Temperature Anisotropy Instabilities Research Articles

The excitation of whistler waves and wave-particle interaction in magnetic reconnection

A 2.5D PIC kinetic code is executed to simulate the excitation of electron whistlers and ion cyclotron whistler (L wave) during magnetic reconnection. Our simulation results suggest that quasi-parallel and anti-parallel whistler waves can be excited in the separatrix region near X-Line and in the outflow region, respectively. Electron whistlers play a crucial role in energy release and transfer, as the small mass of electrons allows them to move rapidly under the perturbation of a magnetic field during magnetic reconnection. The L wave excited near the separatrix region far from X-Line, it propagate obliquely to the local magnetic field. Velocity distribution functions (VDFs) indicate that these waves are associated with temperature anisotropy instabilities of electron-beams (ion-beam). The electron whistler in the separatrix region is excited by cyclotron resonance due to the electron beam. The electron whistler observed in the outflow region is generated by Landau resonance owing to beam-mode, where the v∥ is close to the phase speed vph of the wave. Regarding L wave, their excitation mechanism is attributed to significant perpendicular temperature anisotropy instability (Ti⊥>Ti∥) of ion-beam, and satisfies cyclotron resonance condition. Meanwhile, our simulation results illustrate that the existence of energy conversion between particles and waves during the wave excitation by using the Wave-Particle Interaction Analyzer (WPIA).

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.

Statistical Study of the Energetic Electron Microinjections at the High‐Latitude Magnetosphere

AbstractUnderstanding the formation of the seed population for the energetic electrons trapped within the Earth's Van Allen radiation belts has been under debate for decades. The magnetic reconnection in the Earth's magnetotail during the substorms is the main process of accelerating the electrons to the tens to hundreds of keV. These electrons are further injected toward the radiation belts, where they get further accelerated to relativistic energies. Recently, it has been suggested that another source could come from the dayside diamagnetic cavities where electrons and ions can be locally energized to hundreds of keV energies. It has been shown that the physical mechanism within the cavities can create a strong acceleration perpendicular to magnetic field, which can lead to temperature anisotropy and drift mirror instability. The electron fluxes localized within the troughs of the mirror mode waves exhibit the counter‐streaming “microinjection” signature. To investigate the origin of microinjections and their dependence on solar wind conditions, here we have performed an event search and a statistical study of their properties encompassing a total of ∼165 hr (47 microinjection events) of Magnetospheric Multiscale observations at the pre‐dusk sector high‐latitude boundary layer. The ultralow frequency range magnetic field fluctuations coincided with the counter‐streaming energetic electron fluxes. For most events, the interplanetary magnetic field was duskward and anti‐sunward; over 60% of these microinjections satisfy the criteria of the drift mirror instability, which indicates the temperature anisotropy could play an important role for the microinjection.

Instabilities Driven by Proton Temperature Anisotropy in the Presence of Alpha Particles: Implications for Proton-temperature-anisotropy Constraint in the Solar Wind

In situ measurements reveal that proton temperature anisotropy is ubiquitous in the solar wind. Various plasma instabilities have been proposed to regulate the distribution of the proton temperature anisotropy in the solar wind; detailed constraint processes are still unclear. In this paper, we study the effects of alpha beams on both the forward and backward proton temperature anisotropy instabilities at parallel and oblique propagation with the Vlasov theory, and compare the theoretical results with the Wind observation. As the alpha-beam drift velocity v α /v A increases, the growth rates of forward Alfvén/ion-cyclotron (FA/IC) and backward magnetosonic/whistler (BM/W) instabilities increase, those of backward Alfvén/ion-cyclotron (BA/IC) and forward magnetosonic/whistler (FM/W) instabilities decrease, and those of the mirror and forward Alfvén wave (FAW) instabilities are nearly constant. In particular, there are different constraining mechanisms on the distribution of proton temperature anisotropy for different values of the alpha-beam drift velocity. The proton temperature anisotropy instability together with the alpha beam can provide a potential explanation for the distribution of the proton temperature anisotropy in the solar wind.

Distribution of alpha temperature anisotropy in the slow and fast solar wind: WIND observations and Vlasov theory

Context. Alpha particles in the solar wind frequently show temperature anisotropies, which can be constrained by various kinetic instabilities. However, the detailed constraint mechanism remains unclear, especially in different types of solar wind. Aims. Using data obtained by the Wind spacecraft between 2005 and 2015, we investigate the distribution of alpha temperature anisotropy and the correlation with background plasma parameters in the slow and fast solar wind. Methods. We present a comparison between observations of the alpha temperature anisotropy by the Wind spacecraft and theoretical results obtained with the linear Vlasov theory. Results. The results show that the distribution of the alpha temperature anisotropy depends not only on the parallel temperature ratio of alpha to proton Tα∥/Tp∥ and the proton temperature anisotropy Tp⊥/Tp∥, but also on the drift velocity of alpha to proton vα/vA and the solar wind velocity vsw. In particular, there are clearly different constraining mechanisms on the distribution of alpha temperature anisotropy in the slow and fast solar wind. Conclusions. The alpha temperature anisotropy instability together with different types of free energy can be effective for regulating the alpha temperature anisotropy in the slow and fast solar wind.

Magnetic Helicity Associated with the Proton Temperature Anisotropy Instabilities in the Presence of an Imbalanced Solar Wind Turbulence

Some of the most common processes in the solar wind, such as turbulence and wave generation by instabilities, are associated with spectral magnetic helicity. Therefore, the helicity is a convenient tool to investigate these processes. We use three-dimensional nonlinear kinetic simulations with particle ions and fluid electrons to analyze the magnetic helicity produced by proton temperature anisotropy instabilities coexisting with an ambient turbulence. The symmetry of the unstable system is violated by alpha-particle streaming with respect to protons along the mean magnetic field. At the same time, the turbulent fluctuations are also imbalanced by a nonzero cross-helicity. We show that in the nonlinear phase of the instability the resulting helicity structure is different from the prediction of the linear theory. In particular, it contains sign reversals and multiple domains of nonzero helicity. The turbulence generates its own magnetic helicity signature, which extends over a wide range of angles around the direction perpendicular to the mean magnetic field, and can have a sign the same as or opposite to that of the instability. These findings are consistent with the observed helicity spectra in the solar wind.

Temperature Anisotropy Instabilities Stimulated by the Solar Wind Suprathermal Populations

This review paper compiles recent results obtained by the present group of authors describing the effects of suprathermal populations present in space plasmas (up to a few keVs) on temperature anisotropy instabilities. Of particular interest are the electromagnetic cyclotron and firehose excitations, which play a major role in limiting temperature anisotropy, resulting, for instance, from the adiabatic expansion of the solar wind. Relying on a rigorous modeling and interpretation of the observed velocity distributions, both theoretical models and numerical simulations indicate a systematic stimulation of these excitations in the presence of suprathermal populations of electrons or protons. Moreover, the enhanced fluctuations react back on particles, and determine a faster and deeper relaxation of their anisotropy. The present comparative analysis suggests that previous studies, considering only quasi-thermal low-energy populations, may have significantly underestimated these excitations and their implications in various applications in space plasmas.

Stochastic Electron Acceleration by Temperature Anisotropy Instabilities under Solar Flare Plasma Conditions

Abstract Using 2D particle-in-cell plasma simulations, we study electron acceleration by temperature anisotropy instabilities, assuming conditions typical of above-the-loop-top sources in solar flares. We focus on the long-term effect of T e,⊥ > T e,∥ instabilities by driving the anisotropy growth during the entire simulation time through imposing a shearing or a compressing plasma velocity (T e,⊥ and T e,∥ are the temperatures perpendicular and parallel to the magnetic field). This magnetic growth makes T e,⊥/T e,∥ grow due to electron magnetic moment conservation, and amplifies the ratio ω ce/ω pe from ∼0.53 to ∼2 (ω ce and ω pe are the electron cyclotron and plasma frequencies, respectively). In the regime ω ce/ω pe ≲ 1.2–1.7, the instability is dominated by oblique, quasi-electrostatic modes, and the acceleration is inefficient. When ω ce/ω pe has grown to ω ce/ω pe ≳ 1.2–1.7, electrons are efficiently accelerated by the inelastic scattering provided by unstable parallel, electromagnetic z modes. After ω ce/ω pe reaches ∼2, the electron energy spectra show nonthermal tails that differ between the shearing and compressing cases. In the shearing case, the tail resembles a power law of index α s ∼ 2.9 plus a high-energy bump reaching ∼300 keV. In the compressing runs, α s ∼ 3.7 with a spectral break above ∼500 keV. This difference can be explained by the different temperature evolutions in these two types of simulations, suggesting that a critical role is played by the type of anisotropy driving, ω ce/ω pe, and the electron temperature in the efficiency of the acceleration.

Intermittency and Ion Temperature–Anisotropy Instabilities: Simulation and Magnetosheath Observation

Abstract Weakly collisional space plasmas are rarely in local thermal equilibrium and often exhibit non-Maxwellian electron and ion velocity distributions that lead to the growth of microinstabilities—that is, enhanced electric and magnetic fields at relatively short wavelengths. These instabilities play an active role in the evolution of space plasmas, as does ubiquitous broadband turbulence induced by turbulent structures. This study compares certain properties of a 2.5-dimensional particle-in-cell (PIC) simulation for the forward cascade of Alfvénic turbulence in a collisionless plasma against the same properties of turbulence observed by the Magnetospheric Multiscale Mission spacecraft in the terrestrial magnetosheath. The PIC simulation is of decaying turbulence that develops both coherent structures and anisotropic ion velocity distributions with the potential to drive kinetic scale instabilities. The uniform background magnetic field points perpendicular to the plane of the simulation. Growth rates are computed from linear theory using the ion temperature anisotropies and ion beta values for both the simulation and the observations. Both the simulation and the observations show that strong anisotropies and growth rates occur highly intermittently in the plasma, and the simulation further shows that such anisotropies preferentially occur near current sheets. This suggests that, though microinstabilities may affect the plasma globally, they act locally and develop in response to extreme temperature anisotropies generated by turbulent structures. Further studies will be necessary to understand why there is an apparent correlation between linear instability theory and strongly intermittent turbulence.

Combined Whistler Heat Flux and Anisotropy Instabilities in Solar Wind

AbstractWhistler heat flux and electron temperature anisotropy instabilities are spontaneously generated in the expanding solar wind. The present study investigates the interplay between the two unstable modes and how each contributes to the dynamical evolution of macroscopic quantities. Whistler heat flux instability is excited in the forward direction along the heat flow, whereas anisotropy instability propagates in both backward and forward directions. Velocity moment‐based quasi‐linear theory is employed in order to investigate the saturation behavior of these instabilities, and it is shown that under certain conditions, the forward propagating whistler heat flux instability dominates, while for other conditions, especially when the temperature anisotropies are sufficiently high, it is shown that the backward mode may also contribute to the dynamical process, and even dominate. These saturation behaviors cannot be predicted by linear growth rate alone and that they imply that in the global‐kinetic models of the solar wind, bidirectional nature of these instabilities must be carefully taken into account.

Parallel-propagating Fluctuations at Proton-kinetic Scales in the Solar Wind Are Dominated By Kinetic Instabilities

Abstract We use magnetic helicity to characterize solar wind fluctuations at proton-kinetic scales from Wind observations. For the first time, we separate the contributions to helicity from fluctuations propagating at angles quasi-parallel and oblique to the local mean magnetic field, . We find that the helicity of quasi-parallel fluctuations is consistent with Alfvén-ion cyclotron and fast magnetosonic-whistler modes driven by proton temperature anisotropy instabilities and the presence of a relative drift between α-particles and protons. We also find that the helicity of oblique fluctuations has little dependence on proton temperature anisotropy and is consistent with fluctuations from the anisotropic turbulent cascade. Our results show that parallel-propagating fluctuations at proton-kinetic scales in the solar wind are dominated by proton temperature anisotropy instabilities and not the turbulent cascade. We also provide evidence that the behavior of fluctuations at these scales is independent of the origin and macroscopic properties of the solar wind.

Effects of Alpha–Proton Differential Flow on Proton Temperature Anisotropy Instabilities in the Solar Wind: Wind Observations

Abstract Plasma kinetic waves and alpha–proton differential flow are two important subjects on the topic of solar wind evolution. Based on the Wind data during 2005–2015, this paper reports that the occurrence of electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency significantly depends on the direction of alpha–proton differential flow . As rotates from the anti-Sunward direction to the Sunward direction, the occurrence rate of ECWs as well as the percentage of left-handed (LH) polarized ECWs decreases considerably. In particular, it is shown that the dominant polarization changes from LH polarization to right-handed polarization during the rotation. The investigation on proton and alpha particle parameters ordered by the direction of further illustrates that large kinetic energies of alpha–proton differential flow correspond to high occurrence rates of ECWs. These results are consistent with theoretical predictions for effects of alpha–proton differential flow on proton temperature anisotropy instabilities.

Large‐Amplitude Electromagnetic Ion Cyclotron Waves and Density Fluctuations in the Flank of the Earth's Magnetosheath

AbstractThe electromagnetic ion cyclotron (EMIC) wave usually observed in the Earth's magnetosheath is thought to be generated through the ion temperature anisotropy instability. This paper presents an observation of a long‐lasting large‐amplitude EMIC wave event in the dawnside flank of the magnetosheath by the Magnetospheric Multiscale mission, lasting from 06:33:00 to 12:35:00 UT on 16 April 2018. The wave amplitude is around 0.2 nT as compared to the ambient magnetic field ~12 nT. The characteristic frequency and scale size are around 0.2 Hz and 1,028 km, respectively. Accompanying EMIC waves are density fluctuations, which exhibit both positive and negative correlations with the longitudinal magnetic field. Using the fitted parameters for the ion and electron phase space densities, plasma kinetic theory predicts local excitations of both EMIC and mirror instabilities, which provide the energy of these observed EMIC waves and density fluctuations.

Effects of Thermal Fluctuations on Temperature Anisotropy Instabilities in the Solar Wind

AbstractThe proton temperature anisotropy in the solar wind, when plotted as a two‐dimensional histogram in (β∥,T⊥/T∥) phase space, shows that the bulk of the data are broadly distributed in quasi‐isotropic state, while the fringe of data distribution is apparently regulated by various marginal stability conditions. The physical origin of magnetic field fluctuations in the stable regime is an open question. Among plausible mechanisms is the spontaneous thermal emission process. Discrete particle effects are central to the theory of spontaneous emission and reabsorption of electromagnetic fluctuations in plasmas. The present paper formulates the quasi‐linear theory of temperature anisotropy‐driven instabilities in which discrete particle effects are taken into account. The hybrid simulation is also carried out with varying number of superparticles per cell in order to unveil the underlying relationship between the theoretical and simulated discrete particle effects. The present paper demonstrates that the inherent numerical noise in the particle simulation is not always undesirable but may actually be exploited in order to control the degree of particle discreteness. Such a finding may have a significant implication for simulating the solar wind plasmas.

Temperature anisotropy instabilities stimulated by the interplay of the core and halo electrons in space plasmas

Two central components are revealed by electron velocity distributions measured in space plasmas, a thermal bi-Maxwellian core and a bi-Kappa suprathermal halo. A new kinetic approach is proposed to characterize the temperature anisotropy instabilities driven by the interplay of core and halo electrons. Suggested by the observations in the solar wind, direct correlations of these two populations are introduced as co-variations of the key parameters, e.g., densities, temperature anisotropies, and (parallel) plasma betas. The approach involving correlations enables the instability characterization in terms of either the core or halo parameters and a comparative analysis to depict mutual effects. In the present paper, the instability conditions are described for an extended range of plasma beta parameters, making the new dual approach relevant for a wide variety of space plasmas, including the solar wind and planetary magnetospheres.

Applying Nyquist's method for stability determination to solar wind observations

AbstractThe role instabilities play in governing the evolution of solar and astrophysical plasmas is a matter of considerable scientific interest. The large number of sources of free energy accessible to such nearly collisionless plasmas makes general modeling of unstable behavior, accounting for the temperatures, densities, anisotropies, and relative drifts of a large number of populations, analytically difficult. We therefore seek a general method of stability determination that may be automated for future analysis of solar wind observations. This work describes an efficient application of the Nyquist instability method to the Vlasov dispersion relation appropriate for hot, collisionless, magnetized plasmas, including the solar wind. The algorithm recovers the familiar proton temperature anisotropy instabilities, as well as instabilities that had been previously identified using fits extracted from in situ observations in Gary et al. (2016). Future proposed applications of this method are discussed.

Proton fire hose instabilities in the expanding solar wind

Using two-dimensional hybrid expanding box simulations we study the competition between the continuously driven parallel proton temperature anisotropy and fire hose instabilities in collisionless homogeneous plasmas. For a quasi-radial ambient magnetic field the expansion drives $T_{p\Vert }>T_{p\bot }$ and the system becomes eventually unstable with respect to the dominant parallel fire hose instability. This instability is generally unable to counteract the induced anisotropization and the system typically becomes unstable with respect to the oblique fire hose instability later on. The oblique instability efficiently reduces the anisotropy and the system rapidly stabilizes, while a significant part of the generated electromagnetic fluctuations is damped to protons. As long as the magnetic field is in the quasi-radial direction, this evolution repeats itself and the electromagnetic fluctuations accumulate. For a sufficiently oblique magnetic field the expansion drives $T_{p\bot }>T_{p\Vert }$ and brings the system to the stable region with respect to the fire hose instabilities.

The inhomogeneous ion temperature anisotropy instabilities of magnetic-field-aligned plasma sheared flow

The stability of the magnetic field aligned sheared flow with anisotropic ion temperatures, which have the anisotropic spatial inhomogeneities across the magnetic field and are comparable with or are above the electron temperature, is investigated numerically and analytically. The ion temperatures gradients across the magnetic field affect the instability development only when the inhomogeneous is the ion temperature along the magnetic field irrespective the inhomogeneity of the ion temperature across the magnetic field. In this case, the instability is developed due to the combined effect of the ion Landau damping, velocity shear, ion temperature anisotropy, and anisotropy of the ion temperature gradients. In the case when the ion temperature along the magnetic field is homogeneous, but the ion temperature across the magnetic field is inhomogeneous, the short wavelength instability develops with the wave length less than the thermal ion Larmor radius. This instability excites due to the coupled effect of the ion Landau damping, velocity shear and ion temperature anisotropy.

Heliospheric plasma sheet (HPS) impingement onto the magnetosphere as a cause of relativistic electron dropouts (REDs) via coherent EMIC wave scattering with possible consequences for climate change mechanisms

AbstractA new scenario is presented for the cause of magnetospheric relativistic electron decreases (REDs) and potential effects in the atmosphere and on climate. High‐density solar wind heliospheric plasmasheet (HPS) events impinge onto the magnetosphere, compressing it along with remnant noon‐sector outer‐zone magnetospheric ~10‐100 keV protons. The betatron accelerated protons generate coherent electromagnetic ion cyclotron (EMIC) waves through a temperature anisotropy (T⊥/T|| > 1) instability. The waves in turn interact with relativistic electrons and cause the rapid loss of these particles to a small region of the atmosphere. A peak total energy deposition of ~3 × 1020 ergs is derived for the precipitating electrons. Maximum energy deposition and creation of electron‐ion pairs at 30‐50 km and at < 30 km altitude are quantified. We focus the readers' attention on the relevance of this present work to two climate change mechanisms. Wilcox et al. (1973) noted a correlation between solar wind heliospheric current sheet (HCS) crossings and high atmospheric vorticity centers at 300 mb altitude. Tinsley et al. () has constructed a global circuit model which depends on particle precipitation into the atmosphere. Other possible scenarios potentially affecting weather/climate change are also discussed.

Short-wavelength plasma turbulence and temperature anisotropy instabilities: recent computational progress.

Plasma turbulence consists of an ensemble of enhanced, broadband electromagnetic fluctuations, typically driven by multi-wave interactions which transfer energy in wavevector space via non- linear cascade processes. Temperature anisotropy instabilities in collisionless plasmas are driven by quasi-linear wave–particle interactions which transfer particle kinetic energy to field fluctuation energy; the resulting enhanced fluctuations are typically narrowband in wavevector magnitude and direction. Whatever their sources, short-wavelength fluctuations are those at which charged particle kinetic, that is, velocity-space, properties are important; these are generally wavelengths of the order of or shorter than the ion inertial length or the thermal ion gyroradius. The purpose of this review is to summarize and interpret recent computational results concerning short-wavelength plasma turbulence, short-wavelength temperature anisotropy instabilities and relationships between the two phenomena.