Bursty bulk flow (BBF) events, frequently observed in the magnetotail, carry significant energy and mass from the tail region at distances that are often greater than 20 RE into the near‐Earth plasma ...sheet at ∼10 RE where the flow is slowed and/or diverted. This region at ∼10 RE is referred to as the BBF braking region. A number of possible channels are available for the transfer or dissipation of energy in BBF events including adiabatic heating of particles, the propagation of Alfvén waves out of the BBF braking region and into the auroral region, diverted flow out of the braking region, and energy dissipation within the braking region itself. This study investigates the generation of intense high‐frequency electric field activity observed within the braking region. When present, these intense electric fields have power above the ion cyclotron frequency and almost always contain nonlinear structures such as electron phase space holes and double layers, which are often associated with field‐aligned currents. A hypothesis in which the observed high‐frequency electric field activity is generated by field‐aligned currents resulting from turbulence in the BBF braking region is considered. Although linear Alfvén waves can generate field‐aligned currents, based on theoretical calculations, the required currents are likely not the result of linear waves. Observations from the Time History of Events and Macroscale Interactions during Substorms satellites support the picture of a turbulent plasma leading to the generation of nonlinear kinetic structures. This work provides a possible mechanism for energy dissipation in turbulent plasmas.
Key Points
Strong electric field activity generated by intense field‐aligned currents
Intense currents generated by turbulence in bursty bulk flow braking region
This work provides mechanism for energy dissipation in turbulent plasmas
We report observations of large‐amplitude (>50 mV/m) electric fields primarily associated with bursty bulk flow events. These electric fields reach ~500 mV/m, which are some of the largest electric ...fields (E) observed in the magnetotail. E not only has a larger than expected component perpendicular to the magnetic field but often has an intense parallel component. High time resolution waveforms reveal nonlinear structures such as electron phase‐space holes and double layers, which suggest strong field‐aligned currents or electron beams. Further examination shows that these large‐amplitude electric fields are almost always accompanied by enhanced magnetic field fluctuations. The electric fields are enhanced both above and below the ion cyclotron frequency, whereas the magnetic field fluctuations (δB) are mostly below the ion cyclotron frequency. Analysis of the wave spectra and the Poynting flux suggest that shear Alfvén waves are participating in these events. The Alfvén waves are revealed through the |δE|/|δB| ratio and strong field‐aligned Poynting flux, sometimes reaching nearly 1 mW/m2. This value, when mapped to the low‐altitude auroral region, exceeds 1 W/m2, which is an extreme value for that region. This Alfvénic activity is accompanied by evidence of compressional modes. These observations support a hypothesis whereby intense currents or electron beams, generated by kinetic Alfvénic waves that result from a turbulent cascade in bursty bulk flow (BBF) braking region, may be an energy source for large‐amplitude electric fields. The large‐amplitude electric fields may act as a dissipation mechanism and relax the highly tangled magnetic fields that result from the turbulence. Furthermore, these observations offer strong support that Alfvénic Poynting flux from the BBF braking region can be the energy source for Alfvénic aurora.
Key Points
Observations of large‐amplitude electric fields associated with BBF events
Intense currents of kinetic Alfvén waves generate large‐amplitude E fields
Alfvénic Poynting flux from the BBF braking region as energy source for aurora
Abstract
At the Earth's magnetopause, the Kelvin‐Helmholtz (KH) instability, driven by the persistent velocity shear between the magnetosheath and the magnetosphere, has been frequently observed ...during northward interplanetary magnetic field periods and considered as one of the most important candidates for transporting and mixing plasmas across the magnetopause. However, how this process interacts with magnetic field fluctuations, which persistently exist near the magnetopause, has been less discussed. Here we perform a series of 2‐D fully kinetic simulations of the KH instability at the magnetopause considering a power law spectrum of initial fluctuations in the magnetic field. The simulations demonstrate that when the amplitude level of the initial fluctuations is sufficiently large, the KH instability evolves faster, leading to a more efficient plasma mixing within the vortex layer. In addition, when the spectral index of the initial fluctuations is sufficiently small, the modes whose wavelength is longer than the theoretical fastest growing mode grow dominantly. The fluctuating magnetic field also results in the formation of the well‐matured turbulent spectrum with a −5/3 index within the vortex layer even in the early nonlinear growth phase of the KH instability. The obtained spectral features in the simulations are in reasonable agreement with the features in KH waves events at the magnetopause observed by the Magnetospheric Multiscale mission and conjunctively by the Geotail and Cluster spacecraft. These results indicate that the magnetic field fluctuations may really contribute to enhancing the wave activities especially for longer wavelength modes and the associated mixing at the magnetopause.
Key Points
The 2‐D fully kinetic simulations of magnetopause Kelvin‐Helmholtz instability initially imposing power law field fluctuations are performed
The growth of the instability especially for long wavelength modes is enhanced by the fluctuating field, leading to more efficient mixing
Spectral features obtained from the simulations are in reasonable agreement with past spacecraft observations at the Earth's magnetopause
Three flux ropes associated with near-Earth magnetotail reconnection are analyzed using Magnetospheric Multiscale observations. The flux ropes are Earthward propagating with sizes from ∼3 to 11 ion ...inertial lengths. Significantly different axial orientations are observed, suggesting spatiotemporal variability in the reconnection and/or flux rope dynamics. An electron-scale vortex, associated with one of the most intense electric fields (E) in the event, is observed within one of the flux ropes. This E is predominantly perpendicular to the magnetic field (B); the electron vortex is frozen-in with E × B drifting electrons carrying perpendicular current and causing a small-scale magnetic enhancement. The vortex is ∼16 electron gyroradii in size perpendicular to B and potentially elongated parallel to B. The need to decouple the frozen-in vortical motion from the surrounding plasma implies a parallel E at the structure's ends. The formation of frozen-in electron vortices within reconnection-generated flux ropes may have implications for particle acceleration.
Context.
Switchbacks are discrete angular deflections in the solar wind magnetic field that have been observed throughout the heliosphere. Recent observations by Parker Solar Probe (PSP) have ...revealed the presence of patches of switchbacks on the scale of hours to days, separated by ‘quieter’ radial fields.
Aims.
We aim to further diagnose the origin of these patches using measurements of proton temperature anisotropy that can illuminate possible links to formation processes in the solar corona.
Methods.
We fitted 3D bi-Maxwellian functions to the core of proton velocity distributions measured by the SPAN-Ai instrument onboard PSP to obtain the proton parallel,
T
p,∥
, and perpendicular,
T
p,⊥
, temperature.
Results.
We show that the presence of patches is highlighted by a transverse deflection in the flow and magnetic field away from the radial direction. These deflections are correlated with enhancements in
T
p,∥
, while
T
p,⊥
remains relatively constant. Patches sometimes exhibit small proton and electron density enhancements.
Conclusions.
We interpret that patches are not simply a group of switchbacks, but rather switchbacks are embedded within a larger-scale structure identified by enhanced
T
p,∥
that is distinct from the surrounding solar wind. We suggest that these observations are consistent with formation by reconnection-associated mechanisms in the corona.
The four Magnetospheric Multiscale (MMS) spacecraft recorded the first direct evidence of reconnection exhausts associated with Kelvln-Helmholtz (KH) waves at the duskside magnetopause on 8 September ...2015 which allows for local mass and energy transport across the flank magnetopause. Pressure anisotropy-weighted Walen analyses confirmed in-plane exhausts across 22 of 42 KH-related trailing magnetopause current sheets (CSs). Twenty-one jets were observed by all spacecraft, with small variations in ion velocity, along the same sunward or antisunward direction with nearly equal probability. One exhaust was only observed by the MMS-1,2 pair, while MMS-3,4 traversed a narrow CS (1.5 ion inertial length) in the vicinity of an electron diffusion region. The exhausts were locally 2-D planar in nature as MMS-1, 2 observed almost identical signatures separated along the guide-field. Asymmetric magnetic and electric Hall fields are reported in agreement with a strong guide-field and a weak plasma density asymmetry across the magnetopause CS.
Abstract
We perform a 2.5-dimensional particle-in-cell simulation of a quasi-parallel shock, using parameters for the Earth’s bow shock, to examine electron acceleration and heating due to magnetic ...reconnection. The shock transition region evolves from the ion-coupled reconnection dominant stage to the electron-only reconnection dominant stage, as time elapses. The electron temperature enhances locally in each reconnection site, and ion-scale magnetic islands generated by ion-coupled reconnection show the most significant enhancement of the electron temperature. The electron energy spectrum shows a power law, with a power-law index around 6. We perform electron trajectory tracing to understand how they are energized. Some electrons interact with multiple electron-only reconnection sties, and Fermi acceleration occurs during multiple reflections. Electrons trapped in ion-scale magnetic islands can be accelerated in another mechanism. Islands move in the shock transition region, and electrons can obtain larger energy from the in-plane electric field than the electric potential in those islands. These newly found energization mechanisms in magnetic islands in the shock can accelerate electrons to energies larger than the achievable energies by the conventional energization due to the parallel electric field and shock drift acceleration. This study based on the selected particle analysis indicates that the maximum energy in the nonthermal electrons is achieved through acceleration in ion-scale islands, and electron-only reconnection accounts for no more than half of the maximum energy, as the lifetime of sub-ion-scale islands produced by electron-only reconnection is several times shorter than that of ion-scale islands.
Using observations of Earth's bow shock by the Magnetospheric Multiscale mission, we show for the first time that active magnetic reconnection is occurring at current sheets embedded within the ...quasi‐parallel shock's transition layer. We observe an electron jet and heating but no ion response, suggesting we have observed an electron‐only mode. The lack of ion response is consistent with simulations showing reconnection onset on sub‐ion time scales. We also discuss the impact of electron heating in shocks via reconnection.
Plain Language Summary
For the first time, we document an observation of magnetic reconnection occurring at Earth's bow shock. The observations have been made by NASA's Magnetospheric Multiscale mission while the bow shock is under a “quasi‐parallel” geometry, which typically results in a highly disordered structure. Models of shock waves in space plasmas do not currently account for reconnection. This therefore introduces a new avenue of research into how shocks can repartition energy when slowing the solar wind from supersonic to subsonic flow. The observations also introduce a new regime for magnetic reconnection, for which we observe only an electron response at an ion scale reconnecting structure. This work will also attract interest from the broader astrophysics community, as reconnection at shocks may influence cosmic ray generation.
Key Points
Reconnecting current sheets have been observed at a quasi‐parallel bow shock
The ion‐scale current sheet exhibits only an electron jet and heating, with no ion response
Consistent with kinetic simulations, reconnection relaxes complexity in the shock transition region
We briefly review helicity dynamics, inverse and bidirectional cascades in fluid and magnetohydrodynamic (MHD) turbulence, with an emphasis on the latter. The energy of a turbulent system, an ...invariant in the nondissipative case, is transferred to small scales through nonlinear mode coupling. Fifty years ago, it was realized that, for a two‐dimensional fluid, energy cascades instead to larger scales and so does magnetic excitation in MHD. However, evidence obtained recently indicates that, in fact, for a range of governing parameters, there are systems for which their ideal invariants can be transferred, with constant fluxes, to both the large scales and the small scales, as for MHD or rotating stratified flows, in the latter case including quasi‐geostrophic forcing. Such bidirectional, split, cascades directly affect the rate at which mixing and dissipation occur in these flows in which nonlinear eddies interact with fast waves with anisotropic dispersion laws, due, for example, to imposed rotation, stratification, or uniform magnetic fields. The directions of cascades can be obtained in some cases through the use of phenomenological arguments, one of which we derive here following classical lines in the case of the inverse magnetic helicity cascade in electron MHD. With more highly resolved data sets stemming from large laboratory experiments, high‐performance computing, and in situ satellite observations, machine learning tools are bringing novel perspectives to turbulence research. Such algorithms help devise new explicit subgrid‐scale parameterizations, which in turn may lead to enhanced physical insight, including in the future in the case of these new bidirectional cascades.
Plain Language Summary
Turbulent flows are prevalent in Geophysics and Space Physics. They are complex and involve interactions between fluctuations at widely separated scales, with the energy expected in the general case to flow only to small scales where it is dissipated. It was found recently that, contrary to such expectations, energy can go in substantial amounts to both the small and large scales, in the presence of magnetic fields, as applicable to space plasmas, and for rotating stratified flows as encountered in the atmosphere and the oceans. This result implies that the amount of energy available for dissipation may differ from flow to flow, and simple scaling arguments allow for predictions that are backed up by results stemming from direct numerical simulations. One should incorporate this bidirectional cascade phenomenon in the turbulence models used for global computations of geophysical and astrophysical media. Furthermore, machine learning tools may prove useful in deriving such enhanced models in their capacity to interrogate the large data bases that already exist for such complex flows.
Key Points
Magnetic helicity displays an inverse cascade to large scales which, in electron MHD, can be justified with a simple phenomenological model
Total energy in MHD or rotating stratified turbulence has constant‐flux cascades to small and large scales, affecting mixing and dissipation
With these results, needed modifications to subgrid scale turbulence models will be enhanced using tools from big data and machine learning
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