Electron Energization Research Articles

Energy Transport and Conversion Above a Bright Discrete Auroral Arc.

Magnetically connected observations of particle distributions and luminosity from the Reimei spacecraft are used to examine energy transport and conversion occurring above a discrete auroral arc. By combining imaging and in situ measurements it is shown how transverse electromagnetic and kinetic energy fluxes measured along the spacecraft trajectory converge across geomagnetic field-lines into the acceleration region. It is shown how cross-field energy transport is facilitated by the formation of vortices along the length of the arc. From an integration over the vertical extent of the acceleration region it is shown that the transverse and field-aligned flow of energy into the arc locally supports the dissipation needed to power the electron acceleration observed. Estimates of gradients in electromagnetic and kinetic energy flows show how field-aligned electron energization in the acceleration region is supported by the divergence of Poynting flux along and across the background magnetic field.

Alfvén wave generation and electron energization in the KiNET-X sounding rocket mission

Active plasma experiments can be used to strongly perturb the space plasma environment. During the early phase of a chemical release (e.g., few to several seconds), the injected plasma cloud can excite a variety of waves rather than acting as “inert” tracer particles. It is during this early phase of the release that fundamental plasma processes can be studied. For example, the Trigger [Holmgren et al., J. Geophys. Res. 85, 5043 (1980)] and recent KINetic-scale Energy and momentum Transport eXperiment (KiNET-X) missions were both designed to study processes related to auroral electron energization. Early experiments relied primarily on ground-based optics to diagnose the plasma interaction. Advances in optical sensors have dramatically improved imaging capability of both the ion and neutral components of the injected cloud; therefore, optics remain an important part of these types of experiments. However, advances in plasma (fields and particles) instruments have enabled a new generation of possible experiments from the sounding rocket platform. In this article, we discuss previous sounding rocket (and orbital) active experiments, the related science objectives, and an overview of select results from the KiNET-X rocket mission. Specifically, KiNET-X produced an Alfvénic perturbation, a variety of high frequency waves, energized thermal electrons, and produced a field-aligned electron beam of ∼ 200 eV. The electron energization indicates non-ideal coupling of the injected barium cloud with the ambient ionospheric plasma.

Electron Energization by Ion Density Enhancement in the Martian Magnetotail—MAVEN Observations

It has been long observed at Mars that electron fluxes are enhanced during the tail current sheet crossings, of which the cause is not well understood. We use a novel approach to reveal one of the electron energization mechanisms with observations from the Mars Atmospheric and Volatile EvolutioN (MAVEN) mission. We find the field-aligned potential, derived from comparing electron distribution functions, to be approximately linearly correlated with the logarithmic values of the local total ion density. This is the expected behavior of an ambipolar electrostatic potential. The large amplitude of potential (tens to hundreds of V) is a result of both a significant density gradient (1 order of magnitude) and the high electron temperature (tens of eV) in the tail. Such a mechanism is not limited to ion density enhancements at current sheet crossings but can be present anywhere that large ion density gradients and hot electrons are present.

Electron Influence on the Parallel Proton Firehose Instability in 10-moment, Multifluid Simulations

Instabilities driven by pressure anisotropy play a critical role in modulating the energy transfer in space and astrophysical plasmas. For the first time, we simulate the evolution and saturation of the parallel proton firehose instability using a multifluid model without adding artificial viscosity. These simulations are performed using a 10-moment, multifluid model with local and gradient relaxation heat-flux closures in high-β proton–electron plasmas. When these higher-order moments are included and pressure anisotropy is permitted to develop in all species, we find that the electrons have a significant impact on the saturation of the parallel proton firehose instability, modulating the proton pressure anisotropy as the instability saturates. Even for lower β's more relevant to heliospheric plasmas, we observe a pronounced electron energization in simulations using the gradient relaxation closure. Our results indicate that resolving the electron pressure anisotropy is important to correctly describe the behavior of multispecies plasma systems.

Direct observation of ion cyclotron damping of turbulence in Earth’s magnetosheath plasma

Plasma turbulence plays a key role in space and astrophysical plasma systems, enabling the energy of magnetic fields and plasma flows to be transported to particle kinetic scales at which the turbulence dissipates and heats the plasma. Identifying the physical mechanisms responsible for the dissipation of the turbulent energy is a critical step in developing the predictive capability for the turbulent heating needed by global models. In this work, spacecraft measurements of the electromagnetic fields and ion velocity distributions by the Magnetospheric Multiscale (MMS) mission are used to generate velocity-space signatures that identify ion cyclotron damping in Earth’s turbulent magnetosheath, in agreement with analytical modeling. Furthermore, the rate of ion energization is directly quantified and combined with a previous analysis of the electron energization to identify the dominant channels of turbulent dissipation and determine the partitioning of energy among species in this interval.

Simultaneous Proton and Electron Energization during Macroscale Magnetic Reconnection

The results of simulations of magnetic reconnection accompanied by electron and proton heating and energization in a macroscale system are presented. Both species form extended power-law distributions that extend nearly three decades in energy. The primary drive mechanism for the production of these nonthermal particles is Fermi reflection within evolving and coalescing magnetic flux ropes. While the power-law indices of the two species are comparable, the protons overall gain more energy than electrons, and their power law extends to higher energy. The power laws roll into a hot thermal distribution at low energy with the transition energy occurring at lower energy for electrons compared with protons. A strong guide field diminishes the production of nonthermal particles by reducing the Fermi drive mechanism. In solar flares, proton power laws should extend down to tens of keV, far below the energies that can be directly probed via gamma-ray emission. Thus, protons should carry much more of the released magnetic energy than expected from direct observations.

Electron Acceleration via Secondary Reconnection in the Separatrix Region of Magnetopause Reconnection

AbstractMagnetic reconnection is a fundamental process known to play a crucial role in electron acceleration and heating, however, the mechanism of electron energization during reconnection is still not fully understood. This study introduces a novel electron acceleration mechanism in which electrons can be accelerated by secondary reconnection in the separatrix region. The secondary reconnection occurs in a thin current sheet resulted from the shear of the out‐of‐plane Hall magnetic fields of the primary magnetopause reconnection. It results in the intense electron energy fluxes toward the primary X‐line. This mechanism will likely be an important piece in the puzzle of particle acceleration by reconnection.

Enhanced Efficiency of Solar Wind Electron Interaction with Whistlers Caused by Switchback-related Magnetic Dips

Through test particle simulations based on solar wind observations by the Parker Solar Probe (PSP) mission, we demonstrate that a magnetic gradient can significantly enhance the efficiency of scattering and energization of the strahl electrons by quasi-parallel whistlers, through the phase trapping effect due to the gyrosurfing mechanism. We identify quasi-linear and nonlinear regimes of these interactions for different combinations of wave amplitude (B w /B 0) and the strength of the magnetic field gradient with magnetic field depletion level (B h /B 0) as a proxy. Nonlinear effects are observed for B w /B 0 ≳ 10−3 and B h /B 0 ≳ 0.1. We estimated the extending of the resonant energy range due to the wave and the magnetic field gradient interplay and demonstrated that these mechanisms result in the broadening of the strahl electron pitch-angle distribution typically observed in situ. The combination of parallel whistlers collocated with a magnetic gradient is frequently observed by PSP in magnetic dips at the edges of magnetic switchbacks. Our results indicate that these mechanisms may be highly relevant for pitch-angle scattering of the strahl electrons and regulating the heat flux near the Sun at heliocentric distances of 30–45 R S . Specifically, core and halo electrons may experience a 10% increase in their initial energy, and the majority of strahl electrons may be scattered (by an average of 30°) into the hot and trapped plasma inside magnetic dips.

Applications of Fast Magnetic Reconnection Models to the Atmospheres of the Sun and Protoplanetary Disks

Partially ionized plasmas consist of charged and neutral particles whose mutual collisions modify magnetic reconnection compared with the fully ionized case. The collisions alter the rate and locations of the magnetic dissipation heating and the distribution of energies among the particles accelerated into the nonthermal tail. We examine the collisional regimes for the onset of fast reconnection in two environments: the partially ionized layers of the solar atmosphere, and the protoplanetary disks that are the birthplaces for planets around young stars. In both these environments, magnetic nulls readily develop into resistive current sheets in the regime where the charged and neutral particles are fully coupled by collisions, but the current sheets quickly break down under the ideal tearing instability. The current sheets collapse repeatedly, forming magnetic islands at successively smaller scales, until they enter a collisionally decoupled regime where the magnetic energy is rapidly turned into heat and charged-particle kinetic energy. Small-scale, decoupled fast reconnection in the solar atmosphere may lead to preferential heating and energization of ions and electrons that escape into the corona. In protoplanetary disks such reconnection causes localized heating in the atmospheric layers that produce much of the infrared atomic and molecular line emission observed with the Spitzer and James Webb Space Telescopes.

Determination of Outer Radiation Belt MeV Electron Energization Rates and Delayed Response Times to Solar Wind Driving During Geomagnetic Storms

Determination of Outer Radiation Belt MeV Electron Energization Rates and Delayed Response Times to Solar Wind Driving During Geomagnetic Storms

Laser-driven ion and electron acceleration from near-critical density gas targets: Towards high-repetition rate operation in the 1 PW, sub-100 fs laser interaction regime

Ion acceleration from gaseous targets driven by relativistic-intensity lasers was demonstrated as early as the late 1990s, yet most of the experiments conducted to date have involved picosecond-duration, Nd:glass lasers operating at low repetition rate. Here, we present measurements on the interaction of ultraintense (≈1020Wcm−2, 1 PW), ultrashort (≈70fs) Ti:Sa laser pulses with near-critical (≈1020cm−3) helium gas jets, a debris-free targetry with the potential for future compatibility with high (≈1 Hz) repetition rate operation. We provide evidence of α particles being forward accelerated up to ≈2.7−MeV energy with a total flux of ≈1011sr−1 as integrated over >0.1−MeV energies and detected within a 0.5−mrad solid angle. We also report on on-axis emission of relativistic electrons with an exponentially decaying spectrum characterized by a ≈10−MeV slope, i.e., five times larger than the standard ponderomotive scaling. The total charge of these electrons with energy above 2 MeV is estimated to be of ≈1nC, corresponding to ≈0.1% of the laser drive energy. In addition, we observe the formation of a plasma channel, extending longitudinally across the gas density maximum and expanding radially with time. These results are well captured by large-scale particle-in-cell simulations, which reveal that the detected fast ions most likely originate from reflection off the rapidly expanding channel walls. The latter process is predicted to yield ion energies in the MeV range, which compare well with the measurements. Finally, direct laser acceleration is shown to be the dominant mechanism behind the observed electron energization. Published by the American Physical Society 2024

A computational model for ion and electron energization during macroscale magnetic reconnection

A set of equations is developed that extends the macroscale magnetic reconnection simulation model kglobal to include particle ions. The extension from earlier versions of kglobal, which included only particle electrons, requires the inclusion of the inertia of particle ions in the fluid momentum equation. The new equations will facilitate the exploration of the simultaneous non-thermal energization of ions and electrons during magnetic reconnection in macroscale systems. Numerical tests of the propagation of Alfvén waves and the growth of firehose modes in a plasma with anisotropic electron and ion pressure are presented to benchmark the new model.

Scale-invariant breathing oscillations and transition of the electron energization mechanism in magnetized discharges

Scale-invariant breathing oscillations are observed in similar magnetized discharges at different spatiotemporal scales via fully kinetic particle-in-cell simulations. With an increase in the similarity invariant B/p, i.e., the ratio of magnetic field to pressure, breathing oscillations are triggered, leading to an appreciable time-averaged potential fall outside the sheath. With the onset and development of breathing oscillations, the electron energization mechanism shifts from sheath energization to direct Ohmic heating in the ionization region due to the change in the potential fall inside and outside the cathode sheath. Based on the scale invariance of the Boltzmann equation and its collision term, the characteristics of breathing oscillations and the transition of the electron energization mechanism are confirmed to be scale-invariant under similar discharge conditions.

Observation of Energetic Electron Near the Electron Diffusion Region of Magnetic Reconnection

AbstractMagnetic reconnection can effectively convert magnetic energy into plasma energy, and accelerate electrons. In this article, the electrons with energies up to 150 keV are observed in the separatrix region and near the electron diffusion region (EDR) detected by the Magnetospheric Multiscale mission in the magnetotail. Combined with the electron pitch‐angle distribution, the electrons in these two regions have a striking behavior: the low‐energy electrons (<∼10 keV) move mainly toward the X‐line, while the energetic electrons (69–139 keV) move mainly away from the X‐line. In the EDR, the energy of electrons can reach up to 10 keV and there is a notable enhancement in the flux of electrons in the direction perpendicular to the magnetic field, which implies the presence of acceleration processes occurring in the EDR, leading to the energization of electrons. Furthermore, the energy spectrum of non‐thermal electron with energies above 6 keV shows a power law distribution in this event, suggesting the occurrence of multiple acceleration processes rather than a single energization mechanism. These findings underscore the EDR's role as a crucial region for electron acceleration during magnetic reconnection. The study provides essential clues about the mechanisms driving electron acceleration, contributing to our understanding of space weather phenomena and the broader dynamics of plasma physics in space environments.

Electron Heating in 2D Particle-in-cell Simulations of Quasi-perpendicular Low-beta Shocks

We measure the thermal electron energization in 1D and 2D particle-in-cell simulations of quasi-perpendicular, low-beta (β p = 0.25) collisionless ion–electron shocks with mass ratio m i/m e = 200, fast Mach number Mms=1 –4, and upstream magnetic field angle θ Bn = 55°–85° from the shock normal nˆ . It is known that shock electron heating is described by an ambipolar, B -parallel electric potential jump, Δϕ ∥, that scales roughly linearly with the electron temperature jump. Our simulations have Δϕ∥/(0.5miush2)∼0.1 –0.2 in units of the pre-shock ions’ bulk kinetic energy, in agreement with prior measurements and simulations. Different ways to measure ϕ ∥, including the use of de Hoffmann–Teller frame fields, agree to tens-of-percent accuracy. Neglecting off-diagonal electron pressure tensor terms can lead to a systematic underestimate of ϕ ∥ in our low-β p shocks. We further focus on two θ Bn = 65° shocks: a Ms=4 ( MA=1.8 ) case with a long, 30d i precursor of whistler waves along nˆ , and a Ms=7 ( MA=3.2 ) case with a shorter, 5d i precursor of whistlers oblique to both nˆ and B ; d i is the ion skin depth. Within the precursors, ϕ ∥ has a secular rise toward the shock along multiple whistler wavelengths and also has localized spikes within magnetic troughs. In a 1D simulation of the Ms=4 , θ Bn = 65° case, ϕ ∥ shows a weak dependence on the electron plasma-to-cyclotron frequency ratio ω pe/Ωce, and ϕ ∥ decreases by a factor of 2 as m i/m e is raised to the true proton–electron value of 1836.

Electron and Proton Energization in 3D Reconnecting Current Sheets in Semirelativistic Plasma with Guide Magnetic Field

Using 3D particle-in-cell simulation, we characterize energy conversion, as a function of guide magnetic field, in a thin current sheet in semirelativistic plasma, with relativistic electrons and subrelativistic protons. There, magnetic reconnection, the drift-kink instability (DKI), and the flux-rope kink instability all compete and interact in their nonlinear stages to convert magnetic energy to plasma energy. We compare fully 3D simulations with 2D in two different planes to isolate reconnection and DKI effects. In zero guide field, these processes yield distinct energy conversion signatures: ions gain more energy than electrons in 2Dxy (reconnection), while the opposite is true in 2Dyz (DKI), and the 3D result falls in between. The flux-rope instability, which occurs only in 3D, allows more magnetic energy to be released than in 2D, but the rate of energy conversion in 3D tends to be lower. Increasing the guide magnetic field strongly suppresses DKI, and in all cases slows and reduces the overall amount of energy conversion; it also favors electron energization through a process by which energy is first stored in the motional electric field of flux ropes before energizing particles. Understanding the evolution of the energy partition thus provides insight into the role of various plasma processes, and is important for modeling radiation from astrophysical sources such as accreting black holes and their jets.

Electron energization in reconnection: Eulerian vs Lagrangian perspectives

Particle energization due to magnetic reconnection is an important unsolved problem for myriad space and astrophysical plasmas. Electron energization in magnetic reconnection has traditionally been examined from a particle, or Lagrangian, perspective using particle-in-cell (PIC) simulations. Guiding-center analyses of ensembles of PIC particles have suggested that Fermi (curvature drift) acceleration and direct acceleration via the reconnection electric field are the primary electron energization mechanisms. However, both PIC guiding-center ensemble analyses and spacecraft observations are performed in an Eulerian perspective. For this work, we employ the continuum Vlasov–Maxwell solver within the Gkeyll simulation framework to reexamine electron energization from a kinetic continuum, Eulerian, perspective. We separately examine the contribution of each drift energization component to determine the dominant electron energization mechanisms in a moderate guide-field Gkeyll reconnection simulation. In the Eulerian perspective, we find that the diamagnetic and agyrotropic drifts are the primary electron energization mechanisms away from the reconnection x-point, where direct acceleration dominates. We compare the Eulerian (Vlasov Gkeyll) results with the wisdom gained from Lagrangian (PIC) analyses.

Electron scale coherent structure as micro accelerator in the Earth’s magnetosheath

Turbulent energy dissipation is a fundamental process in plasma physics that has not been settled. It is generally believed that the turbulent energy is dissipated at electron scales leading to electron energization in magnetized plasmas. Here, we propose a micro accelerator which could transform electrons from isotropic distribution to trapped, and then to stream (Strahl) distribution. From the MMS observations of an electron-scale coherent structure in the dayside magnetosheath, we identify an electron flux enhancement region in this structure collocated with an increase of magnetic field strength, which is also closely associated with a non-zero parallel electric field. We propose a trapping model considering a field-aligned electric potential together with the mirror force. The results are consistent with the observed electron fluxes from ~50 eV to ~200 eV. It further demonstrates that bidirectional electron jets can be formed by the hourglass-like magnetic configuration of the structure.

Effect of nonlinearity, magnetic islands on turbulence and observation of electron energization, temperature anisotropy at Earth’s magnetopause (magnetosphere)

Space missions’ observations have shown that waves such as lower hybrid waves (LHWs), whistler waves, and kinetic Alfven waves play a vital role in magnetic reconnection, turbulence, and particle acceleration. This paper studies the effect of nonlinearity and the magnetic islands on lower hybrid turbulence and current sheets in Earth’s magnetopause region. The evolution of electromagnetic LHW has been studied with numerical model using pseudo-spectral method for spatial integration and finite difference method with modified predictor-corrector approach for temporal integration. We have considered both ion and electron dynamics and included electromagnetic and warm plasma effects in our model. The study outcomes reveal that both the nonlinear effects and magnetic islands are responsible for the evolution of LHWs and current sheets to a chaotic and turbulent state. We have also used the semi-analytical model to elaborate on the physics behind the localization. Finally, the nonlinear model with field perturbations (magnetic islands) is further used to elaborate on the electron energization and temperature anisotropy near reconnection regions. We have also discussed the relevance of model predictions in the context of the MMS mission observations at Earth’s magnetopause.

Galileo Observation of Electron Spectra Dawn‐Dusk Asymmetry in the Middle Jovian Magnetosphere: Evidence for Convection Electric Field

AbstractIn Jupiter's middle magnetosphere, debate remains on the controlling physics of electron acceleration to the ultrarelativistic regime. Some local‐time asymmetric processes regulating MeV electrons may have their hold across electron spectra, leaving imprints down to 10–100s keV energies. Spectra at these lower energies can be measured with finite energy resolution, which is hardly achieved for MeV electrons. We investigate 10–100s keV electron spectra using Galileo measurements. We show that at 15–20 RJ, the power‐law exponents exhibit a previously unexpected dawn‐dusk asymmetry. This asymmetry is more prominent for 100s‐keV spectra, persistent but enhanced from 1996 to 2001. These match the theory of transport driven by a dawn‐to‐dusk electric field. Contrary to past expectations of dawn‐to‐dusk electric field at Io plasma torus (IPT) extending outward from 6 RJ, the origin of the field we infer may be completely different, despite its orientation and impacts on electron energization similar with the IPT one.