Spatial and high-time-resolution properties of the velocities, magnetic field, and 3-D electric field within plasma turbulence are examined observationally using data from the Magnetospheric ...Multiscale mission. Observations from a Kelvin-Helmholtz instability (KHI) on the Earth's magnetopause are examined, which both provides a series of repeatable intervals to analyze, giving better statistics, and provides a first look at the properties of turbulence in the KHI. For the first time direct observations of both the high-frequency ion and electron velocity spectra are examined, showing differing ion and electron behavior at kinetic scales. Temporal spectra exhibit power law behavior with changes in slope near the ion gyrofrequency and lower hybrid frequency. The work provides the first observational evidence for turbulent intermittency and anisotropy consistent with quasi two-dimensional turbulence in association with the KHI. The behavior of kinetic-scale intermittency is found to have differences from previous studies of solar wind turbulence, leading to novel insights on the turbulent dynamics in the KHI.
We present a model of electromagnetic drift waves in the current sheet adjacent to magnetic reconnection at the subsolar magnetopause. These drift waves are potentially important in governing 3‐D ...structure of subsolar magnetic reconnection and in generating turbulence. The drift waves propagate nearly parallel to the X line and are confined to a thin current sheet. The scale size normal to the current sheet is significantly less than the ion gyroradius and can be less than or on the order of the wavelength. The waves also have a limited extent along the magnetic field (B), making them a three‐dimensional eigenmode structure. In the current sheet, the background magnitudes of B and plasma density change significantly, calling for a treatment that incorporates an inhomogeneous plasma environment. Using detailed examination of Magnetospheric Multiscale observations, we find that the waves are best represented by series of electron vortices, superimposed on a primary electron drift, that propagate along the current sheet (parallel to the X line). The waves displace or corrugate the current sheet, which also potentially displaces the electron diffusion region. The model is based on fluid behavior of electrons, but ion motion must be treated kinetically. The strong electron drift along the X line is likely responsible for wave growth, similar to a lower hybrid drift instability. Contrary to a classical lower hybrid drift instability, however, the strong changes in the background B and no, the normal confinement to the current sheet, and the confinement along B are critical to the wave description.
Key Points
Drift waves are potentially important in governing 3D structure of subsolar magnetic reconnection and in generating turbulence
Drift waves displace or corrugate the current sheet and potentially displace the electron diffusion region of magnetic reconnection
Parallel electric fields arise in the drift waves
Magnetospheric Multiscale observations are used to probe the structure and temperature profile of a guide field reconnection exhaust ~100 ion inertial lengths downstream from the X‐line in the ...Earth's magnetosheath. Asymmetric Hall electric and magnetic field signatures were detected, together with a density cavity confined near 1 edge of the exhaust and containing electron flow toward the X‐line. Electron holes were also detected both on the cavity edge and at the Hall magnetic field reversal. Predominantly parallel ion and electron heating was observed in the main exhaust, but within the cavity, electron cooling and enhanced parallel ion heating were found. This is explained in terms of the parallel electric field, which inhibits electron mixing within the cavity on newly reconnected field lines but accelerates ions. Consequently, guide field reconnection causes inhomogeneous changes in ion and electron temperature across the exhaust.
Plain Language Summary
Plasma heating and energization by magnetic reconnection is a fundamental process in space, solar, astrophysical, and planetary plasmas. Most reconnecting current sheets do not exhibit perfectly antialigned magnetic fields and a so‐called guide field is often present. Using new experimental data from NASA's Magnetospheric Multiscale mission, this article shows that far from the X‐line during guide field reconnection, the heating is substantially modified from the typically studied antiparallel case. More specifically, the new multipoint, high time resolution Magnetospheric Multiscale measurements of a guide field reconnection exhaust in the Earth's magnetosheath reveal inhomogenous ion and electron heating and cooling. This uncovers in new detail the structure of the exhaust, including predicted density cavity structure and electron holes, and indicates the importance of the parallel electric field. The results are important for the general understanding of reconnection heating and energization. The results will be of immediate and timely interest to the Geophysical Research Letters (GRL) community and beyond.
Key Points
A guide field reconnection exhaust was encountered by MMS in the magnetosheath ~100 ion inertial lengths downstream from the X‐line
A density cavity forms on one edge of the exhaust with embedded electron jetting toward the X‐line and electron holes on the cavity edge
The parallel electric field causes electron cooling and ion heating in the cavity and inhomogeneous temperature profiles across the exhaust
The Magnetospheric Multiscale Mission observes, in detail, charged particle heating and substantial nonthermal acceleration in a region of strong turbulence ( , where is the magnetic field) that ...surrounds a magnetic reconnection X-line. Magnetic reconnection enables magnetic field annihilation in a volume that far exceeds that of the diffusion region. The formidable magnetic field annihilation breaks into strong, intermittent turbulence with magnetic field energy as the driver. The strong, intermittent turbulence appears to generate the necessary conditions for nonthermal acceleration. It creates intense, localized currents ( ) and unusually large-amplitude electric fields ( ). The combination of turbulence-generated and results in a significant net positive mean of , which signifies particle energization. Ion and electron heating rates are such that they experience a fourfold increase from their initial temperature. Importantly, the strong turbulence also generates magnetic holes or depletions in that can trap particles. Trapping considerably increases the dwell time of a subset of particles in the turbulent region, which results in significant nonthermal particle acceleration. The direct observation of strong turbulence that is enabled by magnetic reconnection with nonthermal particle acceleration has far-reaching implications, since turbulence in plasmas is pervasive and may occupy significant volumes of the interstellar medium and intergalactic space. For example, strong turbulence from magnetic field annihilation in the supernova nebulae may dominate large volumes. As such, this observed energization process could plausibly contribute to the supply and development of the cosmic-ray spectrum.
Large‐amplitude electric fields (>50 mV/m) typical to bursty bulk flow (BBF) braking regions of the Earth's magnetotail can accelerate energetic electrons and ions to many times their initial thermal ...energies. We follow up on the Usanova and Ergun (2022), https://doi.org/10.1029/2022JA030336, study of electron energization and examine wave and plasma observations from the THEMIS satellites over four tail seasons to investigate the transfer of BBF energy to ions by turbulent electric fields. The results show that the large‐amplitude electric fields are accompanied by an ion temperature increase of ∼50% when compared to times when the turbulence is not observed. Electric field turbulence is also associated with a roughly ten‐fold increase in temperature fluctuations and a five‐fold increase in variations of energetic ion fluxes. We discuss the contribution of this turbulent energy transfer process to the dynamics of energetic ions in the magnetosphere.
Plain Language Summary
Bursty bulk flows are high‐speed ion flows propagating toward Earth from the reconnection sites. On their approach to Earth, they decelerate and divert, while generating a turbulent cascade through which their energy dissipates. We use data from NASA's THEMIS satellites to show that high‐amplitude turbulent electric fields are produced through this energy dissipation process, which, in turn, transfer energy to ions. Further, we discuss the contribution of this turbulent energy transfer to the energetic ion dynamics in the inner magnetosphere.
Key Points
Large‐amplitude electric fields are linked to a 1,000% increase in ion temperature fluctuations and a 500% increase in ion flux variations
The effect on ion temperature is smaller than on electron temperature, being 50% versus 300%
The accelerated energetic ions may contribute to ring current and plasmasheet energization
We report observations of a Bursty Bulk Flow (BBF) penetrating close to the outer edge of the radiation belt. The turbulent BBF braking region is characterized by ion velocity fluctuations, magnetic ...field (B) variations, and intense electric fields (E). In this event, energetic (>100 keV) electron and ion fluxes are appreciably enhanced. Importantly, fluctuations in energetic electrons and ions suggest local energization. Using correlation distances and other observed characteristics of turbulent E, test‐particle simulations support local energization by E that favors higher‐energy electrons and leads to an enhanced energetic shoulder and tail in the electron distributions. The energetic shoulder and tail could be amplified to MeV energies by adiabatic transport into the radiation belt where |B| is higher. This analysis suggests that turbulence generated by BBFs can, in part, supply energetic particles to the outer radiation belt and that turbulence can be a significant contributor to particle acceleration.
Plain Language Summary
Bursty Bulk Flows are thought to be the earthward‐traveling exhaust from magnetic reconnection events in the Earth's magnetotail. These plasma flows slow and divert as they approach Earth and, in doing so, can generate strong plasma turbulence. The electric field turbulence, in turn, appears to energize electrons and ions. The primary finding of this research is that the electron energization favors electrons that already have high energy, and therefore results in "acceleration" in which a relatively few particles receive a disproportionate share of the energy. Furthermore, turbulent regions of bursty bulk flows are shown to penetrate to the edge of the outer radiation belt. As such, bursty bulk flows are potentially a significant source or seed population for radiation belt electrons.
Key Points
A bursty bulk flow near the outer radiation belt displays turbulent electric fields and enhanced fluxes of energetic ions and electrons
Electrons appear to be locally accelerated by turbulent electric fields forming an energetic shoulder in the distribution
Turbulent electric fields in the Bursty Bulk Flow braking region favors energization of the highest energy electrons
We present Magnetospheric Multiscale observations of an electron‐scale reconnecting current sheet in the mixing region along the trailing edge of a Kelvin‐Helmholtz vortex during southward ...interplanetary magnetic field conditions. Within this region, we observe intense electrostatic wave activity, consistent with lower‐hybrid waves. These waves lead to the transport of high‐density magnetosheath plasma across the boundary layer into the magnetosphere and generate a mixing region with highly compressed magnetic field lines, leading to the formation of a thin current sheet associated with electron‐scale reconnection signatures. Consistencies between these reconnection signatures and a realistic, local, fully‐kinetic simulation modeling this current sheet indicate a temporal evolution of the observed electron‐scale reconnection current sheet. The multi‐scale and inter‐process character of this event can help us understand plasma mixing connected to the Kelvin‐Helmholtz instability and the temporal evolution of electron‐scale reconnection.
Plain Language Summary
Like wind blowing over water, the stream of ionized gas released from the Sun, called the solar wind, can lead to waves and rolled‐up vortex structures at the boundary of Earth's magnetosphere, called the magnetopause. These so‐called Kelvin‐Helmholtz waves have been shown to be connected to various different plasma processes on different scales. This multi‐scale and multi‐process character makes them an ideal candidate to study the relation between these processes from both spacecraft observations and simulations. By using spacecraft data from the Magnetospheric Multiscale mission, which was designed for the study of small‐scale plasma processes in Earth's magnetosphere, we show observations of electron‐scale magnetic reconnection, an explosive energy conversion process in plasmas, in a region along the trailing edge of these waves. These observations shed new light on the multi‐scale and multi‐process character of the Kelvin‐Helmholtz instability and the energy conversion processes along its boundary.
Key Points
A reconnecting electron‐scale current sheet is observed by Magnetospheric Multiscale (MMS) in mixing plasma along the trailing edge of a Kelvin‐Helmholtz vortex
Realistic 2.5D fully‐kinetic simulation shows reasonable agreement with MMS data
Consistencies between the simulation and MMS indicate a temporal evolution of the reconnecting current sheet
PDF
