hole contributed by the ion vortex can help to further shed light on the mechanism of the long-term survival of magnetic holes in the astrophysical plasma.

Within dipolarization fronts (DFs) in the Earth's magnetotail, significant magnetic energy is converted to plasma energy, and a significant portion of the electrons and ions therein are accelerated to suprathermal energies. The mechanism that produces these suprathermal particles while simultaneously reducing magnetic field energy is poorly understood, however. We use two‐dimensional particle‐in‐cell simulations to explore this process in conventional flux bundle‐type DFs, which are formed by single X line reconnection and connected to the Earth, and in newly proposed flux rope‐type DFs, which are formed and bracketed by two X lines. In flux bundle‐type DFs, electrons are betatron accelerated near the Bz peak, and ions are energized through reflection at the front. In flux rope‐type DFs, most suprathermal electrons and ions are confined to the flux rope's magnetic structure and are accelerated through repeated reflections at the structure's two ends.

In this paper, with the in-situ observations from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes we report a wavy dipolarization front (DF) event, where the DF has different magnetic structures and electron distributions at different $y$ positions in the Geocentric Solar Magnetospheric (GSM) coordinates. At $y \sim2.1R_{E}$ ( $R_{E}$ is the radius of Earth), the DF has a relatively simple structure, which is similar to that of a conventional DF. At $y \sim3.0R_{E}$ , the DF is revealed to have a multiple DF structure, where the plasma exhibits a vortex flow. Such a wavy DF could be the results of the interchange instability. The different structure of such a wavy DF at different sites has a great effect on electron acceleration. Fermi acceleration can occur at the site of the DF with a simple or multiple DF structure, while betatron acceleration as a local process has the contribution to energetic electrons only at the site of the DF with a simple structure.

Magnetic reconnection, a fundamental plasma process, releases magnetic energy and converts it to particle energy, by accelerating and heating ions and electrons. This energy conversion plays an important role in the Earth's magnetotail. A two-dimensional particle-in-cell simulation is performed to study such a conversion in a magnetotail topology, one with a nonzero Bz, and the energy conversion is found to be more efficient in the earthward outflow than in the tailward outflow. Such earthward–tailward asymmetry is manifested not only in j·E but also in Poynting flux, Hall electromagnetic fields, bulk kinetic energy flux, enthalpy flux, heat flux, bulk acceleration, heating, and suprathermal particle energization, all of which are more prevalent on the earthward side. Such asymmetries are consistent with spacecraft observations reported in the literature. Our study shows that in the magnetotail, most of the energy converted by reconnection flows predominantly toward the Earth and has the potential of being geoeffective, rather than being expelled to the solar wind by the tailward flow. The energy conversion asymmetry arises from the presence of the non-zero normal magnetic field, the stronger lobe magnetic field, and the stronger cross-tail current earthward of the reconnection site in the pre-reconnecting thin current sheet.

We perform a case study on the evolution of the current sheet in different regions around the dipolarization front (DF), including magnetic‐dip preceding the DF, front at the DF, and magnetic pileup region (MPR) behind the DF based on magnetospheric multiscale (MMS) observation on July 31, 2017. In this event, MMS1 stays inside the current sheet during the whole bursty bulk flow (BBF) interval. Our analysis reveals that the cross‐tail current sheet at the DF is rolled up, signified by the depression (−Vz/−Bz) at the dip and elevation (+Vz/+Bz) at the front. The minimum variance analysis on the magnetic field method is applied to obtain the normal direction of the current sheet. The result confirms the roll‐up, that is, downward at the depressed current sheet and upward at the elevated current sheet. The current sheet roll‐up at the DF is asymmetric, with steeper elevation than depression. The elevation angle of the elevated current sheet is evaluated to be ∼30°. Strong duskward and predominantly perpendicular J spike (∼90 nA/m2) concentrate at the interface between the dip and the front. The strength of the current of the J‐spike is about nine/three times the current at the dip/front. The front is characterized by positive E·J. In the dip/MPR, no such preference is seen. Ion/Electron pitch angle distributions exhibit significant and different evolutions in the roll‐up current sheet from dip to front, including their energy‐dependence and distributions. Finally, the roll‐up current sheet could decelerate BBF and change the flow structure. The potential significance of the roll‐up current sheet on BBF evolution is emphasized.

The dipolarization front (DF), usually observed near the leading edge of a bursty bulk flow (BBF), is thought to carry an intense current sufficient to modify the large‐scale near‐Earth magnetotail current system. However, the physical mechanism of the current generation associated with DFs is poorly understood. This is primarily due to the limitations of conventional plasma instruments which are unable to provide a sufficient number of unaliased 3‐D distribution functions on the timescale of the DF, which usually travels past a spacecraft in only a few seconds. It is thus almost impossible to unambiguously determine the detailed plasma structure of the DF at the usual temporal resolution of such instruments. Here we present detailed plasma measurements using the Cluster Plasma Electron and Current Experiment and Cluster Ion Spectrometry‐Composition and Distribution Function ion data for an event during which it was possible to observe the full pitch angle distribution at a cadence of 1/4 s. The observations clearly show details of plasma substructure within the DF, including the presence of field‐aligned electron beams. In this event, the current density carried by the electron beam is much larger than the current obtained from the curlometer method. We also suggest that the field‐aligned current around the DF obtained from the curlometer method may have been misinterpreted in previous studies. Our results imply that the nature of the DF current system needs to be revisited using high‐resolution particle measurements, such as those observations shortly to be available from the Magnetospheric Multiscale mission.

<p>At the leading edges of reconnection jets in the magnetotail, commonly referred to as Dipolarization Fronts (DF), strong fluctuations in the electric field δE and the magnetic field δB are observed. Recent results from a fully kinetic PIC simulation (Nakamura et al., 2019) demonstrate that a Lower Hybrid Drift Instability-driven (LHDI) disturbance at the DF front region can be responsible for these electric and magnetic field fluctuations. These findings are well in line with an observed event (Liu at al., 2018), comparable to the simulated plasma conditions. However, a general experimental validation under a wider range of conditions yet remains absent. The present work experimentally investigates δE and δB fluctuations for a selection of DF events between July 2017 and September 2018 using Magnetospheric Multiscale (MMS) mission data. Aiming for a statistical approach, the analysis consists of a quantitative evaluation of dynamic wave power spectra of both δE and δB in the lower hybrid frequency range. Furthermore, propagation properties of associated wave structures are analyzed and related to present plasma conditions. Findings include the identification of peak wave power occurrence times relative to the magnetic DF structure and the associated density gradient.</p>

Magnetic reconnection often initiates abruptly and then rapidly progresses to a nonlinear quasi‐steady state. While satellites frequently detect reconnection events, ascertaining whether the system has achieved steady‐state or is still evolving in time remains challenging. Here, we propose that the relatively rapid opening of the reconnection separatrices within the electron diffusion region serves as an indicator of the growth phase of reconnection. The opening of the separatrices is produced by electron flows diverging away from the neutral line downstream of the X‐line and flowing around a dipolarization front. This flow pattern leads to characteristic spatial structures in the electron pressure‐strain interaction that could be a useful indicator for the growth phase of a reconnection event. We employ two‐dimensional particle‐in‐cell numerical simulations of anti‐parallel magnetic reconnection to validate this prediction. We find that the signature discussed here, alongside traditional reconnection indicators, can serve as a marker of the growth phase. This signature is potentially accessible using multi‐spacecraft single‐point measurements, such as with NASA's Magnetospheric Multiscale satellites in Earth's magnetotail. Applications to other settings where reconnection occurs are also discussed.

Ion heating during a substorm on 15 February 2008, starting at 0348 UT, is studied with a new approach recently described in Ashour‐Abdalla et al. (2015). The general conditions of the magnetotail are obtained from a global magnetohydrodynamic (MHD) model and are used to drive a full kinetic particle‐in‐cell (PIC) simulation of a 3‐D region of the tail. Within the kinetic box, the ions, the electrons, and the fields evolve self‐consistently. The large scales are captured by the MHD model and the small scales by the PIC model based on the MHD state. This approach is used to study ion heating. Different heating mechanisms were analyzed by examining the velocity distributions at different locations. In the x direction heating occurs as the reconnection‐generated ion jet interacts with the environment in which it propagates. The heating is found mostly in the separatrices and increases downstream of the reconnection region. In the y direction the heating is less intense and is found near the dipolarization fronts. It occurs as ions become magnetized and gyrotropize the distribution function. In addition, ions can be heated in the y direction by the reconnection electric field near the reconnection site. In the z direction the ions are heated by the formation of beams moving along z between the separatrices.