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An understanding of how turbulent energy is partitioned between ions and electrons in weakly collisional plasmas is crucial for modeling many astrophysical systems. Using theory and simulations of a four-dimensional reduced model of low-beta gyrokinetics (the “Kinetic Reduced Electron Heating Model”), we investigate the dependence of collisionless heating processes on plasma beta and imbalance (normalized cross-helicity). These parameters are important because they control the helicity barrier, the formation of which divides the parameter space into two distinct regimes with remarkably different properties. In the first, at lower beta and/or imbalance, the absence of a helicity barrier allows the cascade of injected power to proceed to small (perpendicular) scales, but its slow cascade rate makes it susceptible to significant electron Landau damping, in some cases leading to a marked steepening of the magnetic spectra on scales above the ion Larmor radius. In the second, at higher beta and/or imbalance, the helicity barrier halts the cascade, confining electron Landau damping to scales above the steep “transition-range” spectral break, resulting in dominant ion heating. We formulate quantitative models of these processes that compare well to simulations in each regime, and combine them with results of previous studies to construct a simple formula for the electron–ion heating ratio as a function of beta and imbalance. This model predicts a “winner takes all” picture of low-beta plasma heating, where a small change in the fluctuations' properties at large scales (the imbalance) can cause a sudden switch between electron and ion heating.

To understand the ion injection for diffusive shock acceleration in space and astrophysical shock waves, such as in planetary bow shocks and supernova remnant shocks, ion heating and acceleration in high-Alfvén-Mach-number (MA) quasi-parallel shock waves are studied by means of full particle-in-cell (PIC) simulations and theory. We perform 2D and 3D PIC simulations in the regime of MA ≥ 10, where shock-driven turbulence contains a number of current sheets and magnetic reconnection sites. The nonresonant ion–ion beam mode grows in the shock transition region, which creates current sheets, and some of them drive magnetic reconnection. The ion temperature in magnetic islands produced by ion-coupled reconnection becomes significantly higher than those in the surrounding regions. This temperature enhancement is due to two physical reasons: the reduction of the number density of the incident ions in the nonresonant wave, and the energization and heating in the magnetic islands. Ions that enter magnetic islands are energized by the Hall electric field pointing toward the island center. Ions are also energized by the motional electric field produced in the outer regions of the islands and the current sheets. The energy increase rate of ions by these energization mechanisms is much larger than that of the conventional shock drift acceleration. These energetic ions are unmagnetized, and they escape from the shock transition region toward the upstream region; therefore, ions in shocks with reconnecting current sheets can be injected into diffusive shock acceleration more efficiently than those in laminar shock waves.

Efficient ion cyclotron range of frequencies (ICRF) wave heating requires good wave coupling at the plasma edge and good radio frequency power absorption in the plasma core. This study reviews recent progress in improving these two aspects of ICRF heating with the new two-strap antennas through various experiments and simulations on the Experimental Advanced Superconducting Tokamak (EAST). Our study shows that the ICRF coupling can be significantly improved by decreasing the parallel wave number, increasing the local scrape-off layer (SOL) density by midplane gas puffing, and increasing the global SOL density by decreasing the separatrix–antenna distance. It can also be improved by increasing the core plasma density, changing the divertor strike point position, and optimizing the antenna phasing. The core ICRF power absorption can be increased by optimizing the cyclotron resonance position and minority ion concentration and by applying new heating schemes such as three-ion heating. Although some of the methods have been previously studied on other devices, improving ICRF coupling by shifting the divertor strike point was tested on EAST for the first time. Quantitative characterization of these methods and the conclusions drawn from this study can provide important insights for achieving more efficient ICRF heating in current and future fusion machines.

We survey 20 reconnection outflow events observed by Magnetospheric MultiScale in the low-β and high-Alfvén-speed regime of the Earth’s magnetotail to investigate the scaling of ion bulk heating produced by reconnection. The range of inflow Alfvén speeds (800–4000 km s−1) and inflow ion β (0.002–1) covered by this study is in a plasma regime that could be applicable to the solar corona and flare environments. We find that the observed ion heating increases with increasing inflow (upstream) Alfvén speed, V A, based on the reconnecting magnetic field and the upstream plasma density. However, ion heating does not increase linearly as a function of available magnetic energy per particle, miVA2 . Instead, the heating increases progressively less as miVA2 rises. This is in contrast to a previous study using the same data set, which found that electron heating in this high-Alfvén-speed and low-β regime scales linearly with miVA2 , with a scaling factor nearly identical to that found for the low-V A and high-β magnetopause. Consequently, the ion-to-electron heating ratio in reconnection exhausts decreases with increasing upstream V A, suggesting that the energy partition between ions and electrons in reconnection exhausts could be a function of the available magnetic energy per particle. Finally, we find that the observed difference in ion and electron heating scaling may be consistent with the predicted effects of a trapping potential in the exhaust, which enhances electron heating, but reduces ion heating.

Finite-amplitude low-frequency Alfvén waves (AWs) are ubiquitous in space plasmas, where they play a key role in the transport and dissipation of energy, particularly in the heating of ions in the solar corona and solar wind. In this study, we investigate the nonlinear interaction between ions and obliquely propagating AWs. When the wave amplitude and propagation angle lie within specific ranges, ion motion becomes chaotic. We quantify this behavior using the maximum Lyapunov exponent (λm) and define a new parameter, the chaos ratio (CR), to describe the fraction of chaotic particles across different initial states. The global chaos threshold is determined as the contour CR = 0.01. Analysis of magnetic moment variations reveals that the physical origin of chaos is pitch-angle scattering induced by wave-driven field-line curvature (WFLC), which disrupts adiabatic invariance and leads to stochastic ion energization. The onset condition for chaos can be expressed by an effective relative curvature radius, Peff. < 25. This analytical criterion delineates the boundary of the chaotic region in the (kx, kz, Bw) parameter space and agrees well with numerical results. The identified WFLC mechanism provides a new physical pathway for converting macroscale Alfvénic disturbances into microscopic ion heating. This analysis offers a simplified model that illustrates a plausible ion energization mechanism in Alfvénic turbulent plasmas, including those associated with solar wind switchbacks and coronal fluctuations. These results highlight a universal chaotic process that may underlie stochastic heating in heliospheric and astrophysical plasmas.

Finite-amplitude low-frequency Alfvén waves are commonly found in space plasmas and play a crucial role in ion heating. The nonlinear interaction between oblique Alfvén wave spectrum and ions is studied. We find that, as the number of wave modes increases, ions are more likely to exhibit chaotic motion and experience stochastic heating. We extend the effective relative curvature radius (Peff) criterion—recently proposed by us for monochromatic waves—to continuous wave spectra and confirm that the chaos threshold (Peff ≲ 25) remains valid for the breakdown of magnetic moment conservation and the onset of chaos. This criterion is more easily satisfied with increasing wave modes, implying that stochastic heating is nearly ubiquitous in solar wind and coronal Alfvénic turbulence. We identify a three-stage anisotropic heating process: preferential perpendicular heating at early times, quasi-isotropic heating at intermediate times, and preferential parallel heating at late times, which we quantitatively explain using a uniform solid-angle distribution model. We further derive an analytical scaling law for the stochastic heating rate, Q/(ΩimivA2)=H(α)v˜3B˜w3ω˜1ω˜1+Δω˜ , governed by wave amplitude, frequency, bandwidth, propagation angle, and initial ion drift speed in wave reference frame. This heating arises from wave-induced field line curvature, providing a new physical picture for ion kinetic heating in astrophysical plasmas.

Anomalous ion heating is frequently observed to accompany magnetic reconnection, yet there is little consensus on its origin. Instead of the usual velocity-space analysis, we use phase-space analysis to exhaustively explain how ions are nonthermally energized during collisionless, antiparallel magnetic reconnection. There are both ordered and disordered aspects in the process; the former is explained in terms of conservative quantities, and the latter is explained by demonstrating chaos through a direct calculation of Lyapunov exponents. The former induces “multibeam-like heating” in all three directions, whereas the latter induces stochastic bulk heating. Profiles of the ion temperature tensor components during reconnection can be easily understood by the phase-space distributions of ions in different motional stages.

Parker Solar Probe observes unexpectedly prevalent switchbacks, which are rapid magnetic field reversals that last from seconds to hours, in the inner heliosphere, posing new challenges to understanding their nature, origin, and evolution. In this work, we investigate the thermal states, electron pitch-angle distributions, and pressure signatures of both inside and outside the switchbacks, separating a switchback into spike, transition region (TR), and quiet period (QP). Based on our analysis, we find that the proton temperature anisotropies in TRs seem to show an intermediate state between spike and QP plasmas. The proton temperatures are more enhanced in the spike than in the TR and QP, but the alpha temperatures and alpha-to-proton temperature ratios show the opposite trend to the proton temperatures, implying that the preferential heating mechanisms of protons and alphas are competing in different regions of switchbacks. Moreover, our results suggest that the electron-integrated intensities are almost the same across the switchbacks, but the electron pitch-angle distributions are more isotropic inside than outside switchbacks, implying switchbacks are intact structures, but strong scattering of electrons happens inside switchbacks. In addition, the examination of pressures reveals that the total pressures are comparable through an individual switchback, confirming switchbacks are pressure-balanced structures. These characteristics could further our understanding of ion heating, electron scattering, and the structure of switchbacks.

Recently, electron‐only reconnection, in which there is no obvious ion bulk flow and ion heating, has been pervasively observed in the Earth's magnetosphere. In this Letter, we realize electron‐only reconnection with a guide field in the Keda Linear Magnetized Plasma (KLMP) device. By measuring the magnetic field, we identify unambiguously a distorted quadrupolar structure of the magnetic field in the out‐of‐plane direction. At the same time, electrons are obviously heated in the current sheet with the half‐width about 0.8 electron inertial length. The maximum velocity of the estimated electron flow in the reconnection plane is about eight Alfvén speed. Plain Language Summary Magnetic reconnection, a fundamental process during which magnetic field lines break and reconfigure their connectivity, can convert magnetic energy into plasma kinetic and thermal energy in space and laboratory plasmas. A standard reconnection model with both ion and electron dynamics has been established to describe the magnetic reconnection observed in space and laboratory plasmas. Recently, a new type of magnetic reconnection has been pervasively observed in space, called electron‐only reconnection, in which there is no obvious ion bulk flow and ion heating. Here, we realize electron‐only reconnection experiments in the Keda Linear Magnetized Plasma (KLMP) device. By measuring the magnetic fields in a two‐dimensional reconnection plane, we find that the magnetic field in the out‐of‐plane direction has a distorted quadrupolar structure. Additionally, electrons are obviously heated in a thin electron‐scale current sheet, and the peak velocity of the electron flow is estimated to be about eight Alfvén speed. Key Points We realize electron‐only reconnection with a guide field in the KLMP device The magnetic field in the out‐of‐plane direction has a distorted quadrupolar structure Electrons are obviously heated in a thin electron‐scale current sheet with the half‐width about 0.8 electron inertial length

Standard magnetic reconnection couples with both ions and electrons on different scales. Recently, a new type of magnetic reconnection, electron‐only reconnection without the coupling of ions, has been observed in various plasma environments. Standard reconnection typically has a reconnection outflow velocity of about one Alfvén speed. According to the scaling analysis, the electron outflow velocity is expected to be about one electron Alfvén speed in electron‐only reconnection. However, observations and simulations both find that the electron outflows are much slower than the electron Alfvén speed. In this letter, by performing two‐dimensional particle‐in‐cell simulations, we show that this is because ions play a role in electron‐only reconnection. The ions move slower than the electrons in the outflow direction, and such charge separation forms an in‐plane Hall electric field, which prevents the electrons from being accelerated to the electron Alfvén speed in electron‐only reconnection. Plain Language Summary Magnetic reconnection is a fundamental plasma process during which magnetic field line topologies change and magnetic energy transfers to plasma kinetic and thermal energy. In standard collisionless magnetic reconnection, ions and electrons are both heated and accelerated to form high‐speed outflow. Theoretical analyses have been performed to successfully explain the fast outflow speed in standard magnetic reconnection. Recently, a new type of magnetic reconnection, electron‐only reconnection (in which there is no obvious ion heating and acceleration) has been reported to occur in various plasma environments. However, the same scaling analyses performed in electron‐only reconnection overestimate the outflow speed in the electron‐only reconnection by a factor of ∼10. Here, by performing two‐dimensional (2‐D) particle‐in‐cell (PIC) simulations, we show that the overestimation is due to the neglect of the role of ions. Because of the in‐plane Hall electric field caused by charge separation, electron outflow in electron‐only reconnection cannot be accelerated to the expected value. Key Points Electrons are not accelerated to electron Alfvén speed in electron‐only magnetic reconnection Deceleration of electron outflow caused by in‐plane Hall electric field should not be ignored