Multiscale Currents Observed by MMS in the Flow Braking Region.

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.

This paper addresses the question of the contribution of azimuthally localized flow channels and magnetic field dipolarizations embedded in them in the global dipolarization of the inner magnetosphere during substorms. We employ the high‐resolution Lyon‐Fedder‐Mobarry global magnetosphere magnetohydrodynamic model and simulate an isolated substorm event, which was observed by the geostationary satellites and by the Magnetospheric Multiscale spacecraft. The results of our simulations reveal that plasma sheet flow channels (bursty bulk flows, BBFs) and elementary dipolarizations (dipolarization fronts, DFs) occur in the growth phase of the substorm but are rare and do not penetrate to the geosynchronous orbit. The substorm onset is characterized by an abrupt increase in the occurrence and intensity of BBFs/DFs, which penetrate well earthward of the geosynchronous orbit during the expansion phase. These azimuthally localized structures are solely responsible for the global (in terms of the magnetic local time) dipolarization of the inner magnetosphere toward the end of the substorm expansion. Comparison with the geostationary satellites and Magnetospheric Multiscale data shows that the properties of the BBFs/DFs in the simulation are similar to those observed, which gives credence to the above results. Additionally, the simulation reveals many previously observed signatures of BBFs and DFs, including overshoots and oscillations around their equilibrium position, strong rebounds and vortical tailward flows, and the corresponding plasma sheet expansion and thinning.

We present a comprehensive analysis of magnetic and velocity fluctuations in Earth's magnetotail plasma sheet based on observations from the Magnetospheric Multiscale (MMS) mission during its 2017 magnetotail campaign. Utilizing a novel Hybrid Filter–Decision Tree Model (HFDTM), we systematically classify the plasma sheet (X < −10 R_E in Geocentric Solar Ecliptic coordinates) into four key regions: the current sheet (CS), central plasma sheet (CPS), plasma sheet boundary layer (PSBL), and tail lobe. Within each region, we examine fluctuation dynamics across three critical flow regimes, including stagnant (V < 50 km s−1), sub‐Alfvénic (50 km s−1 ≤ V < VA), and super‐Alfvénic (V ≥ VA). Our key findings reveal: (a) Anisotropy Transition: Magnetic field anisotropy reverses with increasing flow speed, shifting from near‐isotropic values (ΔB∥/ΔB⊥ ≈ 1.1) under stagnant conditions to strongly perpendicular‐dominated distributions (∼0.4) in the super‐Alfvénic regime; (b) Multimodal Heating: Multi‐peak structures in the thermal energy (ET) spectrum, along with the co‐evolution of thermal (HT) and kinetic (HV) enstrophy from the CS to the PSBL, reveal a dual‐pathway heating mechanism involving both kinetic and magnetic energy transfer; and (c) Correlation Structure: Across all regions and regimes, weak‐to‐moderate velocity–magnetic field correlations dominate, with enhanced V∥‐B∥ correlations under super‐Alfvénic flows. Collectively, these results identify the plasma sheet as a distinct turbulent regime, governed by localized energization mechanisms (e.g., reconnection, substorm dipolarization, and flow braking), marking a departure from the Alfvénic turbulence paradigm observed in the solar wind.

We show that the spin axis offset of a fluxgate magnetometer can be calibrated with an electron drift instrument (EDI) and that the required input time interval is relatively short. For missions such as Cluster or the upcoming Magnetospheric Multiscale (MMS) mission the spin axis offset of a fluxgate magnetometer could be determined on an orbital basis. An improvement of existing methods for finding spin axis offsets via comparison of accurate measurements of the field magnitude is presented, that additionally matches the gains of the two instruments that are being compared. The technique has been applied to EDI data from the Cluster Active Archive and fluxgate magnetometer data processed with calibration files also from the Cluster Active Archive. The method could prove to be valuable for the MMS mission because the four MMS spacecraft will only be inside the interplanetary field (where spin axis offsets can be calculated from Alfvénic fluctuations) for short periods of time and during unusual solar wind conditions.

We report on the transient strong flow shear (large northward/southward electric field) events accompanied by energetic ion beams and localized field‐aligned currents observed at the boundary of the premidnight plasma sheet by Cluster in the Northern Hemisphere and Geotail in the Southern Hemisphere. The events took place associated with plasma sheet expansion during a substorm interval, with the main positive bay onset at 1155 UT on 10 October 2001. Typical timescales of these events were 1–5 minutes. Cluster multipoint analysis showed that the field‐aligned currents consist of upward and downward current layers, the latter located at the outermost edge of the plasma sheet and concentrated in a region with a thickness of 1600 km. Low‐energy proton flow suggested that the electric field was southward at the outer part and northward at the inner part, with a magnitude exceeding 10 mV/m. The electric field reversal region also corresponds to the boundary between beam‐like electrons and more isotropic electron distributions. Geotail observed corresponding plasma and field disturbances simultaneously inside the plasma sheet. We suggest that the strong bipolar electric fields could be related to the Hall effect of the transient reconnection process tailward of Cluster and Geotail and to the leading edge of the plasma flow jetting Earthward from the reconnection region.

Using the novel Magnetospheric Multiscale (MMS) mission data accumulated during the 2019 MMS Solar Wind Turbulence Campaign, we calculate the Taylor microscale (λ T) of the turbulent magnetic field in the solar wind. The Taylor microscale represents the onset of dissipative processes in classical turbulence theory. However, an accurate estimation of the Taylor scale from spacecraft data is usually difficult due to low time cadence, the effect of time decorrelation, and other factors. Previous reports were based either entirely on the Taylor frozen-in approximation, which conflates time dependence, or were obtained using multiple data sets, which introduces sample-to-sample variation of plasma parameters, or had interspacecraft distances that were larger than the present study. The unique configuration of linear formation with logarithmic spacing of the four MMS spacecraft, during the campaign, enables a direct evaluation of the λ T from a single data set, independent of the Taylor frozen-in approximation. A value of λ T ≈ 7000 km is obtained, which is about three times larger than the previous estimates.

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.

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.

An analysis of the Magnetospheric Multiscale (MMS) spacecraft data shows the presence of slow electrostatic solitary waves (SESWs) in the Earth’s plasma sheet, which have been interpreted as slow electron holes (SEHs). An alternative mechanism based on slow ion-acoustic solitons is proposed for these SESWs. The SESWs are observed in the region where double humped ion distributions and hot electrons co-exist. Our theoretical model considers the plasma in the SESW region to consist of hot electrons with a vortex distribution, core Maxwellian protons drifting parallel to the magnetic field, B and beam protons drifting anti-parallel to B. Parallel propagating nonlinear ion-acoustic waves are studied using the Sagdeev pseudopotential technique. The analysis yields four types of modes, namely, two slow ion-acoustic (SIA1 and SIA2) solitons and two fast ion-acoustic (FIA1 and FIA2) solitons. All solitons have positive potentials. Except the FIA1 solitons which propagate parallel to B; the other three types propagate anti-parallel to B. Good agreement is found between the amplitudes of electrostatic potential, the electric field, the widths and speed of SIA1 and SIA2 solitons, and the observed properties of SESWs by the MMS spacecraft.

We study spectral features of ion velocity and magnetic field correlations in the magnetosheath and in the solar wind using data from the Magnetospheric Multiscale (MMS) spacecraft. High-resolution MMS observations enable the study of the transition of these correlations between their magnetofluid character at larger scales into the subproton kinetic range, previously unstudied in spacecraft data. Cross-helicity, angular alignment, and energy partitioning is examined over a suitable range of scales, employing measurements based on the Taylor frozen-in approximation as well as direct two-spacecraft correlation measurements. The results demonstrate signatures of alignment at large scales. As kinetic scales are approached, the alignment between v and b is destroyed by demagnetization of protons.

In order to determine particle velocities and electric field in the frame of the magnetic structure, one first needs to determine the velocity of the magnetic structure in the frame of the spacecraft observations. Here, we demonstrate two methods to determine a two‐dimensional magnetic structure velocity for the magnetic reconnection event observed in the magnetotail by the Magnetospheric Multiscale (MMS) spacecraft on July 11, 2017, Spatio‐Temporal Difference (STD) and the recently developed polynomial reconstruction method. Both of these methods use the magnetic field measurements; the reconstruction technique also uses the current density measured by the particle instrument. We find rough agreement between the results of our methods and with other velocity determinations previously published. We also explain a number of features of STD and show that the polynomial reconstruction technique is most likely to be valid within a distance of 2 spacecraft spacings from the centroid of the MMS spacecraft. Both of these methods are susceptible to contamination by magnetometer calibration errors.

Multiple-satellite measurements of magnetic field and/or energetic particles during four magnetospheric substorms are presented. The data were obtained from Ogo 5 and Vela 4A, located near local midnight at about the same distance from the neutral sheet but separated by 3–10 RE in the radial direction. The substorm expansion phases all had multiple onsets, typically observed as multiple bursts of Pi 2 magnetic pulsations (which we use for accurate timing of each onset), auroral zone bay intensifications, and low-latitude positive bay increases. A minimum in the plasma sheet thickness was probably formed at Xsm ∼ −15 RE in association with the formation of an X-type neutral line. Earthward of this line, successive substorm onset signatures were observed in an almost one-to-one relationship with ground Pi 2 bursts: a short (∼5 min) burst of magnetic fluctuations confined to the plasma sheet, a plasma sheet expansion, and a field vector rotation toward a more dipolar orientation. These onsets, which occurred at 10- to 15-min intervals, may have been caused by impulsive enhancements of field line reconnection. Since the plasma sheet was thinning before each new onset, it appears that these enhancements were triggered in the tail each time the plasma sheet became very thin, causing a disruption of the cross-tail electric currents. In the tailward part of the plasma sheet (Xsm ∼ −18 RE) the plasma sheet was thinning down or remained thin until well after the last near-earth expansion, when a recovery occurred. The near-tail observations are consistent with a model in which the associated tailward motion of the neutral line started when the reconnection rate exceeded the earthward flux return rate, producing a tailward pressure which forced the neutral line to move tailward. A comparison with what appears to be a ‘contracted oval substorm’ shows that, even though this substorm apparently took place within a more limited local time sector, it had all the usual substorm features in the tail.

Substorm-associated energetic ion (E ≅ 45 keV) flows at the plasma sheet boundary layer: A dawn-dusk flow reversal

A simple theory of the magnetostatic equilibrium of the magnetospheric plasma sheet is proposed. It takes into consideration the effects of the large‐scale dawn‐to‐dusk electric field and developed medium‐scale turbulence. This makes it possible to describe the main features of the plasma sheet dynamics. The suggested model assumes that the magnetic field lines are not equipotential and that the regular velocity of a plasma parcel is much lower than the chaotic velocity. Hence the plasma is intensely mixed. One of the main consequences of such mixing is the experimentally observed temperature equalization across the plasma sheet. The obtained dependence of plasma pressure on the magnetic vector potential permits one to solve the Grad‐Shafranov equation and reconstruct, in the tail approximation, the magnetic field line configuration for the case of inhomogeneous distribution of the dawn‐to‐dusk electric field along the tail. The thickness of the plasma sheet in the model is determined by the magnitudes of the regular and stochastic electric fields, their scales, and plasma temperature. A quasi three‐dimensional model of the plasma sheet is developed. The substorm dynamics of the plasma sheet is analyzed. The model can explain the thinning of the plasma sheet without variations in the plasma pressure at the tail axis (thinning without compression) during the substorm growth phase. The plasma sheet expansion during substorm expansion phase is related to the growth of turbulent fluctuations.

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.