https://escholarship.org/content/qt1xm794rm/qt1xm794rm.pdf?t=p2njav

Skyrmions in magnetic multilayers

The solar wind is a magnetized plasma and as such exhibits collective plasma behavior associated with its characteristic spatial and temporal scales. The characteristic length scales include the size of the heliosphere, the collisional mean free paths of all species, their inertial lengths, their gyration radii, and their Debye lengths. The characteristic timescales include the expansion time, the collision times, and the periods associated with gyration, waves, and oscillations. We review the past and present research into the multi-scale nature of the solar wind based on in-situ spacecraft measurements and plasma theory. We emphasize that couplings of processes across scales are important for the global dynamics and thermodynamics of the solar wind. We describe methods to measure in-situ properties of particles and fields. We then discuss the role of expansion effects, non-equilibrium distribution functions, collisions, waves, turbulence, and kinetic microinstabilities for the multi-scale plasma evolution.

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The multi-scale nature of the solar wind

/pdf/the-multi-scale-nature-of-the-solar-wind-56fzt1kp8f.pdf

We report the results of a direct comparison between different kinetic models of collisionless plasma turbulence in two spatial dimensions. The models considered include a first principles fully-kinetic (FK) description, two widely used reduced models [gyrokinetic (GK) and hybrid-kinetic (HK) with fluid electrons], and a novel reduced gyrokinetic approach (KREHM). Two different ion beta ($\\beta_i$) regimes are considered: 0.1 and 0.5. For $\\beta_i=0.5$, good agreement between the GK and FK models is found at scales ranging from the ion to the electron gyroradius, thus providing firm evidence for a kinetic Alfv\\'en cascade scenario. In the same range, the HK model produces shallower spectral slopes, presumably due to the lack of electron Landau damping. For $\\beta_i=0.1$, a detailed analysis of spectral ratios reveals a slight disagreement between the GK and FK descriptions at kinetic scales, even though kinetic Alfv\\'en fluctuations likely still play a significant role. The discrepancy can be traced back to scales above the ion gyroradius, where the FK and HK results seem to suggest the presence of fast magnetosonic and ion Bernstein modes in both plasma beta regimes, but with a more notable deviation from GK in the low-beta case. The identified practical limits and strengths of reduced-kinetic approximations, compared here against the fully-kinetic model on a case-by-case basis, may provide valuable insight into the main kinetic effects at play in turbulent collisionless plasmas, such as the solar wind.

/pdf/fully-kinetic-versus-reduced-kinetic-modeling-of-1intx1o3qy.pdf

Fully Kinetic versus Reduced-kinetic Modeling of Collisionless Plasma Turbulence

The present paper comprises a review of kinetic instabilities that may be operative in the solar wind, and how they influence the dynamics thereof. The review is limited to collective plasma instabilities driven by the temperature anisotropies. To limit the scope even further, the discussion is restricted to the temperature anisotropy-driven instabilities within the model of bi-Maxwellian plasma velocity distribution function. The effects of multiple particle species or the influence of field-aligned drift will not be included. The field-aligned drift or beam is particularly prominent for the solar wind electrons, and thus ignoring its effect leaves out a vast portion of important physics. Nevertheless, for the sake of limiting the scope, this effect will not be discussed. The exposition is within the context of linear and quasilinear Vlasov kinetic theories. The discussion does not cover either computer simulations or data analyses of observations, in any systematic manner, although references will be made to published works pertaining to these methods. The scientific rationale for the present analysis is that the anisotropic temperatures associated with charged particles are pervasively detected in the solar wind, and it is one of the key contemporary scientific research topics to correctly characterize how such anisotropies are generated, maintained, and regulated in the solar wind. The present article aims to provide an up-to-date theoretical development on this research topic, largely based on the author’s own work.

Kinetic instabilities in the solar wind driven by temperature anisotropies

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.

/pdf/dshark-a-dispersion-relation-solver-for-obliquely-313a7fg1zr.pdf

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

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 $k d_i \lesssim 1$, 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 $k d_i \lesssim 1$.

https://pure.mpg.de/pubman/item/item_3011596_3/component/file_3015377/Horaites_Stability.pdf

Stability analysis of core-strahl electron distributions in the solar wind

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

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

/pdf/leopard-a-grid-based-dispersion-relation-solver-for-9smiejexld.pdf

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

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017JA025066

Patrick Astfalk

Papers

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

Stability analysis of core-strahl electron distributions in the solar wind

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

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

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