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Register a new userMagnetospheric Multiscale Observation of Kinetic Signatures in the Alfven Vortex
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Alfven vortex is a multi-scale nonlinear structure which contributes to intermittency of turbulence. Despite previous explorations mostly on the spatial properties of the Alfven vortex (i.e., scale, orientation, and motion), the plasma characteristics within the Alfven vortex are unknown. Moreover, the connection between the plasma energization and the Alfven vortex still remains unclear. Based on high resolution in-situ measurement from the Magnetospheric Multiscale (MMS) mission, we report for the first time, distinctive plasma features within an Alfven vortex. This Alfven vortex is identified to be two-dimensional ($k_{bot} gg k_{|}$) quasi-monopole with a radius of ~10 proton gyroscales. Its magnetic fluctuations $delta B_{bot}$ are anti correlated with velocity fluctuations $delta V_{bot}$, thus the parallel current density $j_{|}$ and flow vorticity $omega_{|}$ are anti-aligned. In different part of the vortex (i.e., edge, middle, center), the ion and electron temperatures are found to be quite different and they behave in the reverse trend: the ion temperature variations are correlated with $j_{|}$, while the electron temperature variations are correlated with $omega_{|}$. Furthermore, the temperature anisotropies, together with the non-Maxwellian kinetic effects, exhibit strong enhancement at peaks of $|omega_{|}| (|j_{|}|)$ within the vortex. Comparison between observations and numerical/theoretical results are made. In addition, the energy-conversion channels and the compressibility associated with the Alfven vortex are discussed. These results may help to understand the link between coherent vortex structures and the kinetic processes, which determines how turbulence energy dissipate in the weakly-collisional space plasmas.
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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.
Using observational data from the emph{Magnetospheric Multiscale} (MMS) Mission in the Earths 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 Karman 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 Karman 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 Earths magnetosheath is about $1000$ times larger than the cascade rate in the pristine solar wind.
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.
The nature of the plasma wave modes around the ion kinetic scales in highly Alfvenic slow solar wind turbulence is investigated using data from the NASAs Parker Solar Probe taken in the inner heliosphere, at 0.18 Astronomical Unit (AU) from the sun. The joint distribution of the normalized reduced magnetic helicity ${sigma}_m ({theta}_{RB}, {tau})$ is obtained, where ${theta}_{RB}$ is the angle between the local mean magnetic field and the radial direction and ${tau}$ is the temporal scale. Two populations around ion scales are identified: the first population has ${sigma}_m ({theta}_{RB}, {tau}) < 0$ for frequencies (in the spacecraft frame) ranging from 2.1 to 26 Hz for $60^{circ} < {theta}_{RB} < 130^{circ}$, corresponding to kinetic Alfven waves (KAWs), and the second population has ${sigma}_m ({theta}_{RB}, {tau}) > 0$ in the frequency range [1.4, 4.9] Hz for ${theta}_{RB} > 150^{circ}$, corresponding to Alfven ion Cyclotron Waves (ACWs). This demonstrates for the first time the co-existence of KAWs and ACWs in the slow solar wind in the inner heliosphere, which contrasts with previous observations in the slow solar wind at 1 AU. This discrepancy between 0.18 and 1 AU could be explained, either by i) a dissipation of ACWs via cyclotron resonance during their outward journey, or by ii) the high Alfvenicity of the slow solar wind at 0.18 AU that may be favorable for the excitation of ACWs.
Foreshock transients are ion kinetic structures in the ion foreshock. Due to their dynamic pressure perturbations, they can disturb the bow shock and magnetosphere-ionosphere system. They can also accelerate particles contributing to shock acceleration. However, it is still unclear how exactly they form. Recent particle-in-cell simulations point out the important role of electric field and Hall current in the formation process. To further examine this, we use data from the Magnetospheric Multiscale (MMS) mission to apply case studies on two small (1000-2000 km) foreshock transient events that just started to form. In event 1 where MMS were in a tetrahedral formation, we show that the current density configuration, which determined the magnetic field profile, was mainly driven by Hall currents generated by demagnetized foreshock ions. The resulting time variation of the magnetic field induced electric field that drove cold plasma moving outward with magnetic field lines. In event 2 where MMS were in a string-of-pearls formation, we analyze the evolution of field and plasma parameters. We show that the magnetic flux and mass flux were transported outward from the core resulting in the steepening of the boundary. The steepened boundary, which trapped more foreshock ions and caused stronger demagnetization of foreshock ions, nonlinearly further enhanced the Hall current. Based on our observations, we propose a physical formation process that the positive feedback of foreshock ions on the varying magnetic field caused by the foreshock ion Hall current enables an instability and the growth of the structure.
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