Gabriel Torralba Paz

Cosmic-ray acceleration processes in astrophysical plasmas are often investigated with fully kinetic or hybrid kinetic numerical simulations, which enable us to describe a detailed microphysics of particle energization mechanisms. Tracing of individual particles in such simulations is especially useful in this regard. However, visually inspecting particle trajectories introduces a significant amount of bias and uncertainty, making it challenging to pinpoint specific acceleration mechanisms. Here, we present a novel approach utilizing neural networks to assist in the analysis of individual particle data. We demonstrate the effectiveness of this approach using the dataset from our recent particle-in-cell (PIC) simulations of non-relativistic perpendicular shocks, which consists of 252 000 electrons, each characterized by their position, momentum, and electromagnetic field at particle’s position, recorded in a time series of 1200 time steps. These electrons cross a region affected by the electrostatic Buneman instability, and a small percentage of them attain high energies. We perform classification, regression, and anomaly detection algorithms on the dataset by using a convolutional neural network, a multi-layer perceptron, and an autoencoder. Despite the noisy and imbalanced dataset, all methods demonstrate the capability to differentiate between thermal and accelerated electrons with remarkable accuracy. The proposed methodology may considerably simplify particle classification in large-scale PIC and hybrid simulations.

Strong nonrelativistic shocks are known to accelerate particles up to relativistic energies. However, for diffusive shock acceleration, electrons must have a highly suprathermal energy, implying the need for very efficient preacceleration. Most published studies consider shocks propagating through homogeneous plasma, which is an unrealistic assumption for astrophysical environments. Using 2D3V particle-in-cell simulations, we investigate electron acceleration and heating processes at nonrelativistic high-Mach-number shocks in electron-ion plasma with a turbulent upstream medium. For this purpose, slabs of plasma with compressive turbulence are simulated separately and then inserted into shock simulations, which require matching of the plasma slabs at the interface. Using a novel procedure of matching electromagnetic fields and currents, we perform simulations of perpendicular shocks setting different intensities of density fluctuations (≲10%) in the upstream region. The new simulation technique provides a framework for studying shocks propagating in turbulent media. We explore the impact of the fluctuations on electron heating, the dynamics of upstream electrons, and the driving of plasma instabilities. Our results indicate that while the presence of turbulence enhances variations in the upstream magnetic field, their levels remain too low to significantly influence the behavior of electrons at perpendicular shocks.

Strong non-relativistic shocks are known to accelerate particles up to relativistic energies. However, for Diffusive Shock Acceleration electrons must have a highly suprathermal energy, implying a need for very efficient pre-acceleration. Most published studies consider shocks propagating through homogeneous plasma, which is an unrealistic assumption for astrophysical environments. Using 2D3V particle-in-cell simulations, we investigate electron acceleration and heating processes at non-relativistic high-Mach-number shocks in electron-ion plasma with a turbulent upstream medium. For this purpose slabs of plasma with compressive turbulence are separately simulated and then inserted into shock simulations, which requires matching of the plasma slabs at the interface. Using a novel procedure of matching electromagnetic fields and currents, we perform simulations of perpendicular shocks setting different intensities of density fluctuations ($\lesssim 10\%$) in the upstream. The new simulation technique provides a framework for studying shocks propagating in turbulent media. We explore the impact of the fluctuations on electron heating, the dynamics of upstream electrons, and the driving of plasma instabilities. Our results indicate that while the presence of the turbulence enhances variations in the upstream magnetic field, their levels remain too low to influence significantly the behavior of electrons at perpendicular shocks.