Satellite measurements suggest that space plasmas often exhibit bi‐kappa particle distributions with high‐energy tails instead of simple Maxwellians. The presence of suprathermal particles significantly alters the plasmas' dispersion properties compared to purely Maxwellian scenarios. In the past, wave propagation in magnetized, bi‐kappa plasmas was almost exclusively addressed for parallel propagating modes only. To enable a systematic study of both parallel and oblique wave propagation, the new kinetic dispersion relation solver Dispersion Solver for Homogeneous Plasmas with Anisotropic Kappa Distributions (DSHARK) was developed and is presented in this work. DSHARK is an iterative root‐finding algorithm which is based on Summers et al. (1994) who derived the dielectric tensor for plasmas with bi‐kappa‐distributed particles. After a brief discussion of kappa distributions, we present the kinetic theory and the numerical methods implemented in DSHARK and verify the code by considering several test cases. Then, we apply DSHARK to the oblique firehose instability to initiate a more extensive work which will be addressed in the future. A systematic investigation of the dispersion properties of bi‐kappa‐distributed plasmas is expected to lead to a deeper understanding of wave propagation and instability growth in the solar wind.

In this work, we analyze the kinetic stability of a solar wind electron distribution composed of core and strahl subpopulations. The core is modeled by a drifting Maxwellian distribution, while the strahl is modeled by an analytic function recently derived in (Horaites et al. 2018) from the collisional kinetic equation. We perform a numerical linear stability analysis using the LEOPARD solver (Astfalk & Jenko 2017), which allows for arbitrary gyrotropic distribution functions in a magnetized plasma. In contrast with previous reports, we do not find evidence for a whistler instability directly associated with the electron strahl. This may be related to the more realistic shape of the electron strahl distribution function adopted in our work, as compared to previous studies. We however find that for typical solar wind conditions, the core-strahl distribution is unstable to the kinetic Alfvén and magnetosonic modes. The maximum growth rates for these instabilities occur at wavenumbers kd i 1 (where d i is the ion inertial length), at moderately oblique angles of propagation, thus providing a potential source of kinetic-scale turbulence. We therefore suggest that if the whistler modes are invoked to explain anomalous scattering of strahl particles, these modes may appear as a result of nonlinear mode coupling and turbulent cascade originating at scales kd i 1.

Mirror instability driven by the temperature anisotropy of protons can offer a plausible explanation for the mirror‐like fluctuations observed in planetary magnetosheaths. In the present paper we invoke a realistic kinetic approach which can reproduce nonthermal features of plasma particles reported by the observations, i.e., temperature anisotropies and suprathermal populations. Seeking accuracy, a numerical analysis is performed using an advanced code named DSHARK, recently proposed to resolve the linear dispersion and stability for an arbitrary propagation in bi‐Kappa distributed electron‐proton plasmas. The stimulating effect of the anisotropic bi‐Maxwellian electrons reported in Remya et al. (2013, https://doi.org/10.1002/jgra.50091) is markedly enhanced in the presence of suprathermal electrons described by the bi‐Kappa distribution functions. The influence of suprathermal protons is more temperate, but overall, present results demonstrate that these sources of free energy provide natural conditions for a stimulated mirror instability, more efficient than predicted before and capable to compete with other instabilities (e.g., the electromagnetic ion‐cyclotron instability) and mechanisms of relaxation.

Particle velocity distributions measured in collisionless space plasmas often show strong deviations from idealized model distributions. Despite this observational evidence, linear wave analysis in space plasma environments such as the solar wind or Earth's magnetosphere is still mainly carried out using dispersion relation solvers based on Maxwellians or other parametric models. To enable a more realistic analysis, we present the new grid‐based kinetic dispersion relation solver LEOPARD (Linear Electromagnetic Oscillations in Plasmas with Arbitrary Rotationally‐symmetric Distributions) which no longer requires prescribed model distributions but allows for arbitrary gyrotropic distribution functions. In this work, we discuss the underlying numerical scheme of the code and we show a few exemplary benchmarks. Furthermore, we demonstrate a first application of LEOPARD to ion distribution data obtained from hybrid simulations. In particular, we show that in the saturation stage of the parallel fire hose instability, the deformation of the initial bi‐Maxwellian distribution invalidates the use of standard dispersion relation solvers. A linear solver based on bi‐Maxwellians predicts further growth even after saturation, while LEOPARD correctly indicates vanishing growth rates. We also discuss how this complies with former studies on the validity of quasilinear theory for the resonant fire hose. In the end, we briefly comment on the role of LEOPARD in directly analyzing spacecraft data, and we refer to an upcoming paper which demonstrates a first application of that kind.

We report the first satellite measurement of the ultralow frequency (ULF) wave growth rate in the upstream region of the Earth's bow shock. We employ the two identical ARTEMIS spacecraft orbiting the Moon to characterize crescent‐shaped reflected ion beams and relatively monochromatic ULF waves. The event presented here features spacecraft separation of ∼2.5 Earth radii (0.9 ± 0.1 wavelengths) in the solar wind flow direction along a nearly radial interplanetary magnetic field. The ULF wave growth rate is estimated and found to fall within dispersion solver predictions during the initial growth time. Observed frequencies and wave numbers are also within the predicted range. Other ULF wave properties such as the phase speed, obliquity, and polarization are consistent with expectations from resonant beam instability theory and prior satellite measurements. These results will inform future missions near bow and interplanetary shocks as well as future nonlinear studies related to turbulence and dissipation in the heliosphere.