Plasma Oscillations and Kinetic Instabilities at Ion Scales in the Expanding Magnetosheath

Abstract

We investigate a relationship between a decaying spectrum of magnetic field perturbations and kinetic instabilities in a slowly expanding magnetosheath plasma using two-dimensional hybrid expanding simulations in (X, Y) space. We impose an initial ambient magnetic field B ₀/B ₀ = (1, 0, 0), and superimpose a flat spectrum of Alfvénic fluctuations with average amplitudes equal to δ B = ∣B - B ₀∣ ~ 0.2B ₀. The expansion leads to the development of temperature anisotropy A _p = T _p⊥/T _p∥ > 1 (T _p⊥, T _p∥ are plasma temperatures perpendicular and parallel with respect to B ₀, respectively) and the system becomes unstable with respect to the ion cyclotron and mirror instabilities. The onset of mirror instability in the early stage of the simulation leads to the development of filaments that fill the simulation volume. The filaments represent pressure-balanced structures as they develop in the form of magnetic field (δ B) perturbations balanced with corresponding density perturbations. Further expansion of the system leads to the thinning of these filaments, which gradually become extremely narrow. The filamentation maintains the power spectral density in δ B _⊥ at ${P}_{{{\boldsymbol{B}}}_{\perp }}\sim {10}^{-3}{B}_{0}^{2}{d}_{p0}$ (here d _p0 is the proton inertial length) over the range 0.2 < kd _p0 < 1.1. The filaments remain well developed in the form of density, magnetic field, and temperature anisotropy perturbations at t ~ 10,000/Ω _p0 (here Ω _p0 is the initial proton gyrofrequency) and slowly dissipate when the expansion continues further.