We report on the observations of an electron vortex magnetic hole corresponding to a new type of coherent structure in the turbulent magnetosheath plasma using the Magnetospheric Multiscale 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 core region and a peak in the outer region of the magnetic hole. The estimated size of the magnetic hole is about 0.23 i (∼30 e) in the quasi-circular cross-section perpendicular to its axis, where i and e are respectively the proton and electron gyroradius. There are no clear enhancements seen in high-energy electron fluxes. However, there is an enhancement in the perpendicular electron fluxes at 90° pitch angle inside the magnetic hole, implying that the electrons 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 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.
Selected Time History of Events and Macroscale Interactions During Substorms observations at medium latitudes of highly oblique and high‐amplitude chorus waves are presented and analyzed. The ...presence of such very intense waves is expected to have important consequences on electron energization in the magnetosphere. An analytical model is therefore developed to evaluate the efficiency of the trapping and acceleration of energetic electrons via Landau resonance with such nearly electrostatic chorus waves. Test‐particle simulations are then performed to illustrate the conclusions derived from the analytical model, using parameter values consistent with observations. It is shown that the energy gain can be much larger than the initial particle energy for 10 keV electrons, and it is further demonstrated that this energy gain is weakly dependent on the density variation along field lines.
Key Points
Chorus may propagate in a quasi‐electrostatic mode
The parallel component of wave electric field is about 25%
The large parallel wave electric field can trap electrons into Landau resonance
We present the first quantified measure of the energy dissipation rates, due to wave‐particle interactions, in the transition region of the Earth's collisionless bow shock using data from the Time ...History of Events and Macroscale Interactions during Substorms spacecraft. Our results show that wave‐particle interactions can regulate the global structure and dominate the energy dissipation of collisionless shocks. In every bow shock crossing examined, we observed both low‐frequency (<10 Hz) and high‐frequency (≳10 Hz) electromagnetic waves throughout the entire transition region and into the magnetosheath. The low‐frequency waves were consistent with magnetosonic‐whistler waves. The high‐frequency waves were combinations of ion‐acoustic waves, electron cyclotron drift instability driven waves, electrostatic solitary waves, and whistler mode waves. The high‐frequency waves had the following: (1) peak amplitudes exceeding δB∼ 10 nT and δE∼ 300 mV/m, though more typical values were δB∼ 0.1–1.0 nT and δE∼ 10–50 mV/m; (2) Poynting fluxes in excess of 2000 μW m−2 (typical values were ∼1–10 μW m−2); (3) resistivities > 9000 Ω m; and (4) associated energy dissipation rates >10 μW m−3. The dissipation rates due to wave‐particle interactions exceeded rates necessary to explain the increase in entropy across the shock ramps for ∼90% of the wave burst durations. For ∼22% of these times, the wave‐particle interactions needed to only be ≤ 0.1% efficient to balance the nonlinear wave steepening that produced the shock waves. These results show that wave‐particle interactions have the capacity to regulate the global structure and dominate the energy dissipation of collisionless shocks.
Key PointsMicroscopic wave‐particle interactions can regulate macroscopic shock structureHigh‐frequency large‐amplitude waves are ubiquitous in collisionless shocksWave‐particle interactions are the end result of nearly all dissipation pathways
Turbulent plasmas generate intense current structures, which have long been suggested as magnetic reconnection sites. Recent Magnetospheric Multiscale observations in Earth's magnetosheath revealed a ...novel form of reconnection where the dynamics only couple to electrons, without ion involvement. It was suggested that such dynamics were driven by magnetosheath turbulence. In this study, the fluctuations are examined to determine the properties of the turbulence and if a signature of reconnection is present in the turbulence statistics. The study reveals statistical properties consistent with plasma turbulence with a correlation length of ∼10 ion inertial lengths. When reconnection is more prevalent, a steepening of the magnetic spectrum occurs at the length scale of the reconnecting current sheets. The statistics of intense currents suggest the prevalence of electron-scale current sheets favorable for electron reconnection. The results support the hypothesis that electron reconnection is driven by turbulence and highlight diagnostics that may provide insight into reconnection in other turbulent plasmas.
The Magnetospheric Multiscale mission has observed electron whistler waves at the center and at the edges of magnetic holes in the dayside magnetosheath. The magnetic holes are nonlinear mirror ...structures since their magnitude is anticorrelated with particle density. In this article, we examine the growth mechanisms of these whistler waves and their interaction with the host magnetic hole. In the observations, as magnetic holes develop and get deeper, an electron population gets trapped and develops a temperature anisotropy favorable for whistler waves to be generated. In addition, the decrease in magnetic field magnitude and the increase in density reduce the electron resonance energy, which promotes the electron cyclotron resonance. To investigate this process, we used expanding box particle-in-cell simulations to produce the mirror instability, which then evolve into magnetic holes. The simulation shows that whistler waves can be generated at the center and edges of magnetic holes, which reproduces the primary features of the MMS observations. The simulation shows that the electron temperature anisotropy develops in the center of the magnetic hole once the mirror instability reaches its nonlinear stage of evolution. The plasma is then unstable to whistler waves at the minimum of the magnetic field structures. In the saturation regime of mirror instability, when magnetic holes are developed, the electron temperature anisotropy appears at the edges of the holes and electron distributions become more isotropic at the magnetic field minimum. At the edges, the expansion of magnetic holes decelerates the electrons, which leads to temperature anisotropies.
Waves around the lower hybrid frequency are frequently observed at Earth's magnetopause and readily reach very large amplitudes. Determining the properties of lower hybrid waves is crucial because ...they are thought to contribute to electron and ion heating, cross‐field particle diffusion, anomalous resistivity, and energy transfer between electrons and ions. All these processes could play an important role in magnetic reconnection at the magnetopause and the evolution of the boundary layer. In this paper, the properties of lower hybrid waves at Earth's magnetopause are investigated using the Magnetospheric Multiscale mission. For the first time, the properties of the waves are investigated using fields and direct particle measurements. The highest‐resolution electron moments resolve the velocity and density fluctuations of lower hybrid waves, confirming that electrons remain approximately frozen in at lower hybrid wave frequencies. Using fields and particle moments, the dispersion relation is constructed and the wave‐normal angle is estimated to be close to 90° to the background magnetic field. The waves are shown to have a finite parallel wave vector, suggesting that they can interact with parallel propagating electrons. The observed wave properties are shown to agree with theoretical predictions, the previously used single‐spacecraft method, and four‐spacecraft timing analyses. These results show that single‐spacecraft methods can accurately determine lower hybrid wave properties.
Key Points
The velocity and density fluctuations of magnetopause lower hybrid waves are resolved, showing that electrons are approximately frozen in
Lower hybrid wave dispersion relation and wave‐normal angle are computed from fields and particle measurements
Single‐ and multi‐spacecraft methods yield consistent lower hybrid wave properties, confirming the accuracy of single‐spacecraft methods
Large‐amplitude waves near the electron plasma frequency are found by the Magnetospheric Multiscale (MMS) mission near Earth's magnetopause. The waves are identified as Langmuir and upper hybrid (UH) ...waves, with wave vectors either close to parallel or close to perpendicular to the background magnetic field. The waves are found all along the magnetopause equatorial plane, including both flanks and close to the subsolar point. The waves reach very large amplitudes, up to 1 V m−1, and are thus among the most intense electric fields observed at Earth's magnetopause. In the magnetosphere and on the magnetospheric side of the magnetopause the waves are predominantly UH waves although Langmuir waves are also found. When the plasma is very weakly magnetized only Langmuir waves are likely to be found. Both Langmuir and UH waves are shown to have electromagnetic components, which are consistent with predictions from kinetic wave theory. These results show that the magnetopause and magnetosphere are often unstable to intense wave activity near the electron plasma frequency. These waves provide a possible source of radio emission at the magnetopause.
Key Points
Large‐amplitude upper hybrid and Langmuir waves frequently occur at Earth's magnetopause, reaching a maximum amplitude of 1 V m−1
The waves are quasi‐electrostatic but electromagnetic properties are observed
The upper hybrid and Langmuir wave properties are consistent with predictions from linear kinetic theory
Kinetic‐size magnetic holes (KSMHs) in the turbulent magnetosheath are statistically investigated using high time resolution data from the Magnetospheric Multiscale mission. The KSMHs with short ...duration (i.e., <0.5 s) have their cross section smaller than the ion gyroradius. Superposed epoch analysis of all events reveals that an increase in the electron density and total temperature significantly increases (resp. decrease) the electron perpendicular (resp. parallel) temperature and an electron vortex inside KSMHs. Electron fluxes at ~90° pitch angles with selective energies increase in the KSMHs are trapped inside KSMHs and form the electron vortex due to their collective motion. All these features are consistent with the electron vortex magnetic holes obtained in 2‐D and 3‐D particle‐in‐cell simulations, indicating that the observed KSMHs seem to be best explained as electron vortex magnetic holes. It is furthermore shown that KSMHs are likely to heat and accelerate the electrons.
Key Points
Kinetic‐size magnetic holes are statistical investigated by MMS
Observed kinetic‐size magnetic holes seem to be best explained as electron vortex magnetic holes
Kinetic‐size magnetic holes are likely to heat and accelerate the electrons
Plain Language Summary
A nonlinear energy cascade in magnetized turbulent plasmas leads to the formation of different coherent structures which are thought to play an important role in dissipating energy and transporting particles. This study statistically investigate one new type of coherent structure, named electron vortex magnetic hole, used by Magnetospheric Multiscale data. It reveals the common features of this structure, including an increase in the electron density and total temperature, significantly increase (resp. decrease) the electron perpendicular (resp. parallel) temperature and an electron vortex inside these holes. The increase of electron temperature inside the holes indicates that these holes are likely to heat and accelerate the electrons. This gives new clue for energy dissipation in turbulent plasmas.
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