Electron Temperature Anisotropy Research Articles

Multi-dimensional incoherent Thomson scattering system in PHAse Space MApping (PHASMA) facility.

A multi-dimensional incoherent Thomson scattering diagnostic system capable of measuring electron temperature anisotropies at the level of the electron velocity distribution function (EVDF) is implemented on the PHAse Space MApping facility to investigate electron energization mechanisms during magnetic reconnection. This system incorporates two injection paths (perpendicular and parallel to the axial magnetic field) and two collection paths, providing four independent EVDF measurements along four velocity space directions. For strongly magnetized electrons, a 3D EVDF comprised of two characteristic electron temperatures perpendicular and parallel to the local magnetic field line is reconstructed from the four measured EVDFs. Validation of isotropic electrons in a single magnetic flux rope and a steady-state helicon plasma is presented.

Increased signal separation upgrade permits preliminary electron anisotropy measurements with Heliotron J multi-pass Thomson diagnostic.

A signal separation system is constructed on the multi-pass Thomson scattering system of Heliotron J to solve the problem of overlapping scattered light signals for the electron temperature anisotropy measurement. The phenomenon of overlapping scattered light signals is relieved by operating the signal separation system. A Raman scattering experiment is undertaken to verify the separation effect of the signal separation system. Two scattered light signals corresponding to two adjacent incidences of one laser shot were extended to 104ns. Moreover, we applied the multi-pass Thomson scattering system with signal separation system to the electron temperature anisotropy measurement. No anisotropy was observed within the error bars in the initial experiment.

Influence of electron temperature anisotropy on wave mode propagation and power deposition characteristics in helicon plasma

As the core issue in helicon discharge, the physical mechanism behind the high ionization rate phenomenon is still not fully understood. Based on the warm plasma dielectric tensor model which contains both the particle drift velocity and temperature anisotropy effect, by employing the general dispersion relation of electromagnetic waves propagating in magnetized and uniform plasma with typical helicon discharge parameter conditions, wave mode propagation characteristic and collisional, cyclotron and Landua damping induced wave power deposition properties of azimuthally symmetric mode are theoretically investigated. Systematic analysis shows the following findings. 1) Under typical helicon plasma parameter conditions, i.e. wave frequency <i>ω</i>/(2π)=13.56 MHz, ion temperature is one tenth of the electron temperature, and for a given magnetic field <i>B</i><sub>0</sub> (or wave frequency <i>ω</i>), there exists a critical wave frequency <i>ω</i><sub>cr</sub> (or magnetic field <i>B</i><sub>0,cr</sub>), above which (or below <i>B</i><sub>0,cr</sub>) the damping of the <i>n = </i>1, 2, 3 cyclotron harmonics begins to increase sharply. 2) For the electron temperature isotropic case, the attenuation constants of different harmonics start to increase significantly and monotonically at different thresholds of magnetic field, while the phase constant abruptly increases monotonically from the beginning of the parameter interval. On the other hand, for the electron temperature anisotropic case, both the phase constant and attenuation constant have peaking phenomenon, i.e. the attenuation constant begins to increase sharply at a certain value of <i>B</i><sub>0</sub> and meanwhile the phase constant presents a maximum value near the same value of magnetic field, thus the phase constant starts to keep constant at a certain value of <i>B</i><sub><i>0</i></sub> and meanwhile the attenuation constant has a maximum value near this same value of magnetic field. 3) For the wave power deposition properties, under electron temperature anisotropy conditions, power deposition due to collisional damping of Trivelpiece-Gould (TG) wave plays a dominant role in a low field (<i>B</i><sub>0 </sub>= 48 Gs) (1 Gs = 10<sup>–4</sup> T); by considering the electron finite Larmor radius (FLR) effect, the power deposition of TG wave presents a maximum value at a certain point of parallel electron temperature<i> T</i><sub>e,</sub><sub>//</sub>; with the decrease of <i>T</i><sub>e,⊥</sub>/<i>T</i><sub>e,</sub><sub>//</sub>, the maximum value of power deposition increases gradually. All these findings are very important in further revealing the physical mechanism behind the high ionization rate in helicon plasma.

The Effects of Substorm Injection of Energetic Electrons and Enhanced Solar Wind Ram Pressure on Whistler‐Mode Chorus Waves: A Statistical Study

AbstractWhistler‐mode chorus waves in the inner magnetosphere are typically excited by an electron temperature anisotropy. The anisotropy can be driven by two sources: particle injections from the tail (such as during substorms) and the solar wind ram pressure on the dayside magnetosphere. Based on 5 years of data from Van Allen Probe A, we have separately studied the effects of substorm injection of energetic ∼10–100 keV electrons and their gradient and curvature drifts () and enhanced solar wind ram pressure () on the generation of whistler‐mode chorus waves. We use time‐modified and indices to take into account time delays. We find that during the period of large but small , chorus waves are mainly observed in the midnight through dawn to noon sectors ( MLT 13) near the magnetic equator (|MLAT| < 10°) at L = 4.5–6.5. With an increase in both the chorus occurrence rates and the wave amplitudes increase. While under the condition of enhanced but small , chorus waves are preferentially detected on the dayside ( MLT 14) in a wide range of latitudes (|MLAT| < 20°) at outer L‐shells (L = 5.5–6.5). With the increase of , chorus occurrence rates also increase, while the amplitude remains relatively constant. Our study supports the two mechanisms for chorus excitation in the Earth’s magnetosphere and provides a better understanding of the global distribution and properties of chorus waves.

Electron Temperature Anisotropy Effects on Alpha/Proton Instability in the Solar Wind

In situ recordings by the solar Wind spacecraft reveal the ubiquitousness of alpha particles, whose drift velocities to the background proton vα are generally less than or equal to the local Alfvén velocity vA. The alpha beam instability plays a significant role in the alpha beam deceleration in the solar wind; nonetheless, the detailed mechanism of deceleration remains unclear. By using the linear Vlasov equation of the PDRK/B0 solver, the present work investigates the kinetic instability caused by both the alpha beam and the electron temperature anisotropy in the solar wind and assesses the effects of the electron temperature anisotropy on such instability. The results show that both anisotropic electrons and alpha beams lead to the excitation of several plasma waves, and the wave frequency, growth rate, and polarization properties are sensitive to the electron temperature anisotropy (Te⊥/Te‖), the parallel electron beta (βe‖), and the alpha beam drift velocity (vα/vA). With an excess parallel temperature Te⊥/Te‖<1, the parallel magnetosonic/whistler (PM/W), parallel Alfvén wave (PAW), and oblique Alfvén/ion cyclotron (OA/IC) instabilities could be generated, while for an excess perpendicular temperature Te⊥/Te‖>1, the PM/W, OA/IC, parallel whistler (PW), and kinetic Alfvén wave (KAW) instabilities could grow. In the region of Te⊥/Te‖<1, the thresholds of the PM/W, PAW, and OA/IC instabilities extend to lower drift velocity vα/vA. In the region of Te⊥/Te‖>1, the thresholds of the PM/W and OA/IC instabilities increase, while those of the PW and KAW instabilities are shifted to lower vα/vA. The current study presents a comprehensive overview for alpha beam instabilities that limit the alpha beam drift velocity in the solar wind.

Whistler turbulence vs. whistler anisotropy instability: Particle-in-cell simulation and statistical analysis

Particle-in-Cell simulations and statistical analysis are carried out to study the dynamic evolution of a collisionless, magnetized plasma with co-existing whistler turbulence and electron temperature anisotropy as the initial condition, and the competing consequences of whistler turbulence cascade and whistler anisotropy instability growth. The results show that the operation of the whistler instability within whistler turbulence has almost no effects on the fluctuating magnetic field energy and intermittency generated by turbulence. However, it leads to a small reduction of the magnetic field wavevector anisotropy and a major reduction of the intermittency of electron temperature anisotropy. Hence, while the overall effect from whistler instability is minor as compared to that of whistler turbulence due to its much smaller field energy, the whistler instability may act as a regulation mechanism for kinetic-range turbulence through wave-particle interactions.

Kinetic Generation of Whistler Waves in the Turbulent Magnetosheath.

The Earth's magnetosheath (MSH) is governed by numerous physical processes which shape the particle velocity distributions and contribute to the heating of the plasma. Among them are whistler waves which can interact with electrons. We investigate whistler waves detected in the quasi‐parallel MSH by NASA's Magnetospheric Multiscale mission. We find that the whistler waves occur even in regions that are predicted stable to wave growth by electron temperature anisotropy. Whistlers are observed in ion‐scale magnetic minima and are associated with electrons having butterfly‐shaped pitch‐angle distributions. We investigate in detail one example and, with the support of modeling by the linear numerical dispersion solver Waves in Homogeneous, Anisotropic, Multicomponent Plasmas, we demonstrate that the butterfly distribution is unstable to the observed whistler waves. We conclude that the observed waves are generated locally. The result emphasizes the importance of considering complete 3D particle distribution functions, and not only the temperature anisotropy, when studying plasma wave instabilities.

Electron-Driven Instabilities in the Solar Wind

The electrons are an essential particle species in the solar wind. They often exhibit non-equilibrium features in their velocity distribution function. These include temperature anisotropies, tails (kurtosis), and reflectional asymmetries (skewness), which contribute a significant heat flux to the solar wind. If these non-equilibrium features are sufficiently strong, they drive kinetic micro-instabilities. We develop a semi-graphical framework based on the equations of quasi-linear theory to describe electron-driven instabilities in the solar wind. We apply our framework to resonant instabilities driven by temperature anisotropies. These include the electron whistler anisotropy instability and the propagating electron firehose instability. We then describe resonant instabilities driven by reflectional asymmetries in the electron distribution function. These include the electron/ion-acoustic, kinetic Alfvén heat-flux, Langmuir, electron-beam, electron/ion-cyclotron, electron/electron-acoustic, whistler heat-flux, oblique fast-magnetosonic/whistler, lower-hybrid fan, and electron-deficit whistler instability. We briefly comment on non-resonant instabilities driven by electron temperature anisotropies such as the mirror-mode and the non-propagating firehose instability. We conclude our review with a list of open research topics in the field of electron-driven instabilities in the solar wind.

Observations of Whistler-mode Waves and Large-amplitude Electrostatic Waves Associated with a Dipolarization Front in the Bursty Bulk Flow

Plasma jets and jet fronts are common phenomena in planetary magnetospheres. They are usually associated with many plasma waves and can play a key role in the energy conversion, the excitation of wave emissions, particle acceleration, and the evolution of many astrophysical phenomena, which are major issues in the study of helio-terrestrial space physics. In this paper, we carefully investigated the properties of the whistler-mode wave and large-amplitude electrostatic wave in a plasma jet (bursty bulk flow (BBF)) using the Magnetospheric Multiscale mission data on the Earth's magnetosphere. At the leading part of the BBF, intense whistler-mode waves were observed inside the ion mirror-mode structures, which should be excited by the perpendicular temperature anisotropy of trapping electrons. A small-scale dipolarization front (DF) was then observed at the center of this BBF as a boundary between the leading and trailing parts of the BBF. Behind the DF, both an ion mirror-mode structure and whistler-mode waves disappear, while a large-amplitude electrostatic wave was detected and was associated with the cold ions at the trailing part of the BBF. The electrostatic wave is supposed to be generated by ion beam instability. These results will significantly improve the understanding of the kinetic process associated with the important boundary layer DF within plasma jets. The corresponding wave–particle interaction in space and the plasma environment can be further understood.

Revisiting Ulysses electron data with a triple fit of velocity distributions

Context. Given their uniqueness, the Ulysses data can still provide us with valuable new clues about the properties of plasma populations in the solar wind, and especially about their variations with heliographic coordinates. In the context of kinetic waves and instabilities in the solar wind plasma, the electron temperature anisotropy plays a crucial role. To date, two electron populations (the core and the halo) have been surveyed using anisotropic fitting models, limited in general to the ecliptic observations. Aims. We revisit the electron data reported by the SWOOPS instrument on board the Ulysses spacecraft between 1990 and early 2008. These observations reveal velocity distributions out of thermal equilibrium, with anisotropies (e.g., parallel drifts and/or different temperatures, parallel and perpendicular to the background magnetic field), and quasi-thermal and suprathermal populations with different properties. Methods. We apply a 2D non-linear least squares fitting procedure, using the Levenberg–Marquardt algorithm, to simultaneously fit the velocity electron data (up to a few keV) with a triple model combining three distinct populations: the more central quasi-thermal core, the suprathermal halo, and a second suprathermal population consisting mainly of the electron strahl (or beaming population with a major field-aligned drift). The recently introduced κ-cookbook is used to describe each component with the following anisotropic distribution functions (recipes): Maxwellian distribution, regularized κ-distribution, and generalized κ-distribution. Most relevant are triple combinations selected as best fits (BFs) with minimum relative errors and standard deviations. Results. The number of BFs obtained for each fitting combination is 80.6% of the total number of events (70.7% in the absence of coronal mass ejections). Showing the distribution of the BFs for the entire data set, during the whole interval of time, enables us to identify the most representative fitting combinations associated with either fast or slow winds, and different phases of solar activity. The temperature anisotropy quantified by the best fits is considered a case study of the main parameters characterizing electron populations. By comparison to the core, both suprathermal populations exhibit higher temperature anisotropies, which slightly increase with the energy of electrons. Moreover, these anisotropies manifest different dependences on the solar wind speed and heliographic coordinates, and are highly conditioned by the fitting model. Conclusions. These results demonstrate that the characterization of plasma particles is highly dependent on the fitting models and their combinations, and this method must be considered with caution. However, the multi-distribution function fitting of velocity distributions has a significant potential to advance our understanding of solar wind kinetics and deserves further quantitative analyses.

Effect of electron temperature anisotropy on TEM in reversed-field-pinch plasmas

For the first time in the reversed-field-pinch configuration, trapped electron mode (TEM) with anisotropies of electron temperature and its gradient is studied by solving the gyrokinetic integral eigenmode equation. Detailed numerical analyses indicate that TEM is enhanced by the anisotropy with temperature in the direction perpendicular to the magnetic field that is higher than that in the direction parallel to the magnetic field when the latter is kept constant. However, the enhancement is limited, such that TEM is weakened and even stabilized when the anisotropy is higher than a critical value, due to strong Landau damping. In comparison with the isotropic case, the lower Landau damping with the higher parallel electron temperature makes TEM instability easier to excite, which expands the TEM unstable region in the diagram of density and temperature scale lengths. In addition, it is found that the electron temperature gradient in the perpendicular direction offers a stronger driving force on TEM instability than that in the parallel direction. The overall effects of the temperature gradients of electrons and ions, magnetic shear, safety factor and density gradient on TEM in the presence of the anisotropies are presented in detail.

Electromagnetic Weibel instability in spatial anisotropic electron–ion plasmas

The Weibel instability due to temperature anisotropy of electrons and ions in a plasma in the presence of cold and warm ions is reported. Numerical calculations of the normalized growth rate are carried out when the frequency of electromagnetic waves is greater than or less than the thermal velocity of electrons for typical existing plasma parameters. The normalized growth rate increases with an increasing normalized wave number, and after attaining maxima, it decreases due to thermal effects. Therefore, a parabolic plot is obtained for the growth rate. The threshold values of the growth rate depend on the anisotropy parameters. On increasing the value of the temperature anisotropy ratio of either plasma component, the observed growth rate increases. There is a considerable and contrasting effect of the presence of cold and warm ions on the growth rate of the Weibel instability in the plasma. The addition of cold ions stabilizes the instability and reduces the maximum growth rate values, while the addition of warm ions to the plasma increases the instability with a considerable decrease in the domain of instability. Our theoretical investigations of the effect of temperature anisotropy on the growth rate of the Weibel instability are in good agreement with the existing experimental results.

Analysis of multiscale structures at the quasi-perpendicular Venus bow shock

Context.Solar Orbiter is a European Space Agency mission with a suite of in situ and remote sensing instruments to investigate the physical processes across the inner heliosphere. During the mission, the spacecraft is expected to perform multiple Venus gravity assist maneuvers while providing measurements of the Venusian plasma environment. The first of these occurred on 27 December 2020, in which the spacecraft measured the regions such as the distant and near Venus magnetotail, magnetosheath, and bow shock.Aims.This study aims to investigate the outbound Venus bow shock crossing measured by Solar Orbiter during the first flyby. We study the complex features of the bow shock traversal in which multiple large amplitude magnetic field and density structures were observed as well as higher frequency waves. Our aim is to understand the physical mechanisms responsible for these high amplitude structures, characterize the higher frequency waves, determine the source of the waves, and put these results into context with terrestrial bow shock observations.Methods.High cadence magnetic field, electric field, and electron density measurements were employed to characterize the properties of the large amplitude structures and identify the relevant physical process. Minimum variance analysis, theoretical shock descriptions, coherency analysis, and singular value decomposition were used to study the properties of the higher frequency waves to compare and identify the wave mode.Results.The non-planar features of the bow shock are consistent with shock rippling and/or large amplitude whistler waves. Higher frequency waves are identified as whistler-mode waves, but their properties across the shock imply they may be generated by electron beams and temperature anisotropies.Conclusions.The Venus bow shock at a moderately high Mach number (∼5) in the quasi-perpendicular regime exhibits complex features similar to the Earth’s bow shock at comparable Mach numbers. The study highlights the need to be able to distinguish between large amplitude waves and spatial structures such as shock rippling. The simultaneous high frequency observations also demonstrate the complex nature of energy dissipation at the shock and the important question of understanding cross-scale coupling in these complex regions. These observations will be important to interpreting future planetary missions and additional gravity assist maneuvers.

Cross-scale energy cascade powered by magnetospheric convection

Plasma convection in the Earth’s magnetosphere from the distant magnetotail to the inner magnetosphere occurs largely in the form of mesoscale flows, i.e., discrete enhancements in the plasma flow with sharp dipolarizations of magnetic field. Recent spacecraft observations suggest that the dipolarization flows are associated with a wide range of kinetic processes such as kinetic Alfvén waves, whistler-mode waves, and nonlinear time-domain structures. In this paper we explore how mesoscale dipolarization flows produce suprathermal electron instabilities, thus providing free energy for the generation of the observed kinetic waves and structures. We employ three-dimensional test-particle simulations of electron dynamics one-way coupled to a global magnetospheric model. The simulations show rapid growth of interchanging regions of parallel and perpendicular electron temperature anisotropies distributed along the magnetic terrain formed around the dipolarization flows. Unencumbered in test-particle simulations, a rapid growth of velocity-space anisotropies in the collisionless magnetotail plasma is expected to be curbed by the generation of plasma waves. The results are compared with in situ observations of an isolated dipolarization flow at one of the Magnetospheric Multiscale Mission spacecraft. The observations show strong wave activity alternating between broad-band wave activity and whistler waves. With estimated spatial extent being similar to the characteristic size of the temperature anisotropy patches in our test-particle simulations, the observed bursts of the wave activity are likely to be produced by the parallel and perpendicular electron energy anisotropies driven by the dipolarization flow, as suggested by our modeling results.

Electron Thermalization and Electrostatic Turbulence Caused by Flow Reversal in Dipolarizing Flux Tubes

Abstract During magnetic reconnection in Earth’s magnetotail, a dipolarizing flux tube (DFT) is formed and carries large amounts of energy toward the Earth to produce the aurora. Electrons inside the entire DFT are generally hot and tenuous, because they originate from the low-density lobe region and subsequently are heated by reconnection. Here, we report a special DFT hosting both hot-tenuous and cold-dense electrons, and we observe unique electron thermalization and associated electrostatic turbulence inside such a DFT. Analyses of the ion dynamics indicate that formation of the special phenomenon might be due to the flow reversal on the DFT flank, which is found to be an isobaric process in the direction perpendicular to the magnetic field. Correlation analysis shows that electrostatic waves at frequencies of 2–70 Hz are well correlated with the temperature anisotropy of electrons in the range of 300–27,000 eV, and waves at a frequency above one electron gyrofrequency (f ce) have a strong negative correlation with the electron temperature anisotropy as well. This study can improve our understanding of electron dynamics in the magnetotail.

Properties of Electron Distributions in the Martian Space Environment

AbstractElectron and magnetic field measurements from the Mars atmosphere and volatile environment (MAVEN) mission are utilized to study the interaction between Mars and the solar wind. Instruments like the solar wind electron analyzer (SWEA) aboard MAVEN measure properties of the electron environment over a broad range of electron energies. Measurements at low electron energies include contributions from spacecraft photoelectrons and secondary electrons that must be accounted for to accurately characterize the environment. We developed an algorithm to identify and remove secondary electron contamination to improve estimates of electron densities and temperature. We then compiled global maps of average electron density, temperature, and temperature anisotropy under different conditions, considering quasi‐parallel and quasi‐perpendicular bow shocks and upstream solar wind Alfven Mach number. Higher temperature anisotropy is observed for quasi‐perpendicular shock crossings, as expected. We find significant electron temperature anisotropy upstream of the bow shock for quasi‐perpendicular shock crossings, suggesting a heating mechanism, such as that provided by electromagnetic waves. We analyzed the influence of hi and low Alfven Mach number conditions and found the electron plasma beta to be the only electron property significantly affected. We studied the relationship between the electron distribution function and the generation of instabilities and conclude that the upstream Alfven Mach number influences the stability of electron distributions in the Martian environment.

Spectroscopy of the Mirror Instability Growth Rate in Hollow Electrodes Discharge (HED) Plasma

In this work, an experimental study was conducted about the effect of gas pressure on the growth rate of the mirror instability produced in hollow electrodes discharge (HED) plasma in two regions: inter-electrodes gap and internal cathode cavity, by optical emission spectroscopy. Optical emission spectroscopy measurements, at different gas pressures in two regions under study, show that the electron number density (ne) increase with increasing gas pressure from 0.04 to 0.2 Torr. While the electron temperature (Te) decrease with increased gas pressure. In addition, the growth rate increase with increasing electron temperature anisotropy in both regions.

Broadband Electrostatic Waves Behind Dipolarization Front: Observations and Analyses

AbstractUsing high‐resolution MMS data, we report the observations of broadband electrostatic waves associated with parallel electron temperature anisotropy () behind a dipolarization front (DF). These electrostatic waves include electrostatic solitary waves and electron cyclotron waves. To quantify electron anisotropy, we define the parallel flux anisotropy parameter , where is phase space densities at each energy. By performing correlation analyses between parallel flux anisotropies () and power spectral densities (PSDs) of these waves, we find that: (a) in 0.8–20 keV are positively correlated with the parallel electric field fluctuations in 50–3,000 Hz; (b) in 0.8–20 keV are positively correlated with perpendicular electric field fluctuations in all frequency ranges; (c) the correlation between parallel flux anisotropies and perpendicular electric field fluctuations is more significant in this case. We also discuss the possibility of electron firehose instability and wave–particle interactions. Our study promotes the understanding of the properties and dynamics of DFs.

Modulation of Whistler Mode Waves by Ultra‐Low Frequency Wave in a Macroscale Magnetic Hole: MMS Observations

AbstractWe report the quasi‐periodic whistler‐mode waves corresponding to electron temperature anisotropy T⊥/T// < 1 at the center of a macroscale magnetic hole (MH) near the Earth's magnetopause observed by the Magnetospheric Multiscale mission. A significant ultra‐low frequency (ULF) wave of background magnetic field was observed in this MH. The ULF wave dramatically exacerbates the evolution of MH and the formation of the donut‐shaped electron pitch angle distribution (PAD) in the macroscale MH. When the whistler‐mode waves were generated, they are consistent with troughs of the ULF wave and density increases, which is also related to the butterfly type PAD at those moments. The analysis of dispersion relation and cyclotron resonance condition indicate that whistler‐mode waves were mainly generated by the butterfly type PAD of electrons. Our results provides new insights into one possible excitation mechanism of whistler‐mode wave.

Turbulent Magnetogenesis in a Collisionless Plasma

Abstract We demonstrate an efficient mechanism for generating magnetic fields in turbulent, collisionless plasmas. By using fully kinetic, particle-in-cell simulations of an initially nonmagnetized plasma, we inspect the genesis of magnetization, in a nonlinear regime. The complex motion is initiated via a Taylor–Green vortex, and the plasma locally develops strong electron temperature anisotropy, due to the strain tensor of the turbulent flow. Subsequently, in a domino effect, the anisotropy triggers a Weibel instability, localized in space. In such active wave–particle interaction regions, the seed magnetic field grows exponentially and spreads to larger scales due to the interaction with the underlying stirring motion. Such a self-feeding process might explain magnetogenesis in a variety of astrophysical plasmas, wherever turbulence is present.