Electron energization at quasi-perpendicular oblique shocks in Supernova remnants

• Applicant: Professor Dr. Martin Karl Wilhelm Pohl
• Subject Area: Astrophysics and Astronomy
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 451321940

Most of the available information about particle acceleration in astrophysical objects relates to electrons, on account of their high radiation efficiency. Not as much is known, however, as to how electrons get accelerated. Electron participation in diffusive shock acceleration at Supernova-remnant shocks requires that they be pre-accelerated to Lorentz factors above 100. Where and how that happens is the focus of this project. Supernova shocks have nonrelativistic propagation velocities and are characterized by high sonic and Alfv ́enic Mach numbers. The physics of non-relativistic shocks is governed by reflection of particles, the interaction of which with the incoming plasma excites a variety of instabilities upstream of the shock. If the magnetic field is oriented quasi-parallel to the shock normal, ions can travel far and drive instabilities in the far-upstream region. For quasi-perpendicular shocks the reflected ions are not fast enough and essentially conduct only one Larmor orbit, but electrons may stream along the magnetic field toward the far upstream region and drive waves there. We use fully kinetic Particle-In-Cell simulations to study these processes, and electron acceleration in particular, at oblique quasi-perpendicular shocks, building on methods and results obtained for strictly perpendicular shocks. At shocks a fraction of the upstream ion kinetic energy goes to electrons though different channels: shock surfing acceleration, shock drift acceleration, acceleration by shock potential, magnetic reconnection, stochastic interaction with magnetic turbulences, etc. Changing the angle between the large-scale magnetic field and the shock normal, we expect to determine the changes in the contribution of individual acceleration mechanisms to particle energization and hence be able to establish the rate of electron injection into diffusive shock acceleration as function of the shock obliquity. The essential output of the project will be the upstream and downstream electron spectra, the efficiency of electron reflection off the shock, and the type and behaviour of plasma instabilities they drive. We shall also explore the scaling of the results with the ion-to-electron mass ratio used in the simulation. Our results will inform global models of electron acceleration at SNR shocks.It is known that the late-time evolution of shocks involves large-scale disturbance of the shock front and the pre-shock medium that are seeded by faster processes like currents carried by reflected particles in the upstream medium whose existence and properties are well captured with the full PIC method, and that is what we are going to do. We posit that the long-wavelength disturbance is almost inevitable once the current startsbecause there will always be small variations in that current. Our project will establish with what efficiency the currents are generated.

• DFG Programme: Research Grants