Magnetospheric Multiscale Mission Research Articles

Statistical Comparison of Various Dayside Magnetopause Reconnection X‐Line Prediction Models

AbstractReconnection at Earth's magnetopause drives magnetospheric convection and provides mass and energy input into the magnetosphere/ionosphere system thereby driving the coupling between solar wind and terrestrial magnetosphere. Despite its importance, the factors governing the location of dayside magnetopause reconnection are not well understood. Though a few models can predict X‐line locations reasonably well, the underlying physics is still unresolved. In this study we present results from a comparative analysis of 274 magnetic reconnection events as observed by the Magnetospheric Multiscale (MMS) mission to determine what quantities affect the accuracy of such models and are most strongly associated with the occurrence of dayside magnetopause reconnection. We also attempt to determine under what upstream solar wind conditions each global X‐line model becomes least reliable.

Observations of Tilted Electron Vortex Flux Rope in the Magnetic Reconnection Tailward Outflow Region

AbstractWith high‐resolution data from Magnetospheric Multiscale (MMS) mission, an ion‐scale flux rope (FR) with a heavily tilted axis is observed in the tailward outflow of a magnetic reconnection in the terrestrial magnetotail. Combined with the field‐aligned electron distribution and positions of MMS when the X‐line and FR are observed, the tilted axis is inferred to be caused by the extension of the X‐line in the dawn‐dusk direction. J · E′ is negative and electrons are losing energy in the FR. An ion‐scale electron vortex embedded in the plane perpendicular to the axis is observed inside FR. The induced magnetic field generated by the electron vortex has the same direction as the axial component, which can contribute to the axial component and increase the magnetic flux of the FR. Such electron vortex FRs may be an essential carrier of magnetic flux from near‐Earth X‐line to distant X‐line or interplanetary space.

Statistical analysis of stochastic magnetic fluctuations in space plasma based on theMMSmission

ABSTRACTBased on the Magnetospheric Multiscale (MMS) mission we look at magnetic field fluctuations in the Earth’s magnetosheath. We apply the statistical analysis using a Fokker–Planck equation to investigate processes responsible for stochastic fluctuations in space plasmas. As already known, turbulence in the inertial range of hydromagnetic scales exhibits Markovian features. We have extended the statistical approach to much smaller scales in space, where kinetic theory should be applied. Here we study in detail and compare the characteristics of magnetic fluctuations behind the bow shock, inside the magnetosheath, and near the magnetopause. It appears that the first Kramers–Moyal coefficient is linear and the second term is a quadratic function of magnetic increments, which describe drift and diffusion, correspondingly, in the entire magnetosheath. This should correspond to a generalization of Ornstein–Uhlenbeck process. We demonstrate that the second-order approximation of the Fokker–Planck equation leads to non-Gaussian kappa distributions of the probability density functions. In all cases in the magnetosheath, the approximate power-law distributions are recovered. For some moderate scales, we have the kappa distributions described by various peaked shapes with heavy tails. In particular, for large values of the kappa parameter this shape is reduced to the normal Gaussian distribution. It is worth noting that for smaller kinetic scales the rescaled distributions exhibit a universal global scale invariance, consistently with the stationary solution of the Fokker–Planck equation. These results, especially on kinetic scales, could be important for a better understanding of the physical mechanism governing turbulent systems in space and astrophysical plasmas.

Active Spacecraft Potential Control in the MMS Mission: Results From Six Years in Orbit

The four spacecraft of the NASA Magnetospheric Multiscale (MMS) mission carry instruments to reduce the positive potential by means of indium ion beams. Since the start of the nominal mission in September 2015 and until the end of 2021, the instruments active spacecraft potential control (ASPOC) have been actively operating for more than 16 000 h at a nominal emission current of 20 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> A per spacecraft. Based on data from more than six years in orbit with more than 50 000 h in regions of scientific interest, statistical results regarding the potential’s interdependencies with ambient plasma were obtained. This article reports on the derivation of the photo electron energy spectrum from the correlation between the potential and the plasma data obtained by the fast plasma instrument with and without controlled potential. Finally, the time constants during the relaxation of the controlled potential when the active control instrument is turned off, if measured at high time resolution, allow to estimate the electric capacitance of the spacecraft system.

Successive Energy Conversion at a Stepwise Dipolarization Front

Dipolarization fronts (DFs), ion-scale magnetic structures characterized by sharp enhancement of northward magnetic field developed within plasma jets, have been suggested to play a crucial role in the energy transfer chain in the terrestrial magnetotail. Here we present the first observation of successive energy conversion driven by multiple current layers at a stepwise DF, using high-cadence measurements from NASA’s Magnetospheric Multiscale mission. The multiple current layers are adjacent and cause gradual variations of particles and electromagnetic fields, leading to an intense, successive increase of energy conversion rates at the DF in the satellite frame, with electromagnetic field energy being transformed into particle energy. The energy conversion is contributed by both ion and electron currents. The motional electric field drives the energy conversion, while the wave electric field fed by lower hybrid drift instability modulates the energy partition. These results provide new insights into understanding energy transfer in the terrestrial magnetotail.

Formation of Electron Butterfly Distribution by a Contracting Dipolarization Front

AbstractThe electron butterfly distribution, characterized by pitch angles (PA) primarily at 45° and 135°, was rarely observed in Earth's magnetotail. Here using the high‐resolution measurements from Magnetospheric Multiscale mission, we present the observation of electron butterfly distribution in a contracting dipolarization front (DF), and propose a new physical mechanism to explain its formation. Specifically, we discover that the electron butterfly distribution only exhibited in the locally contracted DF and was observed above 1.7 keV. We infer that local contraction of the DF transformed its configuration from a magnetic bottle to an hourglass‐shaped magnetic structure, and the butterfly distribution was formed by the magnetic mirror effect of this magnetic hourglass. Additionally, the theoretically estimated loss cone of the magnetic hourglass fits well with the observations of electrons, validating our inference about the formation mechanism. These findings can improve our understanding of electron dynamics in Earth's magnetosphere.

Interhemispheric Observations of ULF Waves Caused by Foreshock Transients Under Quiet Solar Wind Conditions

AbstractForeshock transient (FT) events are frequently observed phenomena that are generated by discontinuities in the solar wind. These transient events are known to trigger global‐scale magnetic field perturbations (e.g., ULF waves). We report a series of FT events observed by the Magnetospheric Multiscale mission in the upstream bow shock region under quiet solar wind conditions. During the event, ground magnetometers observed significant Pc1 wave activity as well as magnetic impulse events in both hemispheres. Ground Pc1 wave observations show ∼8 min time delay (with some time differences) from each FT event which is observed at the bow shock. We also find that the ground Pc1 waves are observed earlier in the northern hemisphere compared to the southern hemisphere. The observation time difference between the hemispheres implies that the source region of the wave is the off‐equatorial region.

Mesoscale Structure and Properties of the Terrestrial Magnetotail Plasma Sheet From the Magnetospheric Multiscale Mission

AbstractUsing Magnetospheric Multiscale mission (MMS) orbits in the Earth's magnetotail from 2017 to 2020, plasma conditions and the 3D spatial structure of inner‐magnetotail plasma environments (with a focus on the plasma sheet (PS)) are studied with different approaches. Threshold conditions for distinguishing the PS, PS boundary layers, and lobes are derived from the statistical properties of background plasma parameters. Our results support previous studies that employed similar methods using Cluster data. However, stronger currents are observed in both the lobes and PS, likely due to the smaller spacecraft separation (≲70 km) that can resolve thin electron‐scale currents. Threshold conditions are used together with magnetic field and electric field measurements to image the spatial structure of the PS. Results are in good agreement with a global neutral sheet model based on solar wind conditions and magnetospheric configurations. Furthermore, the Earth's dipole tilts toward the Sun around June solstice, which warps the magnetotail as much as ∼2–4 RE in Z geocentric solar magnetospheric. This warping effect is relaxed toward September equinox. Consequently, as MMS travels through the magnetotail from dawn to dusk during this period, there is an apparent dawn‐dusk asymmetry in plasma conditions between June and September. Kink‐like flapping waves and interplanetary magnetic field twisting are other mesoscale processes attributed with a few RE of flaring near the flanks. These findings reveal important insights into the mesoscale structure and dynamics of the magnetotail.

Electron Heating in Shocks: Statistics and Comparison

AbstractSupernova remnant (SNR) shocks are the highest Mach number non‐relativistic shocks in electron‐ion plasmas. These shocks are the most efficient particle accelerators in space. SNR shock parameters are inferred from measurements of electromagnetic radiation from heated and accelerated particles. Temperature of the shock heated electrons is one of the most important parameters in supernova remnant shocks. Knowledge of the downstream electron‐to‐ion temperature ratio or of the ratio of the downstream electron temperature to the incident ion energy is crucial for understanding physics of the very high‐Mach number SNR shocks. Heliospheric shocks have substantially lower Mach numbers than SNR shocks but can be extensively studied in in situ observations with further extrapolation of the findings to higher Mach numbers. Magnetospheric Multiscale mission observations of the Earth bow shock are used to analyze dependence of the electron heating on the shock Mach number. It is found that the ratio of the downstream electron temperature to the incident ion energy decreases with the increase of the Mach number. At high Mach numbers this ratio and stabilizes at about 2.5%. The electron‐to‐ion temperature ratio stabilizes at about 10%. The peak electron temperature occurs at the overshoot maximum, further downstream electrons cool down. The mean ratio of the 4.5 s averages of the downstream and maximum electron temperatures is 0.85. Electron heating does not follow the thermodynamic adiabatic law. The heating and cooling behavior implies that the energy is provided by the overall cross‐shock potential while small‐scale electric fields rapidly isotropize the electron distribution.

MMS Observation of Two‐Step Electron Acceleration at Earth's Bow Shock

AbstractWe use the Magnetospheric Multiscale mission to observe a bi‐directional electron acceleration event in the electron foreshock upstream of Earth's quasi‐perpendicular collisionless bow shock. The acceleration region is associated with a decrease in wave activity, inconsistent with common electron acceleration mechanisms such as Diffusive Shock Acceleration and Stochastic Shock Drift Acceleration. We propose a two‐step acceleration process where an electron field‐aligned beam acts as a seed population further accelerated by a shrinking magnetic bottle process, with the shock acting as the magnetic mirror(s).

Collisional-like dissipation in collisionless plasmas

When collisions are strong in a magnetized plasma, standard closures provide simple representations of dissipation in terms of coefficients of viscosity and resistivity. In the opposite limit of weak collisions, the analogous physical effects that lead to dissipation are present, but the simple approximations to describe them, the closures, are not available in general. But how different are these relationships when collisions are absent? Here, we inquire as to whether the collisionless case admits statistical relationships analogous to the viscous and resistive closures found in collisional plasma. We employ kinetic particle-in-cell simulations of proton–electron plasma as well as in situ observations from the Magnetospheric Multiscale Mission, to examine analogous viscous-like and resistive-like scaling in the weakly collisional regime. Rather surprisingly, we find that, on average, the collisionless cases do exhibit dissipation behavior very similar to the collisional plasmas. It is of theoretical significance that the relationships found are statistical and not deterministic as they are when collisional closures are enforced.

Estimation of the Error in the Calculation of the Pressure‐Strain Term: Application in the Terrestrial Magnetosphere

AbstractCalculating the pressure‐strain terms has recently been performed to quantify energy conversion between the bulk flow energy and the internal energy of plasmas. It has been applied to numerical simulations and satellite data from the Magnetospheric MultiScale Mission. The method requires spatial gradients of the velocity and the use of the full pressure tensor. Here we present a derivation of the errors associated with calculating the pressure‐strain terms from multi‐spacecraft measurements and apply it to previously studied examples of magnetic reconnection at the magnetopause and the magnetotail. The errors are small in a dense magnetosheath event but much larger in the more tenuous magnetotail. This is likely due to larger counting statistics in the dense plasma at the magnetopause than in the magnetotail. The propagated errors analyzed in this work are important to understand uncertainties of energy conversion measurements in space plasmas and have applications to current and future multi‐spacecraft missions.

In Situ Observation of Electron Acceleration by a Double Layer in the Bow Shock

The importance of the electric potential along the field direction in the acceleration of electrons in the quasi-perpendicular bow shock has been proposed. Nevertheless, the mechanism by which electrons are accelerated in the bow shock remains unclear from the perspective of in situ observation. In this study, utilizing data obtained by the Magnetospheric Multiscale mission, we identify a double layer (DL) located within the transition layer of the quasi-perpendicular bow shock. It is found that the electric potential generated by this DL corresponds well with an increase in electron parallel temperature, suggesting that the DL can effectively accelerate the electrons in the bow shock. Furthermore, strong whistler emissions have been detected in the vicinity of this DL. It seems that the electrons accelerated by the DL could potentially supply the necessary free energy to excite the observed whistler waves. Thus, the presence of whistler waves provides indirect evidence of an electron beam generated by the DL in bow shock. Our observations demonstrate that the DL can accelerate electrons effectively and impact the electron dynamics within the bow shock.

Electrostatic lower hybrid turbulence due to magnetic islands and particle heating in magnetopause reconnection regions

This investigation presents a model to understand the magnetopause turbulence in the lower hybrid frequency range at reconnection regions as observed by the magnetospheric multiscale mission. We have modeled 3D electrostatic lower hybrid wave (LHW) in cold magnetized plasma in the presence of magnetic islands. In this model, we have developed the modified nonlinear Schrödinger equation, contemplating that nonlinear effects are due to ponderomotive force. The dynamical equation of the system thus obtained is solved using numerical simulation techniques. The simulation results illustrate the formation and evolution of localized structures and the current sheets. Due to the nonlinear effects and magnetic reconnection, the current sheets become turbulent with time. A semi-analytical model is also given to illustrate the scale size of current sheets. The turbulent power spectra have also been studied. The spectral index of the power spectrum is found to be −2.7 (approximately) in the perpendicular direction and −2.2 (approximately) in the parallel direction, indicating anisotropy. Using the Folker–Plank diffusion equation with the new velocity space diffusion coefficient, we find the distribution function of energetic electrons due to these turbulent structures. We propose that LHW may be responsible for plasma heating in magnetopause.

Wave analysis during energetic electron microinjections: A case study

The Magnetospheric Multi-scale Mission has frequently observed periodic bursts of counterstreaming electrons with energies ranging from ≈ 30 to 500 keV at the Earth's magnetospheric boundary layers, termed “microinjections.” Recently, a source region for microinjections was discovered at the high-latitude magnetosphere where microinjections showed up simultaneously at all energy channels and were organized by magnetic field variation associated with ultra low frequency mirror mode waves (MMWs) with ≈ 5 min periodicity. These MMWs were associated with strong higher frequency electromagnetic wave activity. Here, we have identified some of these waves as electromagnetic ion cyclotron (EMIC) waves. EMIC waves and parallel electric fields often lead to the radiation belt electron losses due to pitch-angle scattering. We show that, for the present event, the EMIC waves are not responsible for scattering electrons into a loss cone, and thus, they are unlikely to be responsible for the observed microinjection signature. We also find that the parallel electric field potentials within the waves are not adequate to explain the observed electrons with &amp;gt;90 keV energies. While whistler waves may contribute to the electron scattering and may exist during this event, there was no burst mode data available to verify this.

Waves in Magnetosheath Jets—Classification and the Search for Generation Mechanisms Using MMS Burst Mode Data

AbstractMagnetosheath jets are localized dynamic pressure enhancements in the magnetosheath. We make use of the high time resolution burst mode data of the Magnetospheric Multiscale mission for an analysis of waves in plasmas associated with three magnetosheath jets. We find both electromagnetic and electrostatic waves over the frequency range from 0 to 4 kHz that can be probed by the instruments on board the MMS spacecraft. At high frequencies we find electrostatic solitary waves, electron acoustic waves, and whistler waves. Electron acoustic waves and whistler waves show the typical properties expected from theory assuming approximations of a homogeneous plasma and linearity. In addition, 0.2 Hz waves in the magnetic field, 1 Hz electromagnetic waves, and lower hybrid waves are observed. For these waves the approximation of a homogeneous plasma does not hold anymore and the observed waves show properties from several different basic wave modes. In addition, we investigate how the various types of waves are generated. We show evidence that, the 1 Hz waves are connected to gradients in the density and magnetic field. The whistler waves are generated by a butterfly‐shaped pitch‐angle distribution and the electron acoustic waves by a cold electron population. The lower hybrid waves are probably generated by currents at the boundary of the jets. As for the other waves we can only speculate about the generation mechanism due to limitations of the instruments. Studying waves in jets will help to address the microphysics in jets which can help to understand the evolution of jets better.

Electron Acceleration by Interaction of Two Filamentary Currents Within a Magnetopause Magnetic Flux Rope

AbstractTwo types of filamentary currents (FCs) were observed inside a magnetic flux rope at the magnetopause by the Magnetospheric Multiscale mission. The first FC is identified as an electron vortex, while the other is a reconnecting current sheet. Stochastic electric fields were generated within the FCs, resulting in electron acceleration up to a few keV, similar to recent simulations of electron acceleration inside vortex, which is a second‐order Fermi acceleration. Furthermore, two FCs propagated at different speeds, causing compression in the region between them. Energetic electrons up to 200 keV were detected in the compressed region and displayed a double power‐law spectrum. Observations suggest that the electrons were mainly accelerated by betatron mechanism in the compressed region. The formation, evolution, and interaction of FCs provide a novel mechanism for electron acceleration. These results clearly show the significance of electron‐scale dynamics within flux rope.

Turbulent Energy Transfer at Dipolarization Fronts

AbstractDipolarization fronts (DFs), ion‐scale magnetic transients characterized by dramatic enhancement of northward magnetic field, have been documented as crucial energy transfer regions in the magnetosphere. DF‐driven energy transfer has hitherto been studied mainly in the laminar regime. Energy transfer driven by turbulent processes, however, remains unclear. Here we perform a comprehensive investigation of turbulent energy transfer (TET) developed at DFs, via using high‐cadence data from Magnetospheric Multiscale mission. We find that: (a) TET is equally governed by energy loads and generators, different from laminar energy transfer which is typically dominated by energy loads; (b) ion and electron currents play comparable roles in driving TET; (c) TET is positively correlated with local magnetic field strength and ion speed; (d) TET shows asymmetric global distributions along the dawn‐dusk direction. These features implicate that TET is primarily related to electromagnetic turbulence at electron‐ion hybrid scales. These new results, uncovering unique characteristics of DF‐driven TET, can deeply advance our understanding of energy budgets in the magnetosphere.

Direct Evidence of Electron Acceleration at the Dipolarization Front

The dramatic changes in the magnetic field at the dipolarization front (DF) provide a suitable environment for electron acceleration, which usually can cause the flux enhancement of energetic electrons behind the front. However, it is unknown whether energetic electrons observed at the DF are energized locally, and which mechanism accelerates the electrons at the DF is unclear. Our study performs a direct quantitative analysis to reveal the acceleration process of energetic electrons at the DF using the high-time-resolution data from NASA's Magnetospheric Multiscale mission. The fluxes of energetic electrons at 90° are enhanced at the front. Under adiabatic conditions, our quantitative analysis indicates that these electrons at the front could be locally accelerated to over 100 keV by betatron acceleration. Eventually, the electron temperature anisotropy formed via the betatron mechanism could provide the free energy to excite whistler waves at the DF. Our quantitative study provides, for the first time, strong direct evidence for the local electron acceleration at the DF.

The Nonorthogonal X-line in a Small Guide-field Reconnection Event in the Magnetotail

Magnetic reconnection is a fundamental plasma process that has been studied with analytical theory, numerical simulations, in situ observations, and laboratory experiments for decades. The models that have been established to describe magnetic reconnection often assume a reconnection plane normal to the current sheet in which an antiparallel magnetic field annihilates. The annihilation points, also known as the X-points, form an x-line, which is believed to be perpendicular to the reconnection plane. Recently, a new study using Magnetospheric Multiscale mission observations has challenged our understanding of magnetic reconnection by providing evidence that the x-line is not necessarily orthogonal to the reconnection plane. In this study we report a second nonorthogonal x-line event with similar features as that in the previous case study, supporting that the sheared x-line phenomenon is not an aberrant event. We employ a detailed directional derivative analysis to identify the x-line direction and show that the in-plane reconnection characteristics are well maintained even with a nonorthogonal x-line. In addition, we find the x-line tends to follow the magnetic field on one side of the current sheet, which suggests an asymmetry across the current sheet. We discuss the possibility that the nonorthogonal x-line arises from an interplay between the two aspects of reconnection: the macroscopic magnetic field topology and microscopic particle kinetics.