Patrick Astfalk

A linear dispersion relation for the hybrid kinetic-ion/fluid-electron model of plasma physics

A dispersion relation for a commonly used hybrid model of plasma physics is developed, which combines fully kinetic ions and a massless-electron fluid description. Although this model and variations of it have been used to describe plasma phenomena for about 40 years, to date there exists no general dispersion relation to describe the linear wave physics contained in the model. Previous efforts along these lines are extended here to retain arbitrary wave propagation angles, temperature anisotropy effects, as well as additional terms in the generalized Ohms law which determines the electric field. A numerical solver for the dispersion relation is developed, and linear wave physics is benchmarked against solutions of a full Vlasov-Maxwell dispersion relation solver. This work opens the door to a more accurate interpretation of existing and future wave and turbulence simulations using this type of hybrid model.

Comparative study of gyrokinetic, hybrid-kinetic and fully kinetic wave physics for space plasmas

A set of numerical solvers for the linear dispersion relations of the gyrokinetic (GK), the hybrid-kinetic (HK), and the fully kinetic (FK) model is employed to study the physics of the KAW and the fast magnetosonic mode in these models. In particular, we focus on parameters that are relevant for solar wind oriented applications (using a homogeneous, isotropic background), which are characterized by wave propagation angles averaging close to 90°. It is found that the GK model, while lacking high-frequency solutions and cyclotron effects, faithfully reproduces the FK wave physics close to, and sometimes significantly beyond, the boundaries of its range of validity. The HK model, on the other hand, is much more complete in terms of high-frequency waves, but owing to its simple electron model it is found to severely underpredict wave damping rates even on ion spatial scales across a large range of parameters, despite containing full kinetic ion physics.

Stability analysis of core–strahl electron distributions 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. We find that for typical solar wind conditions, the core-strahl distribution is unstable to the kinetic Alfven and magnetosonic modes. The maximum growth rates for these instabilities occur at wavenumbers

DSHARK: A dispersion relation solver for obliquely propagating waves in bi-kappa-distributed plasmas

k d_i \lesssim 1

LEOPARD: A grid‐based dispersion relation solver for arbitrary gyrotropic distributions

, at moderately oblique angles of propagation, thus providing a potential source of kinetic-scale turbulence. In contrast with previous reports, we however 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. We therefore suggest that the whistler modes often invoked to explain anomalous scattering of strahl particles could appear as a result of nonlinear mode coupling and turbulent cascade originating at scales

Parallel and oblique firehose instability thresholds for bi‐kappa distributed protons

k d_i \lesssim 1

Stimulated Mirror Instability From the Interplay of Anisotropic Protons and Electrons, and their Suprathermal Populations

.