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

TL;DR

This study uses large-scale particle-in-cell simulations to investigate how oblique whistler waves in the electron foreshock region can pre-accelerate electrons, enhancing their energy before crossing the shock in supernova remnants.

Contribution

The paper demonstrates that non-linear oblique whistler waves can trap and scatter reflected electrons, significantly improving pre-acceleration efficiency compared to previous models.

Findings

Oblique whistler waves reach radii of up to 5 ion-gyroradii.

Non-linear structures confine about 0.8% of upstream electrons near the shock.

Pre-acceleration efficiency is approximately three times higher than stochastic shock drift acceleration.

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 co-move with the upstream flow and as non-linear structures eventually reach radii of up to 5 ion-gyroradii. We show that the inner electromagnetic configuration of the whistlers evolves into complex non-linear structures bound by a strong magnetic field around 4 times the upstream value. Although these non-linear structures do not in general interact with co-spatial 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 3 times more efficient than, stochastic shock drift acceleration.