Pre-acceleration in the Electron Foreshock. II. Oblique Whistler Waves

Abstract

Thermal electrons have gyroradii many orders of magnitude smaller than the finite width of a shock, thus need to be pre-accelerated before they can cross it and be accelerated by diffusive shock acceleration. One region where pre-acceleration may occur is the inner foreshock, which upstream electrons must pass through before any potential downstream crossing. In this paper, we perform a large-scale particle-in-cell simulation that generates a single shock with parameters motivated from supernova remnants. Within the foreshock, reflected electrons excite the oblique whistler instability and produce electromagnetic whistler waves, which comove with the upstream flow and as nonlinear structures eventually reach radii of up to 5 ion-gyroradii. We show that the inner electromagnetic configuration of the whistlers evolves into complex nonlinear structures bound by a strong magnetic field around four times the upstream value. Although these nonlinear structures do not in general interact with cospatial upstream electrons, they resonate with electrons that have been reflected at the shock. We show that they can scatter, or even trap, reflected electrons, confining around 0.8% of the total upstream electron population to the region close to the shock where they can undergo substantial pre-acceleration. This acceleration process is similar to, yet approximately three times more efficient than, stochastic shock drift acceleration.

Electron–Ion Temperature Ratio in Astrophysical Shocks

“…Oblique shocks seem to behave like perpendicular shocks, even in the presence of the electron foreshock generated by the shock-reflected electrons (Morris et al 2023). Lezhnin et al (2021) report T e /T p ∼ 0.5 in a Mach 15 shock at Θ BN = 60°, where Θ BN is the angle between shock normal and the magnetic field.…”

Kinetic Simulations of Nonrelativistic High-mach-number Perpendicular Shocks Propagating in a Turbulent Medium

“…This adds room for particle-turbulence interactions, and hence, changes in the reflection conditions and the electron dynamics, as well as modifications of the instabilities. Moreover, 1D3V (Xu et al 2020;Kumar & Reville 2021) and 2D3V (Morris et al 2023) simulations show that a significant nonthermal electron population can be formed downstream of such shocks, which is well described by a power law. Therefore, at oblique shocks, it is possible to directly investigate the influence of preexisting turbulence on energetic particles.…”

PIC simulations of SNRs shocks with a turbulent upstream medium

“…In this section we present preliminary results from simulations of oblique shocks (see primed runs in Table 2). To investigate the influence of pre-existing density fluctuations, we used similar parameters to ones in [10]. The magnetic field has an out-of-plane configuration: B 0 = 𝐵 0 • (cos 𝜃 Bn , cos 𝜃 Bn 𝜙, sin 𝜃 Bn sin 𝜙), where 𝜃 Bn = 60 • and 𝜙 = 90 • .…”

“…This approach allows to study plasmas from first principles and it is commonly employed to examine shocks in various astrophysical environments [4]. By means of PIC simulations, many studies investigated perpendicular shocks (𝜃 Bn = 90 • ), and recently the oblique ones (𝜃 Bn ≈ 50 • − 75 • ) [5][6][7][8][9][10], where 𝜃 Bn is the angle between the external magnetic field and the shock normal.…”