Electron Temperature Anisotropy Research Articles

Whistler-mode waves in the tail of Mercury’s magnetosphere: A numerical study

Context. Mercury presents a highly dynamic, small magnetosphere in which magnetic reconnection plays a fundamental role. Aim. We aim to model the global characteristics of magnetic reconnection in the Hermean environment. In particular, we focus on waves observed during the third BepiColombo flyby. Method. In this work, we used two fully kinetic three-dimensional (3D) simulations carried out with the iPIC3D code, which models the interaction of the solar wind with the Hermean magnetosphere. For the simulations, we used southward solar wind conditions that allow for a maximum magnetic coupling between the solar wind and the planet. Results. Our simulations show that a significant wave activity, triggered by magnetic reconnection, develops near the diffusion region in the magnetotail and propagates at large scales in the night-side magnetosphere. We see an increase in electron temperature close to the diffusion region and we specifically observe narrowband whistler waves developing near the reconnection region. These waves propagate nearly parallel to the magnetic field at frequency f ∼ 0.5fce. In addition to the electromagnetic component, these waves also exhibit an electrostatic one. Furthermore, we observe a strong electron temperature anisotropy, suggesting it plays a role as the source of these waves.

Statistical Properties of Whistler-mode Waves in the Dayside Terrestrial Space: MMS Observations

Whistler-mode waves have been extensively observed and investigated in terrestrial space. In this study, we present the dynamic response of whistler-mode waves to different solar wind conditions in the dayside terrestrial space based on Magnetospheric Multiscale (MMS) data. Statistical results show that the occurrence rate, amplitudes, and corresponding electron temperature anisotropy of whistler-mode waves increase with P sw in the dayside terrestrial space, which is attributed to the compression of magnetic fields in these magnetosheath and outer magnetosphere. Furthermore, whistler-mode waves under the southward interplanetary magnetic fields (IMFs) show a higher occurrence rate than that under the northward IMFs, mostly corresponding to T e⊥/T e∥ > 1, and have a higher occurrence rate during quasi-radial IMFs. These results present that whistler-mode waves in these magnetosheath and outer magnetosphere are also modulated by the solar wind as clearly as the inner magnetosphere. This work advanced our understanding in the solar–terrestrial interaction.

Observation of electron temperature anisotropy in the magnetic filter of a hot cathode discharge

Instabilities arising from electron temperature in an ambient magnetic field when measured parallel and perpendicular to it could influence plasma stability and have effect on plasma diffusion. A planar Langmuir probe measures this instability in the double plasma device in the magnetic filter field region and it is found that the electron temperature measured parallel and perpendicular to the filter region is not equal. This anisotropy is also found to vary with filament current and working pressure.

Effects of the applied fields' strength on the plasma behavior and processes in E × B plasma discharges of various propellants. II. Magnetic field

The effects of magnetic field intensity on the properties of the plasma discharge and on the underlying phenomena are studied for different propellants' ion mass. The plasma setup represents a 2D radial–azimuthal configuration with perpendicular electric and magnetic fields. The electric field is along the axial direction, and the magnetic field is along the radial direction. The magnetic field intensity is changed from 5 to 30 mT, with 5 mT increments. The studied propellant gases are xenon, krypton, and argon. The simulations are carried out using a reduced-order particle-in-cell code. It is shown that, for all the propellants, the change in the magnetic field intensity yields two distinct plasma regimes, where either the modified two-stream instability (MTSI) or the electron cyclotron drift instability (ECDI) are dominant. A third plasma regime is also observed for cases with moderate values of the magnetic field (15 and 20 mT), where the ECDI and the MTSI co-exist with comparable amplitudes. This described variation of plasma regime becomes clearly reflected in the radial distribution of the axial electron current density and the electron temperature anisotropy. At the relatively low-magnetic-field intensities (5 and 10 mT), the MTSI is mitigated. At relatively high magnitudes of the magnetic field (25 and 30 mT), the MTSI becomes strongly present, a long-wavelength wave mode develops, and the ECDI becomes suppressed. An exception to this latter observation was noticed for xenon, for which the ECDI was observed to be detectable with a notable strength up to the magnetic field value of 25 mT.

Whistler‐Mode Waves Inside Short Large‐Amplitude Magnetic Field Structures: Characteristics and Generation Mechanisms

AbstractShort large‐amplitude magnetic structures (SLAMS) frequently appear near the bow shock. Inside steepening SLAMS, whistler‐mode waves are coherently generated at their leading edges. These waves are crucial for electron dynamics, energy conversion and magnetic reconnection near the shock. Nevertheless, the characteristics and generation of the whistler‐mode waves inside SLAMS are still unclear. In this study, we conducted a statistical analysis of whistler‐mode waves within SLAMS near the Earth's bow shock to investigate their properties and generation mechanisms. We found that these waves are mainly excited by electron temperature anisotropy with 99% of them falling below the nonlinear resonant threshold. The combination of a low background magnetic field and intense steepening at SLAMS' leading edge makes it the main source region of whistler‐mode waves and may induce the nonlinear resonance of whistler‐mode waves.

Impact of external magnetic field on temperature anisotropy driven Weibel instability in a magnetized plasma

In this paper, we investigate the impact of an external magnetic field on the Weibel instability driven by temperature anisotropy in a magnetized plasma. The study provides the influence of electron temperature anisotropy, considering both cold as well as warm ions. We derive expressions for the growth rate Γ (s−1) in each case to quantify the effect of some specific parameters on the instability. For example, the growth rate is plotted against the magnetic field for three distinct cases: considering only electrons, including cold ions, and incorporating warm ions. It is observed that the growth rate decreases as the applied magnetic field increases. Furthermore, the addition of both cold and warm ions to the plasma significantly influences the observed effects. It can be explained as follows: in the presence of cold ions, the growth rate is decreased with an increase in the wave number kz (cm−1). In addition, a decrease in the growth rate is observed with an increasing wave number in the case of warm ions. Therefore, it can be seen that the Weibel instability, propelled by thermal anisotropy, holds significant implications owing to its prevalence in both astrophysical and laboratory plasmas. The findings presented here are expected to contribute to the advancements in magnetic field generation and particle acceleration in various astrophysical phenomena, including gamma-ray bursts, relativistic collisionless shock fronts, radio supernovae, and pulsar winds.

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.

Influence of electron temperature anisotropy on the <i>m</i> = 1 helicon mode power deposition characteristic

As a core phenomenon in helicon discharge, the plasma temperature anisotropy may play a crucial role in helicon wave power deposition. Under radially inhomogeneous plasma circumstances, by employing the warm plasma dielectric tensor model and considering the finite Larmor radius (FLR) effect and plasma temperature anisotropy effect, under the typical helicon discharge parameter conditions, the helicon wave and Trivelpiece-Gould (TG) wave mode coupling characteristic and influence of electron temperature anisotropy on the helicon wave power deposition induced by collisional and Landau damping mechanism are theoretically investigated. Detailed analysis shows that for typical helicon plasma electron temperature <i>T</i><sub>e </sub>= 3 eV and low magnetic field <i>B</i><sub>0</sub> = 48 G, the electron FLR effect should be considered, while the ion FLR effect can be ignored due to its large inertia effect; compared with the <inline-formula><tex-math id="Z-20240326171950">\begin{document}$| n | < 2 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20231759_Z-20240326171950.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20231759_Z-20240326171950.png"/></alternatives></inline-formula> cyclotron harmonics, the contribution of the <inline-formula><tex-math id="Z-20240326171926">\begin{document}$| n | > 1 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20231759_Z-20240326171926.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20231759_Z-20240326171926.png"/></alternatives></inline-formula> harmonics in the calculation of plasma dielectric tensor elements can be ignored due to low magnetic field conditions. For the propagation constant, detailed investigation indicates that the phase constant has a maximum value at a certain radial position, near the same position mode coupling between helicon wave and TG wave happens. Full analysis shows that the power deposition of the <i>m</i> = 1 helicon mode peaks at a certain radial position and increases gradually with the increase of the axial electron temperature. Besides, compared with the Landau damping, the collisional damping plays a dominant role in the power deposition under current parameter conditions; importantly, the electron temperature anisotropy exerts a significant influence on the power deposition characteristic, both the increase and decrease of electron temperature anisotropy factor (<i>χ</i> = <i>T</i><sub>e<i>,</i>⊥</sub>/<i>T</i><sub>e,<i>z</i></sub>) can lead the power deposition intensity to change drastically. All these conclusions are very important for us to understand the discharge mechanism of helicon plasma.

Particle-in-Cell Simulations of Sunward and Anti-sunward Whistler Waves in the Solar Wind

We present particle-in-cell simulations of a combined whistler heat flux and temperature anisotropy instability that is potentially operating in the solar wind. The simulations are performed in a uniform plasma and initialized with core and halo electron populations typical of the solar wind beyond about 0.3 au. We demonstrate that the instability produces whistler-mode waves propagating both along (anti-sunward) and opposite (sunward) to the electron heat flux. The saturated amplitudes of both sunward and anti-sunward whistler waves are strongly correlated with their initial linear growth rates, , where for typical electron betas we have 0.6 ≲ ν ≲ 0.9. We show that because of the relatively large spectral width of the whistler waves, the instability saturates through the formation of quasi-linear plateaus around the resonant velocities. The revealed correlations of whistler wave amplitudes and spectral widths with electron beta and temperature anisotropy are consistent with solar wind observations. We show that anti-sunward whistler waves result in an electron heat flux decrease, while sunward whistler waves actually lead to an electron heat flux increase. The net effect is the electron heat flux suppression, whose efficiency is larger for larger electron betas and temperature anisotropies. The electron heat flux suppression can be up to 10%–60% provided that the saturated whistler wave amplitudes exceed about 1% of the background magnetic field. The experimental applications of the presented results are discussed.

On a plasma sheath with a small normal magnetic field separating regions of oppositely directed magnetic field

The Harris-sheet model provides an elegant solution to the kinetic plasma equation for a steady state 1D current sheet geometry separating regions with oppositely directed magnetic field. However, adding just a small normal magnetic field to the Harris configuration yields thermal streaming of particles into and out of the current sheet, fundamentally changing the form of its kinetic description. The action variable, Jz, associated with the oscillatory orbit motion perpendicular to the current sheet is well conserved and can be applied for solving the kinetic equation in the 1D sheet geometry that includes a small normal magnetic field. Revisiting this problem, we develop a new formalism that permits numerical solutions to be readily obtained for general upstream/asymptotic electron and ion distributions. In particular, we consider the case of isotropic ion pressure and anisotropic bi-Maxwellian electrons. The current sheets are then supported by electron pressure anisotropy. Furthermore, the total current across a particular sheet is set by the fire-hose condition based on the electron pressures normalized by the asymptotic magnetic field pressure. Analytical approximations are obtained for the numerical solutions expressed in terms of the asymptotic electron temperature anisotropy and the ion temperature. We discuss a preliminary application of the framework to the electron diffusion region of anti-parallel magnetic reconnection.

Generation Mechanisms of Hiss‐Like Chorus in the Earth's Magnetosphere

AbstractWhistler mode chorus waves appear either as a series of discrete elements (rising or falling tones) or as banded hiss‐like emissions in the dynamic spectrogram. Although the generation of rising‐tone chorus waves has been extensively studied, few studies have focused on the generation of hiss‐like emissions. In this paper, using the one‐dimensional (1‐D) general curvilinear particle‐in‐cell (gcPIC) simulation model, we have thoroughly studied the generation mechanisms of hiss‐like emissions. Based on the spectrogram, we first classify hiss‐like emissions into four types (I, II, III, and IV). We find that the type I hiss‐like waves are linearly excited by the anisotropic electrons, while other types are excited through the nonlinear process. For the type I waves, they do not undergo a frequency chirping process and have a relatively broadband spectrum. The high proportion of hot electrons or the large temperature anisotropy of hot electrons will decrease the threshold for nonlinear growth and cause the wave amplitudes to exceed the threshold, leading to the nonlinear generation of type II hiss‐like waves. The fresh hot electrons continually injected into the system with a large drift velocity significantly reduces the time interval between the rising tone elements to form a continuous spectrum, which are the type III waves. The intense and long‐lasting hiss‐like emissions, that is, type IV, which are the most frequently observed, should be generated due to the combined effect of the rapid injection and the high proportion of hot electrons (or the large temperature anisotropy). Our findings provide a comprehensive understanding of the generation of hiss‐like chorus waves in the Earth's magnetosphere.

Using Direct Laboratory Measurements of Electron Temperature Anisotropy to Identify the Heating Mechanism in Electron-Only Guide Field Magnetic Reconnection.

Anisotropic electron heating during electron-only magnetic reconnection with a large guide magnetic field is directly measured in a laboratory plasma through insitu measurements of electron velocity distribution functions. Electron heating preferentially parallel to the magnetic field is localized to one separatrix, and anisotropies of 1.5 are measured. The mechanism for electron energization is identified as the parallel reconnection electric field because of the anisotropic nature of the heating and spatial localization. These characteristics are reproduced in a 2D particle-in-cell simulation and are also consistent with numerous magnetosheath observations. A measured increase in the perpendicular temperature along both separatrices is not reproduced by our 2D simulations. This work has implications for energy partition studies in magnetosheath and laboratory reconnection.

Whistler heat flux instability governed interaction of anisotropic beam electrons in electromagnetic Vlasov simulations

The kinetic instability of whistlers in a warm plasma, arising from electron temperature anisotropy with respect to directions parallel and perpendicular to the magnetizing field, is studied. Whistlers resonantly interacting with the electron beams, for example, the fast electrons accelerated by strong parallel electric fields and the so-called runaway electrons in a tokamak, are strong players in the schema of thermalization of stellar winds and mitigation of fast electrons in tokamak disruption events. As an evidence of their role in runaway mitigation, most fusion plasma experiments are found to show a threshold magnetic field strength for the generation of runaways. In many of these examples, the faster primary runaways produce a secondary runaway beam having an avalanche-like non-thermal velocity distribution. The electromagnetic Vlasov simulations presented here self-consistently examine the collisionless interaction of anisotropic electron beams, including an avalanche-like beam distribution, with parallel propagating whistlers and dependence of this process on the magnetic field strength. Analysis of the interaction process includes comparison with the simulations done using more analytically accessible anisotropic bulk and beam electron distributions, namely, the bi-Maxwellian and bi-kappa, for the reference.

Simultaneous Observation of Whistler‐Mode Chorus and Fast Magnetosonic Waves During the Magnetic Peak in the Inner Magnetosphere

AbstractSimultaneous observations of whistler‐mode chorus and magnetosonic (MS) waves are reported within a magnetic peak in the inner magnetosphere for the first time. During the magnetic peak, Van Allen Probe A observes flux enhancements of both ring current electrons and relativistic electrons, while medium‐energy proton fluxes are reduced but high‐energy proton fluxes are raised. Those flux variations of ring current particles are almost concentrated in the perpendicular direction of the background magnetic field, leading to enhancements of the electron temperature anisotropy and a positive gradient of proton velocity distributions. The calculated linear growth rates show that unstable electrons with temperature anisotropic instabilities and unstable protons with ring distribution during the magnetic peak structure can provide free energy for the excitation of whistler‐mode chorus and MS waves, respectively. Our findings suggest that injected particles encountering the magnetic peak structure may form more complex unstable electron and ion distributions driving multiple instabilities simultaneously.

Linear analysis of whistler mode instability in anisotropic q-nonextensive distributed plasmas: a numerical approach

Due to the occurrence of nonextensive particle distributions in different space plasma environments, an analytical and numerical studies of electron temperature anisotropy-driven whistler instability (WI) in the nonextensive magnetized plasma are performed within the framework of kinetic theory for both superextensive (q < 1) and subextensive (q > 1) cases. By taking the nonextensive character of electrons into account (distinguished to nearly all existing studies), the dispersion relation for WI is derived and solved numerically to examine the characteristics of growth rate against the allowed domain of wave number. The effect of various physical parameters such as electron plasma beta , temperature anisotropy (Λ e ), and nonextensivity (q) on the growth rate and frequency of whistler mode are investigated. The growth rate of WI displays a significant dependence on these parameters, i.e. the growth rate increases by taking smaller values of nonextensive parameter q and for the large initial electron plasma beta and temperature anisotropy magnitudes. It is observed that the variation in the frequency of whistler mode is more sensitive to the temperature anisotropy (Λ e ) compared with other physical parameters, i.e. plasma beta and nonextensivity. The present findings also validate the large impact of electron temperature anisotropy on altering the dispersion properties of whistler mode. A detailed comparison of parametric dependence of the growth rate of WI for both superextensive and subextensive plasmas is also presented. Furthermore, a comparative analysis of whistler mode instability in anisotropic q-nonextensive and bi-Maxwellian plasmas is also the part of our present findings.

An Implication of Detecting the Internal Modulation in a Pulsating Aurora: A Conjugate Observation by the Arase Satellite and All‐Sky Imagers

AbstractA physical mechanism to produce pulsating aurora (PsA) has been considered to be the interaction of the electron and the chorus wave generated near the equatorial plane of the magnetosphere. A recent observation of high temporal resolution of chorus waves by the Arase satellite revealed that the presence or absence of the internal modulation of PsA, which is a characteristic sub‐second scintillation at 3 ± 1 Hz within each optical pulsation, is closely related to the discreteness of the element structure of the chorus wave. However, it is still unclear what parameters (or conditions) control the discreteness of the element and the existence of the internal modulation of PsA. In this study, we discuss parameters that determine the presence or absence of the internal modulation of PsA and element structure of chorus by showing a conjugate observation of PsA/chorus by ground‐based cameras and the Arase satellite. During the event, the occurrence of internal modulation increased temporally. The wave data from the satellite show that the repetitive frequency of elements was ∼6 Hz when the internal modulation was indistinct, while the repetitive frequency was ∼3 Hz when the internal modulation was distinct. The particle measurements suggest that this difference was caused by changes in the density and the temperature anisotropy of the hot electron. The internal modulation was clearly observed when the density of hot electrons decreased and the temperature anisotropy relaxed after the injection. Observations of internal modulations from the ground might allow us to estimate the parameters such as energetic electron density and temperature anisotropy in the magnetosphere.

Direct Evidence of Electron Acceleration at the Dipolarization Front

The dramatic changes in the magnetic field at the dipolarization front (DF) provide a suitable environment for electron acceleration, which usually can cause the flux enhancement of energetic electrons behind the front. However, it is unknown whether energetic electrons observed at the DF are energized locally, and which mechanism accelerates the electrons at the DF is unclear. Our study performs a direct quantitative analysis to reveal the acceleration process of energetic electrons at the DF using the high-time-resolution data from NASA's Magnetospheric Multiscale mission. The fluxes of energetic electrons at 90° are enhanced at the front. Under adiabatic conditions, our quantitative analysis indicates that these electrons at the front could be locally accelerated to over 100 keV by betatron acceleration. Eventually, the electron temperature anisotropy formed via the betatron mechanism could provide the free energy to excite whistler waves at the DF. Our quantitative study provides, for the first time, strong direct evidence for the local electron acceleration at the DF.

Direct Observations of Electron Firehose Fluctuations in the Magnetic Reconnection Outflow

AbstractElectron temperature anisotropy‐driven instabilities such as the electron firehose instability (EFI) are especially significant in space collisionless plasmas, where collisions are so scarce that wave–particle interactions are the leading mechanisms in the isotropization of the distribution function and energy transfer. Observational statistical studies provided convincing evidence in favor of the EFI constraining the electron distribution function and limiting the electron temperature anisotropy. Magnetic reconnection is characterized by regions of enhanced temperature anisotropy that could drive instabilities—including the electron firehose instability—affecting the particle dynamics and the energy conversion. However, in situ observations of the fluctuations generated by the EFI are still lacking and the interplay between magnetic reconnection and EFI is still largely unknown. In this study, we use high‐resolution in situ measurements by the Magnetospheric Multiscale spacecraft to identify and investigate EFI fluctuations in the magnetic reconnection exhaust in the Earth's magnetotail. We find that the wave properties of the observed fluctuations largely agree with theoretical predictions of the non‐propagating EF mode. These findings are further supported by comparison with the linear kinetic dispersion relation. Our results demonstrate that the magnetic reconnection outflow can be the seedbed of EFI and provide the first direct in situ observations of EFI‐generated fluctuations.

Parametric analysis of heat flux inhibition in the solar wind: a macroscopic quasilinear approach

Abstract Magnitudes of electron temperature anisotropy and solar wind heat flux are defined with different physical mechanisms e.g. microinstabilities, interparticle collisions, and adiabatic expansion. In the dilute space plasma limit, the present study assumes the interplay between anisotropic core-halo electron components, their relative drift, and relative density of the halo electrons to determine the dynamics of backward and forward-propagating whistler heat flux instabilities along the ambient magnetic field. To investigate the feedback effects of these micro-instabilities in reshaping solar wind distributions and the total heat flux regulation, we formulate quasilinear kinetic equations on the basis of taking the macroscopic velocity moments. For the same input parameters of linear analysis, numerical solutions of the quasilinear equations indicate the time-scale variations, electrons and protons population, wave intensities, and constraints on the heat flux. In future perspective of the global-kinetic solar wind model, the present formalism may be an important step with the inclusion of radial and nonthermal effects.

Influence of Solar Wind Dynamic Pressure on Distribution of Whistler Mode Waves Based on Van Allen Probe Observations

AbstractIn this study, the effect of solar wind dynamic pressure (PSW) on the global distribution of whistler mode waves (including chorus and hiss waves) while the absence of substorm activity is investigated using the data originating from the Van Allen Probes for nearly 5 years. After the effect of substorm activity is excluded, our statistical results indicate that the hiss and chorus waves exhibit different characteristics with the increase of PSW. The amplitudes of chorus waves increase on the dayside with the PSW strengthen, which can be explained through the local enhanced electron temperature anisotropy and more intense emission for the chorus in the inner magnetosphere. Interestingly, the amplitudes of hiss wave are reduced with the increase of PSW. The weakness of hiss waves can be attributed to electrons at high L‐shells which drift out of the magnetosphere on the dayside when the magnetopause's position decreases. It can significantly result in a reduced chorus inside the magnetopause that will evolve into hiss waves. In addition, the upper and lower cutoff frequencies of hiss waves will increase. This new finding is critical for understanding the generation and evolution of whistler mode waves and its responses to the intensity of PSW.