TY  - JOUR
A1  - Morris, Paul J.
A1  - Bohdan, Artem
A1  - Weidl, Martin S.
A1  - Tsirou, Michelle
A1  - Fulat, Karol
A1  - Pohl, Martin
T1  - Pre-acceleration in the electron foreshock. II. oblique whistler waves
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - 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.
Y1  - 2023
U6  - https://doi.org/10.3847/1538-4357/acaec8
SN  - 0004-637X
SN  - 1538-4357
VL  - 944
IS  - 1
PB  - Institute of Physics Publ.
CY  - London
ER  - 
TY  - JOUR
A1  - Morris, Paul J.
A1  - Bohdan, Artem
A1  - Weidl, Martin S.
A1  - Pohl, Martin
T1  - Preacceleration in the Electron Foreshock. I. Electron Acoustic Waves
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - To undergo diffusive shock acceleration, electrons need to be preaccelerated to increase their energies by several orders of magnitude, else their gyroradii will be smaller than the finite width of the shock. In oblique shocks, where the upstream magnetic field orientation is neither parallel nor perpendicular to the shock normal, electrons can escape to the shock upstream, modifying the shock foot to a region called the electron foreshock. To determine the preacceleration in this region, we undertake particle-in-cell simulations of oblique shocks while varying the obliquity and in-plane angles. We show that while the proportion of reflected electrons is negligible for theta (Bn) = 74.degrees 3, it increases to R similar to 5% for theta (Bn) = 30 degrees, and that, via the electron acoustic instability, these electrons power electrostatic waves upstream with energy density proportional to R (0.6) and a wavelength approximate to 2 lambda (se), where lambda (se) is the electron skin length. While the initial reflection mechanism is typically a combination of shock-surfing acceleration and magnetic mirroring, we show that once the electrostatic waves have been generated upstream, they themselves can increase the momenta of upstream electrons parallel to the magnetic field. In less than or similar to 1% of cases, upstream electrons are prematurely turned away from the shock and never injected downstream. In contrast, a similar fraction is rescattered back toward the shock after reflection, reinteracts with the shock with energies much greater than thermal, and crosses into the downstream.
Y1  - 2022
U6  - https://doi.org/10.3847/1538-4357/ac69c7
SN  - 0004-637X
SN  - 1538-4357
VL  - 931
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  -