Modulation of Electron Rolling‐Pin Distribution Behind Anti‐Dipolarization Front

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 hosts regions of enhanced ion temperature anisotropy that could drive instabilities and excite magnetosonic waves. However, the relation between magnetosonic-mode fluctuations and the anti-dipolarization fronts (ADFs, also known as the leading edge of tailward reconnection jets), is still unclear. Here, for the first time, we provide direct observations of magnetosonic-mode fluctuations behind ADF by the magnetospheric multiscale mission. These compressible waves propagate quasi perpendicularly to the background magnetic fields at very slow phase speeds (~30 km/s) and appears simultaneously with parallel ion temperature anisotropy. Such waves were likely a result of oblique firehose instability. An electron rolling-pin distribution is modulated by such waves behind the ADF: at wave troughs (B minimum), perpendicular electron fluxes are high; at wave crests (B maximum), perpendicular electron fluxes are low. Fermi acceleration and magnetic mirror effect contribute to the formation of such distribution jointly. These findings improve our understanding of wave-particle interactions near the ADFs in Earth's magnetosphere.

Energetic particle injections are characterized by dispersive or dispersionless increases in observed particle flux. Observations from National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) mission have revealed transient events displaying injection‐like dispersed reductions in energetic (~30–600 keV) electron flux in the dawnside magnetosphere. Although initially believed to be the result of magnetopause losses, drift tracing of the electrons suggests a source for these drift‐dispersed flux dropouts in the near‐to‐postmidnight magnetotail suggesting that they are likely related to similar signatures previously observed at geosynchronous orbit. We suggest that the dozen examples presented are signatures of “flux‐reduced injections” resulting from earthward injection in the presence of a negative phase space density radial gradient as supported by observed phase space density versus L‐shell profiles. These events also display varying pitch angle responses inconsistent with a singular loss mechanism, leading to the suggestion that they result from preconditioning of the magnetotail source region prior to the injection.

This paper describes the attitude ground system (AGS) design to be used for support of the Magnetospheric MultiScale (MMS) mission. The AGS exists as one component of the mission operations control center. It has responsibility for validating the onboard attitude and accelerometer bias estimates, calibrating the attitude sensors and the spacecraft inertia tensor, and generating a definitive attitude history for use by the science teams. NASA's Goddard Space Flight Center (GSFC) in Greenbelt, Maryland is responsible for developing the MMS spacecraft, for the overall management of the MMS mission, and for mission operations. MMS is scheduled for launch in 2014 for a planned two-year mission. The MMS mission consists of four identical spacecraft flying in a tetrahedral formation in an eccentric Earth orbit. The relatively tight formation, ranging from 10 to 400 km, will provide coordinated observations giving insight into small-scale magnetic field reconnection processes. By varying the size of the tetrahedron and the orbital semi-major axis and eccentricity, and making use of the changing solar phase, this geometry allows for the study of both bow shock and magnetotail plasma physics, including acceleration, reconnection, and turbulence. The mission divides into two phases for science; these phases will have orbit dimensions of 1.2 x 12 Earth radii in the first phase and 1.2x25 Earth radii in the second in order to study the dayside magnetopause and the nightside magnetotail, respectively. The orbital periods are roughly one day and three days for the two mission phases. Each of the four MMS spacecraft will be spin stabilized at 3 revolutions per minute (rpm), with the spin axis oriented near the ecliptic north pole but tipped approximately 2.5 deg towards the Sun line. The main body of each spacecraft will be an eight-sided platform with diameter of 3.4 m and height of 1.2 m. Several booms are attached to this central core: two axial booms of 14.9 m length, two radial magnetometer booms of 5 m length, and four radial wire booms of 60 m length. Attitude and orbit control will use a set of axial and radial thrusters. A four-head star tracker and a slit-type digital Sun sensor (DSS) provide input for attitude determination. In addition, an accelerometer will be used for closed-loop orbit maneuver control. The primary AGS product will be a daily definitive attitude history. Due to power limitations, the star tracker and accelerometer data will not be available at all times. However, tracker data from at least 10 percent of each orbit and continuous DSS data will be provided. An extended Kalman filter (EKF) will be used to estimate the three-axis attitude (i.e., spin axis orientation and spin phase) and rotation rate for all times when the tracker data is valid. For other times, the attitude is generated by assuming a constant angular momentum vector in the inertial frame. The DSS sun pulse will provide a timing signal to maintain an accurate spin phase. There will be times when the Sun is occulted and DSS data is not available. If this occurs at the start or end of a definitive attitude product, then the spin phase will be extrapolated using the mean rate determined by the EKF.

<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>

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.

Velocity distribution functions (VDFs) measured by the Magnetospheric Multiscale (MMS) mission are complex 3D data sets that can be represented as a superposition of multiple beams. Recent work proposed the use of the Gaussian Mixture Model (GMM) to identify different populations. Here we investigate the approach by considering first synthetic distributions made by synthetically creating beams of either Maxwellian distributions or kappa distributions with varying power law index. By varying the inter‐beam average difference and the beam standard deviation we evaluate the ability of the GMM in recognizing correctly the beam. We then apply the method systematically to MMS data in the tail and in the dayside of the Earth's magnetosphere. The approach relies on a GMM algorithm to detect and characterize the number of distinct clusters within a VDF, and a model selection approach based on the Bayesian Information Criterion to determine the optimal number of clusters. The conclusion of the analysis is that the GMM can estimate the complexity of a VDF in terms of the number of optimal beams provided by information theory criteria. By evaluating the complexity of VDFs, we can identify regions of interest within the plasma where significant physical phenomena may be present.

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.

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 investigate the electron acceleration behind dipolarization fronts (DFs) in the magnetotail from −25 R E to −10R E through the examination of the energetic electron energy flux (>30 keV) with the observations from THEMIS satellites. Statistical results of 133 DF events are presented based on the dataset from January to April of the years 2008 and 2009. As the DFs propagate earthward, the acceleration of energetic electrons behind the DFs is found to take place over several R E along the tail. The increase in energetic electron energy flux can reach 2∼4 orders of magnitude. The dominant acceleration mechanisms are different in the mid-tail (X GSM ≤−15R E ) and the near-Earth region (−15 GSM ≤−10R E ). In the mid-tail, the majority of DF events show that the dominant electron acceleration mechanism is betatron acceleration. In the near-Earth region, betatron acceleration is dominant in ∼46% DF events while Fermi acceleration is dominant in ∼39% DF events.

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.

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 instruments for active spacecraft potential control to be launched with the NASA Magnetospheric Multiscale (MMS) mission in 2014 have recently been qualified, and the first two out of a total of eight flight models have completed their acceptance test campaign. At the heart of these instruments are liquid metal ion sources with indium, developed at Forschungsund Technologietransfer GmbH. Earlier versions have been flown in the Equator-S, Cluster, and Double Star missions. The redesign efforts to comply with the stringent requirements of the MMS mission are described in detail, as well as the qualification program for the emitters and the instruments. Starting with mechanical and thermal analyses and tests, continuing with performance and life testing, and finally by analysis of the overall performance in the mission application, it can be demonstrated that the new generation of ion emitters beats the performance of previous versions in all aspects. The description of verification efforts is completed by the results from tests at instrument level of the qualification model and the first flight models.

The Magnetospheric Multiscale (MMS) mission is a major NASA effort to better understand the behavior of Earth's magnetosphere. MMS consists of a cluster of four satellites with many instruments including several high voltage instruments. The operations concept includes the notion of a systems engineer for the Instrument Suite (IS); that is, the collection of instruments in aggregate on each spacecraft and across the fleet. This paper discusses the role of the Instrument Suite Systems Engineer (ISSE) over the course of the mission including the six-month commissioning period, the two-year primary mission, and the ongoing extended mission. Some of the activities the ISSE supports include deployments, initial high voltage operations, maneuvers, operation of the central instrument data processor, and interactions between instruments. The ISSE fills the gap between the Payload Operations Center (POC) and the instrument teams by providing in depth knowledge of the IS and instrument designs and the IS concept of operations put into force by the POC.

We show observations of whistler mode waves in both the low‐latitude boundary layer (LLBL) and on closed magnetospheric field lines during a crossing of the dayside reconnecting magnetopause by the Magnetospheric Multiscale (MMS) mission on 11 October 2015. The whistlers in the LLBL were on the electron edge of the magnetospheric separatrix and exhibited high propagation angles with respect to the background field, approaching 40°, with bursty and nonlinear parallel electric field signatures. The whistlers in the closed magnetosphere had Poynting flux that was more field aligned. Comparing the reduced electron distributions for each event, the magnetospheric whistlers appear to be consistent with anisotropy‐driven waves, while the distribution in the LLBL case includes anisotropic backward resonant electrons and a forward resonant beam at near half the electron‐Alfvén speed. Results are compared with the previously published observations by MMS on 19 September 2015 of LLBL whistler waves. The observations suggest that whistlers in the LLBL can be both beam and anisotropy driven, and the relative contribution of each might depend on the distance from the X line.