Magnetospheric multiscale (MMS) mission attitude ground system design

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.

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.

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.

We present characteristics of current layers in the off‐equatorial near‐Earth plasma sheet boundary observed with high time‐resolution measurements from the Magnetospheric Multiscale mission during an intense substorm associated with multiple dipolarizations. The four Magnetospheric Multiscale spacecraft, separated by distances of about 50 km, were located in the southern hemisphere in the dusk portion of a substorm current wedge. They observed fast flow disturbances (up to about 500 km/s), most intense in the dawn‐dusk direction. Field‐aligned currents were observed initially within the expanding plasma sheet, where the flow and field disturbances showed the distinct pattern expected in the braking region of localized flows. Subsequently, intense thin field‐aligned current layers were detected at the inner boundary of equatorward moving flux tubes together with Earthward streaming hot ions. Intense Hall current layers were found adjacent to the field‐aligned currents. In particular, we found a Hall current structure in the vicinity of the Earthward streaming ion jet that consisted of mixed ion components, that is, hot unmagnetized ions, cold E × B drifting ions, and magnetized electrons. Our observations show that both the near‐Earth plasma jet diversion and the thin Hall current layers formed around the reconnection jet boundary are the sites where diversion of the perpendicular currents take place that contribute to the observed field‐aligned current pattern as predicted by simulations of reconnection jets. Hence, multiscale structure of flow braking is preserved in the field‐aligned currents in the off‐equatorial plasma sheet and is also translated to ionosphere to become a part of the substorm field‐aligned current system.

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.

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.

Observations from the Magnetospheric Multiscale (MMS) mission are used to quantify the maximum effect of magnetospheric H+ and He+ on dayside magnetopause reconnection. A data base of current‐sheet crossings from the first 2 years of the MMS mission is used to identify magnetopause crossings with the highest He+ concentrations. While all of these magnetopause crossings exhibit evidence of plasmaspheric plume material, only half of the crossings are directly associated with plasmaspheric plumes. The He+ density varies dramatically within the magnetosphere adjacent to the magnetopause, with density variations of an order of magnitude on timescales as short as 10 s, the time resolution of the composition instrument on MMS. Plasma wave observations are used to determine the total electron density, and composition measurements are used to determine the mass density in the magnetosheath and magnetosphere. These mass densities are then used with the magnetic field observations to determine the theoretical reduction in the reconnection rate at the magnetopause. The presence of high‐density plasmaspheric plume material at the magnetopause causes transient reductions in the reconnection rate of up to ∼40%.

We identify the electron diffusion region (EDR) of a guide field dayside reconnection site encountered by the Magnetospheric Multiscale (MMS) mission and estimate the terms in generalized Ohm's law that controlled energy conversion near the X‐point. MMS crossed the moderate‐shear (∼130°) magnetopause southward of the exact X‐point. MMS likely entered the magnetopause far from the X‐point, outside the EDR, as the size of the reconnection layer was less than but comparable to the magnetosheath proton gyroradius, and also as anisotropic gyrotropic “outflow” crescent electron distributions were observed. MMS then approached the X‐point, where all four spacecraft simultaneously observed signatures of the EDR, for example, an intense out‐of‐plane electron current, moderate electron agyrotropy, intense electron anisotropy, nonideal electric fields, and nonideal energy conversion. We find that the electric field associated with the nonideal energy conversion is (a) well described by the sum of the electron inertial and pressure divergence terms in generalized Ohms law though (b) the pressure divergence term dominates the inertial term by roughly a factor of 5:1, (c) both the gyrotropic and agyrotropic pressure forces contribute to energy conversion at the X‐point, and (d) both out‐of‐the‐reconnection‐plane gradients (∂/∂M) and in‐plane (∂/∂L,N) in the pressure tensor contribute to energy conversion near the X‐point. This indicates that this EDR had some electron‐scale structure in the out‐of‐plane direction during the time when (and at the location where) the reconnection site was observed.

When the interplanetary magnetic field (IMF) is southward and has a substantial Y component, reconnection at the magnetopause occurs at low latitudes. Under these conditions, the maximum magnetic shear model for the reconnection X‐line at the magnetopause predicts a continuous X‐line stretching from the dawn to dusk terminators. During the solstices, the X‐line is not at the subsolar point and may be located in a region where the magnetosheath bulk flow is super‐Alfvenic. For a fixed IMF direction, the maximum shear model also predicts a stationary X‐line. In response to IMF clock angle changes on the timescale of minutes, the X‐line moves on the same timescale. The stationarity of the reconnection X‐line is testable observationally under certain, restrictive conditions. This stationarity is tested using observations from the Magnetospheric Multiscale mission. For two events, the distance from the Magnetospheric Multiscale spacecraft to the X‐line is constant over several minutes (within relatively large error bars) and the X‐line is also near the location predicted by the maximum magnetic shear model. Thus, the reconnection X‐line at the magnetopause appears to be quasi‐stationary for constant IMF clock angle. These observations also place constraints on the formation and motion of multiple X‐lines at the magnetopause.

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.

Observations at the reconnection site from the Magnetospheric Multiscale mission are used to examine the occurrence of magnetic reconnection along the magnetopause in relation to both the upstream interplanetary magnetic field (IMF) orientation and local magnetosheath variations. While there is a statistically larger number of reconnection intervals observed downstream of the quasi‐parallel bow shock than is accounted for based on the histogram of overall IMF orientations, it is surmised that increased fluctuations downstream of the quasi‐parallel bow shock cause greater large‐scale motion of the magnetopause surface; but do not preferentially enhance the occurrence of steady magnetic reconnection. An examination of the locations of observed reconnection sites relative to model predictions also suggest that enhanced magnetosheath fluctuations do not result in steady reconnection occurring at random locations at the magnetopause.

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.

The Earth’s magnetosheath is a dynamic region and its properties strongly depend on the angle between the bow shock normal and the solar wind magnetic field (θbn). If the shock is quasi-parallel (θbn < 45°), the magnetosheath is magnetically connected to the foreshock, causing strong fluctuations and structures propagating from upstream to downstream. A quasi-perpendicular shock (θbn > 45°) produces a less structured and more stationary magnetosheath characterized by compression and high ion temperature anisotropy. These distinct configurations make it possible to study how different plasma environments affect various processes such as turbulence, heating, and wave-particle interactions. Therefore, such studies require an accurate classification of the magnetosheath. This is not easily achieved, especially close to the magnetopause where the shock crossing for the plasma of interest cannot be observed.Previously, Karlsson et al. (2021) used data from the Cluster mission to propose a promising classification method using local measurements of the magnetic field standard deviation, high-energy ion flux, and ion temperature anisotropy. In this work, we are building on this study and extending it to the Magnetospheric Multiscale (MMS) mission, having a different orbit than Cluster. We compare this local classification to θbn estimated from upstream conditions and well-known bow shock models, and discuss the advantages and disadvantages of the different methods. Reference: Karlsson, T., Raptis, S., Trollvik, H., & Nilsson, H. (2021). Classifying the magnetosheath behind the quasi-parallel and quasi-perpendicular bow shock by local measurements. Journal of Geophysical Research: Space Physics, 126, e2021JA029269.