Search

Studies of solar wind turbulence traditionally employ high-resolution magnetic field data, but high-resolution measurements of ion and electron moments have been possible only recently. We report the first turbulence studies of ion and electron velocity moments accumulated in pristine solar wind by the Fast Particle Investigation instrument onboard the Magnetospheric Multiscale (MMS) Mission. Use of these data is made possible by a novel implementation of a frequency domain Hampel filter, described herein. After presenting procedures for processing of the data, we discuss statistical properties of solar wind turbulence extending into the kinetic range. Magnetic field fluctuations dominate electron and ion velocity fluctuation spectra throughout the energy-containing and inertial ranges. However, a multi-spacecraft analysis indicates that at scales shorter than the ion-inertial length, electron velocity fluctuations become larger than ion velocity and magnetic field fluctuations. The kurtosis of ion velocity peaks around few ion-inertial lengths and returns to near gaussian value at sub-ion scales., Comment: Accepted for publication in The Astrophysical Journal Supplement

Protons (ionized hydrogen) in the solar wind frequently exhibit distinct temperatures ($T_{\perp p}$ and $T_{\parallel p}$) perpendicular and parallel to the plasma's background magnetic-field. Numerous prior studies of the interplanetary solar-wind have shown that, as plasma beta ($\beta_{\parallel p}$) increases, a narrower range of temperature-anisotropy ($R_p\equiv T_{\perp p}\,/\,T_{\parallel p}$) values is observed. Conventionally, this effect has been ascribed to the actions of kinetic microinstabilities. This study is the first to use data from the Magnetospheric Multiscale Mission (MMS) to explore such $\beta_{\parallel p}$-dependent limits on $R_p$ in Earth's magnetosheath. The distribution of these data across the $(\beta_{\parallel p},R_p)$-plane reveals limits on both $R_p>1$ and $R_p<1$. Linear Vlasov theory is used to compute contours of constant growth-rate for the ion-cyclotron, mirror, parallel-firehose, and oblique-firehose instabilities. These instability thresholds closely align with the contours of the data distribution, which suggests a strong association of instabilities with extremes of ion temperature anisotropy in the magnetosheath. The potential for instabilities to regulate temperature anisotropy is discussed., Comment: Submitted to The Astrophysical Journal Letters

Using observational data from the \emph{Magnetospheric Multiscale} (MMS) Mission in the Earth's magnetosheath, we estimate the energy cascade rate using different techniques within the framework of incompressible magnetohydrodynamic (MHD) turbulence. At the energy containing scale, the energy budget is controlled by the von K\'arm\'an decay law. Inertial range cascade is estimated by fitting a linear scaling to the mixed third-order structure function. Finally, we use a multi-spacecraft technique to estimate the Kolmogorov-Yaglom-like cascade rate in the kinetic range, well below the ion inertial length scale. We find that the inertial range cascade rate is almost equal to the one predicted by the von K\'arm\'an law at the energy containing scale, while the cascade rate evaluated at the kinetic scale is somewhat lower, as anticipated in theory~\citep{Yang2017PoP}. Further, in agreement with a recent study~\citep{Hadid2018PRL}, we find that the incompressive cascade rate in the Earth's magnetosheath is about $1000$ times larger than the cascade rate in the pristine solar wind., Comment: Accepted for publication in Astrophysical Journal

In space plasma, various effects of magnetic reconnection and turbulence cause the electron motion to significantly deviate from their Larmor orbits. Collectively these orbits affect the electron velocity distribution function and lead to the appearance of the "non-gyrotropic" elements in the pressure tensor. Quantification of this effect has important applications in space and laboratory plasma, one of which is tracing the electron diffusion region (EDR) of magnetic reconnection in space observations. Three different measures of agyrotropy of pressure tensor have previously been proposed, namely, $A\varnothing_e$, $D_{ng}$ and $Q$. The multitude of contradictory measures has caused confusion within the community. We revisit the problem by considering the basic properties an agyrotropy measure should have. We show that $A\varnothing_e$, $D_{ng}$ and $Q$ are all defined based on the sum of the principle minors (i.e. the rotation invariant $I_2$) of the pressure tensor. We discuss in detail the problems of $I_2$-based measures and explain why they may produce ambiguous and biased results. We introduce a new measure $AG$ constructed based on the determinant of the pressure tensor (i.e. the rotation invariant $I_3$) which does not suffer from the problems of $I_2$-based measures. We compare $AG$ with other measures in 2 and 3-dimension particle-in-cell magnetic reconnection simulations, and show that $AG$ can effectively trace the EDR of reconnection in both Harris and force-free current sheets. On the other hand, $A\varnothing_e$ does not show prominent peaks in the EDR and part of the separatrix in the force-free reconnection simulations, demonstrating that $A\varnothing_e$ does not measure all the non-gyrotropic effects in this case, and is not suitable for studying magnetic reconnection in more general situations other than Harris sheet reconnection., Comment: accepted by Phys. of Plasma

We have used the high-resolution data of the Magnetospheric Multiscale (MMS) mission dayside phase to identify twenty-one previously unreported encounters with the electron diffusion region (EDR), as evidenced by electron agyrotropy, ion jet reversals, and j dot E greater than 0. Three of the new EDR encounters, which occurred within a one-minute-long interval on November 23rd, 2016, are analyzed in detail. These events, which resulted from a relatively low and oscillating magnetopause velocity, contained large electric fields (several tens to hundreds of milliVolts per meter), crescent-shaped electron velocity phase space densities, large currents (greater than 2 microAmperes per square meter), and Ohmic heating of the plasma (near or exceeding 10 nanoWatts per cubic meter). Because of the slow in-and-out motion of the magnetopause, two of these events show the unprecedented mixture of perpendicular and parallel crescents, indicating the first breaking and reconnecting of solar wind and magnetospheric field lines. An extended list of thirty-two EDR or near-EDR events is also included, and demonstrates a wide variety of observed plasma behavior inside and surrounding the reconnection site.

Data from the NASA Magnetospheric Multiscale (MMS) mission are used to investigate asymmetric magnetic reconnection at the dayside boundary between the Earth's magnetosphere and the solar wind (the magnetopause). High-resolution measurements of plasmas, electric and magnetic fields, and waves are used to identify highly localized (~15 electron Debye lengths) standing wave structures with large electric-field amplitudes (up to 100 mV/m). These wave structures are associated with spatially oscillatory dissipation, which appears as alternatingly positive and negative values of J dot E (dissipation). For small guide magnetic fields the wave structures occur in the electron stagnation region at the magnetosphere edge of the EDR. For larger guide fields the structures also occur near the reconnection x-line. This difference is explained in terms of channels for the out-of-plane current (agyrotropic electrons at the stagnation point and guide-field-aligned electrons at the x-line).

Plasma turbulence is investigated using high-resolution ion velocity distributions measured by the Magnetospheric Multiscale Mission (MMS) in the Earth's magnetosheath. The particle distribution is highly structured, suggesting a cascade-like process in velocity space. This complex velocity space structure is investigated using a three-dimensional Hermite transform that reveals a power law distribution of moments. In analogy to hydrodynamics, a Kolmogorov approach leads directly to a range of predictions for this phase-space cascade. The scaling theory is in agreement with observations, suggesting a new path for the study of plasma turbulence in weakly collisional space and astrophysical plasmas.

We report Magnetospheric Multiscale observations of electron pressure gradient electric fields near a magnetic reconnection diffusion region using a new technique for extracting 7.5 ms electron moments from the Fast Plasma Investigation. We find that the deviation of the perpendicular electron bulk velocity from $E \times B$ drift in the interval where the out-of-plane current density is increasing can be explained by the diamagnetic drift. In the interval where the out-of-plane current is transitioning to in-plane current, the electron momentum equation is not satisfied at 7.5 ms resolution., Comment: 6 pages, 4 figures

Collisionless space plasma turbulence can generate reconnecting thin current sheets as suggested by recent results of numerical magnetohydrodynamic simulations. The MMS mission provides the first serious opportunity to check if small ion-electron-scale reconnection, generated by turbulence, resembles the reconnection events frequently observed in the magnetotail or at the magnetopause. Here we investigate field and particle observations obtained by the MMS fleet in the turbulent terrestrial magnetosheath behind quasi-parallel bow shock geometry. We observe multiple small-scale current sheets during the event and present a detailed look of one of the detected structures. The emergence of thin current sheets can lead to electron scale structures where ions are demagnetized. Within the selected structure we see signatures of ion demagnetization, electron jets, electron heating and agyrotropy suggesting that MMS spacecraft observe reconnection at these scales.

MMS observations recently confirmed that crescent-shaped electron velocity distributions in the plane perpendicular to the magnetic field occur in the electron diffusion region near reconnection sites at Earth's magnetopause. In this paper, we re-examine the origin of the crescent-shaped distributions in the light of our new finding that ions and electrons are drifting in opposite directions when displayed in magnetopause boundary-normal coordinates. Therefore, ExB drifts cannot cause the crescent shapes. We performed a high-resolution multi-scale simulation capturing sub-electron skin depth scales. The results suggest that the crescent-shaped distributions are caused by meandering orbits without necessarily requiring any additional processes found at the magnetopause such as the highly asymmetric magnetopause ambipolar electric field. We use an adiabatic Hamiltonian model of particle motion to confirm that conservation of canonical momentum in the presence of magnetic field gradients causes the formation of crescent shapes without invoking asymmetries or the presence of an ExB drift. An important consequence of this finding is that we expect crescent-shaped distributions also to be observed in the magnetotail, a prediction that MMS will soon be able to test., Comment: to appear on J. Geophys. Res

Magnetic holes (MHs), with a scale much greater than \r{ho}i (proton gyroradius), have been widely reported in various regions of space plasmas. On the other hand, kinetic-size magnetic holes (KSMHs), previously called small size magnetic holes (SSMHs), with a scale of the order of magnitude of or less than \r{ho}i have only been reported in the Earth's magnetospheric plasma sheet. In this study, we report such KSMHs in the magnetosheath whereby we use measurements from the Magnetospheric Multiscale (MMS) mission, which provides three-dimensional (3D) particle distribution measurements with a resolution much higher than previous missions. The MHs have been observed in a scale of 10 ~ 20 \r{ho}e (electron gyroradii) and lasted 0.1 ~ 0.3 s. Distinctive electron dynamics features are observed, while no substantial deviations in ion data are seen. It is found that at the 90{\deg} pitch angle, the flux of electrons with energy 34 ~ 66 eV decreased while for electrons of energy 109 ~ 1024 eV increased inside the MHs. We also find the electron flow vortex perpendicular to the magnetic field, a feature self-consistent with the magnetic depression. Moreover, the calculated current density is mainly contributed by the electron diamagnetic drift, and the electron vortex flow is the diamagnetic drift flow. The electron magnetohydrodynamics (EMHD) soliton is considered as a possible generation mechanism for the KSMHs with the scale size of 10 ~ 20 \r{ho}e., Comment: The paper need to be removed as some data is not corrected

We report the observations of an electron vortex magnetic hole corresponding to a new type of coherent structures in the magnetosheath turbulent plasma using the Magnetospheric Multiscale (MMS) mission data. The magnetic hole is characterized by a magnetic depression, a density peak, a total electron temperature increase (with a parallel temperature decrease but a perpendicular temperature increase), and strong currents carried by the electrons. The current has a dip in the center of the magnetic hole and a peak in the outer region of the magnetic hole. The estimated size of the magnetic hole is about 0.23 \r{ho}i (~ 30 \r{ho}e) in the circular cross-section perpendicular to its axis, where \r{ho}i and \r{ho}e are respectively the proton and electron gyroradius. There are no clear enhancement seen in high energy electron fluxes, but an enhancement in the perpendicular electron fluxes at ~ 90{\deg} pitch angles inside the magnetic hole is seen, implying that the electron are trapped within it. The variations of the electron velocity components Vem and Ven suggest that an electron vortex is formed by trapping electrons inside the magnetic hole in the circular cross-section (in the M-N plane). These observations demonstrate the existence of a new type of coherent structures behaving as an electron vortex magnetic hole in turbulent space plasmas as predicted by recent kinetic simulations., Comment: 19 pages, 4 figures

Electrons are accelerated to nonthermal energies at shocks in space and astrophysical environments. While shock drift acceleration (SDA) has been considered a key process of electron acceleration at Earth’s bow shock, it has also been recognized that SDA needs to be combined with an additional stochastic process to explain the observed power-law energy spectra. Here, we show mildly energetic (∼0.5 keV) electrons are locally scattered (and accelerated while being confined) by magnetosonic-whistler waves within the shock transition layer, especially when the shock angle is large (θ(sub Bn) approximately equal or greater than 70°). When measured by the Magnetospheric Multiscale mission at a high cadence, ∼0.5 keV electron flux increased exponentially in the shock transition layer. However, the flux profile was not entirely smooth and the fluctuation showed temporal/spectral association with large-amplitude (δB/B ~ 0.3), low-frequency (approximately equal or less than 0.1 Ω(sub ce) where Ω(sub ce) is the cyclotron frequency), obliquely propagating (θ(sub kB) ~ 30°–60°, where θ(sub kB) is the angle between the wave vector and background magnetic field) whistler waves, indicating that the particles were interacting with the waves. Particle simulations demonstrate that, although linear cyclotron resonances with ∼0.5 keV electrons are unlikely due to the obliquity and low frequencies of the waves, the electrons are still scattered beyond 90° pitch angle by (1) resonant mirroring (transit-time damping), (2) non-resonant mirroring, and (3) subharmonic cyclotron resonances. Such coupled nonlinear scattering processes are likely to provide the stochasticity needed to explain the power-law formation.

We examine the role that ions and electrons play in reconnection using observations from the Magnetospheric Multiscale (MMS) mission on kinetic ion and electron scales, which are much shorter than magnetohydrodynamic scales. This study reports observations with unprecedented high resolution that MMS provides for magnetic field (7.8 ms) and plasma (30 ms for electrons and 150 ms for ions). We analyze and compare approaches to the magnetopause in 2016 November, to the electron diffusion region in the magnetotail in 2017 July followed by a current sheet crossing in 2018 July. Besides magnetic field reversals, changes in the direction of the flow velocity, and ion and electron heating, MMS observed large fluctuations in the electron flow speeds in the magnetotail. As expected from numerical simulations, we have verified that when the field lines and plasma become decoupled a large reconnecting electric field related to the Hall current (1–10 mV/m) is responsible for fast reconnection in the ion diffusion region. Although inertial accelerating forces remain moderate (1–2 mV/m), the electric fields resulting from the divergence of the full electron pressure tensor provide the main contribution to the generalized Ohm’s law at the neutral sheet (as large as 200 mV/m). In our view, this illustrates that when ions decouple electron physics dominates. The results obtained on kinetic scales may be useful for better understanding the physical mechanisms governing reconnection processes in various magnetized laboratory and space plasmas.