High-frequency Magnetic Fluctuations in Space Plasmas and the Role of Electron Landau Damping

While low-frequency plasma fluctuations in the interplanetary space have been successfully described in the framework of classical turbulence, high-frequency fluctuations still represent a challenge for theoretical models. At these scales, kinetic plasma processes are at work, but although some of them have been identified in spacecraft measurements, their global effects on observable quantities are sometimes not fully understood. In this paper we present a new framework to the aim of describing the observed magnetic energy spectrum and directly identify in the data the presence of Landau damping as the main collisionless dissipative process in the solar wind.

Spontaneous Magnetic Fluctuations and Collisionless Regulation of Turbulence in the Earth’s Magnetotail

Among the fundamental and most challenging problems of laboratory, space, and astrophysical plasma physics is to understand the relaxation processes of nearly collisionless plasmas toward quasi-stationary states and the resultant states of electromagnetic plasma turbulence. Recently, it has been argued that solar wind plasma β and temperature anisotropy observations may be regulated by kinetic instabilities such as the ion cyclotron, mirror, electron cyclotron, and firehose instabilities; and it has been argued that magnetic fluctuation observations are consistent with the predictions of the fluctuation–dissipation theorem, even far below the kinetic instability thresholds. Here, using in situ magnetic field and plasma measurements by the THEMIS satellite mission, we show that such regulation seems to occur also in the Earth’s magnetotail plasma sheet at the ion and electron scales. Regardless of the clear differences between the solar wind and the magnetotail environments, our results indicate that spontaneous fluctuations and their collisionless regulation are fundamental features of space and astrophysical plasmas, thereby suggesting the processes is universal.

MAGNETIC FLUCTUATIONS AFFECT CIRCADIAN PATTERNS OF LOCOMOTOR ACTIVITY IN ZEBRAFISH (PRELIMINARY DATA)

The locomotor activity of zebrafish (Danio rerio) has a pronounced, well-studied circadian rhythm. Under constant illumination, the period of free-running locomotor activity in this species usually becomes less than 24 hours. To evaluate the entraining capabilities of slow magnetic variations, zebrafish locomotor activity was evaluated at constant illumination and fluctuating magnetic field with a period of 26.8 hours. Lomb-Scargle periodogram revealed significant free-running rhythms of locomotor activity and related behavioral endpoints with a period close to 27 hours. Obtained results reveal the potential of slow magnetic fluctuations for entrainment of the circadian rhythms in zebrafish. The putative mechanisms responsible for the entrainment are discussed, including the possible role of cryptochromes.

Confinement improvement during detached phase with RMP application in deuterium plasmas of LHD

In order to explore compatibility of good core plasma performance with divertor heat load mitigation, interaction between cold edge plasma and core plasma transport including edge transport barrier (ETB) has been analysed in the divertor detachment discharges of deuterium plasmas in LHD with RMP (resonant magnetic perturbation) field application. The RMP application introduces widened edge stochastic layer and sharp boundary in the magnetic field structure between the confinement region and the edge stochastic layer. The widened edge stochastic layer enhances impurity radiation and provides stable detachment operation as compared with the case without RMP. It is found that ETB is formed at the confinement boundary at the onset of detachment transition. However, as the detachment deepens resistive pressure gradient driven MHD mode is excited, which degrade the ETB. At the same time, however, the core transport decreases to keep global plasma stored energy (Wp) unchanged, showing clearly core-edge coupling. After gradual increase of density fluctuation during the MHD activity, spontaneous increase of Wp and recovery of ETB are observed while the detachment is maintained. Then the coherent MHD mode ceases and ELM like bursts appear. In the improved mode, the impurity decontamination occurs, and the divertor heat load increase slightly. Key controlling physics in the interplay between core and cold edge plasma is discussed. Comparison between deuterium and hydrogen plasmas show that the hydrogen plasmas exhibit similar features as the deuterium ones in terms of density and magnetic fluctuations, impurity decontamination toward higher confinement etc. But most of the features are modest in the hydrogen plasmas and thus no clear confinement mode transition with clear ETB formation is defined. Better global confinement is obtained in the deuterium plasmas than the hydrogen ones at higher radiation level.

Design and first result of combined Langmuir-magnetic probe on J-TEXT tokamak

In order to measure boundary electrostatic and magnetic fluctuations simultaneously, a set of combined Langmuir-magnetic probe (CLMP) has been designed and built on Joint-Texas Experimental Tokamak (J-TEXT). The probe consists of 8 graphite probe pins and a 3D magnetic probe, driven by a mechanical pneumatic device. By means of simulation, the shielding effect of the graphite sleeve on the magnetic fluctuation signal is explored, and the influence of the eddy current was reduced by cutting the graphite sleeve. In the experiment, it has been verified that the mutual inductance of electromagnetic signals can be ignored. And a 70~90kHz electromagnetic mode is observed around the last closed magnetic surface (LCFS). The establishment of CLMP provides data for the exploration of the coupling of electrostatic and magnetic fluctuations.

Properties of Hall-MHD Turbulence at Sub-Ion Scales: Spectral Transfer Analysis

We present results of a multiscale study of Hall-magnetohydrodynamic (MHD) turbulence, carried out on a dataset of compressible nonlinear 2D Hall-MHD numerical simulations of decaying Alfvénic turbulence. For the first time, we identify two distinct regimes of fully developed turbulence. In the first one, the power spectrum of the turbulent magnetic fluctuations at sub-ion scales exhibits a power law with a slope of ∼−2.9, typically observed both in solar wind and in magnetosheath turbulence. The second regime, instead, shows a slope of −7/3, in agreement with classical theoretical models of Hall-MHD turbulence. A spectral-transfer analysis reveals that the latter regime occurs when the energy transfer rate at sub-ion scales is dominated by the Hall term, whereas in the former regime, the governing process is the dissipation (and the system exhibits large intermittency). Results of this work are relevant to the space plasma community, as they may potentially reconcile predictions from theoretical models with results from numerical simulations and spacecraft observations.

Geometry of Magnetic Fluctuations near the Sun from the Parker Solar Probe

Solar wind magnetic fluctuations exhibit anisotropy due to the presence of a mean magnetic field in the form of the Parker spiral. Close to the Sun, direct measurements were not available until the recently launched Parker Solar Probe (PSP) mission. The nature of the anisotropy and geometry of the magnetic fluctuations play a fundamental role in dissipation processes and in the transport of energetic particles in space. Using PSP data, we present measurements of the geometry and anisotropy of the inner heliosphere magnetic fluctuations, from fluid to kinetic scales. The results are surprising and different from 1 au observations. We find that fluctuations evolve characteristically with size scale. However, unlike 1 au solar wind, at the outer scale, the fluctuations are dominated by wavevectors quasi-parallel to the local magnetic field. In the inertial range, average wavevectors become less field aligned, but still remain more field aligned than near-Earth solar wind. In the dissipation range, the wavevectors become almost perpendicular to the local magnetic field in the dissipation range, to a much higher degree than those indicated by 1 au observations. We propose that this reduced degree of anisotropy in the outer scale and inertial range is due to the nature of large-scale forcing outside the solar corona.

MHD and Ion Kinetic Waves in Field-aligned Flows Observed by Parker Solar Probe

Parker Solar Probe (PSP) observed predominately Alfvénic fluctuations in the solar wind near the Sun where the magnetic field tends to be radially aligned. In this paper, two magnetic-field-aligned solar wind flow intervals during PSP’s first two orbits are analyzed. Observations of these intervals indicate strong signatures of parallel/antiparallel-propagating waves. We utilize multiple analysis techniques to extract the properties of the observed waves in both magnetohydrodynamic (MHD) and kinetic scales. At the MHD scale, outward-propagating Alfvén waves dominate both intervals, and outward-propagating fast magnetosonic waves present the second-largest contribution in the spectral energy density. At kinetic scales, we identify the circularly polarized plasma waves propagating near the proton gyrofrequency in both intervals. However, the sense of magnetic polarization in the spacecraft frame is observed to be opposite in the two intervals, although they both possess a sunward background magnetic field. The ion-scale plasma wave observed in the first interval can be either an inward-propagating ion cyclotron wave (ICW) or an outward-propagating fast-mode/whistler wave in the plasma frame, while in the second interval it can be explained as an outward ICW or inward fast-mode/whistler wave. The identification of the exact kinetic wave mode is more difficult to confirm owing to the limited plasma data resolution. The presence of ion-scale waves near the Sun suggests that ion cyclotron resonance may be one of the ubiquitous kinetic physical processes associated with small-scale magnetic fluctuations and kinetic instabilities in the inner heliosphere.

Adaptive Critical Balance and Firehose Instability in an Expanding, Turbulent, Collisionless Plasma

Using a hybrid-kinetic particle-in-cell simulation, we study the evolution of an expanding, collisionless, magnetized plasma in which strong Alfvénic turbulence is persistently driven. Temperature anisotropy generated adiabatically by the plasma expansion (and consequent decrease in the mean magnetic-field strength) gradually reduces the effective elasticity of the field lines, causing reductions in the linear frequency and residual energy of the Alfvénic fluctuations. In response, these fluctuations modify their interactions and spatial anisotropy to maintain a scale-by-scale “critical balance” between their characteristic linear and nonlinear frequencies. Eventually the plasma becomes unstable to kinetic firehose instabilities, which excite rapidly growing magnetic fluctuations at ion-Larmor scales. The consequent pitch-angle scattering of particles maintains the temperature anisotropy near marginal stability, even as the turbulent plasma continues to expand. The resulting evolution of parallel and perpendicular temperatures does not satisfy double-adiabatic conservation laws, but is described accurately by a simple model that includes anomalous scattering. Our results have implications for understanding the complex interplay between macro- and microscale physics in various hot, dilute, astrophysical plasmas, and offer predictions concerning power spectra, residual energy, ion-Larmor-scale spectral breaks, and non-Maxwellian features in ion distribution functions that may be tested by measurements taken in high-beta regions of the solar wind.