Electron Acceleration at Earth's Bow Shock Due to Stochastic Shock Drift Acceleration

We use the Magnetospheric Multiscale mission (MMS) to study electron acceleration at Earth's quasi-perpendicular bow shock to address the long-standing electron injection problem. The observations are compared to the predictions of the stochastic shock drift acceleration (SSDA) theory. Recent studies based on SSDA predict electron distribution being a power law with a cutoff energy that scales with upstream parameters. This scaling law has been successfully tested for a single Earth's bow shock crossing by MMS. Here we extend this study and test the prediction of the scaling law for seven MMS Earth's bow shock crossings with different upstream parameters. A goodness-of-fit test shows good agreement between observations and SSDA theoretical predictions, thus supporting SSDA as one of the most promising candidates for solving the electron injection problem. Collisionless shock waves are an important source of accelerating electrons up to cosmic ray energies throughout our universe. Common electron acceleration mechanisms, explaining the highly relativistic energies observed in cosmic rays, require a population of pre-accelerated electrons up to mildly relativistic energies of around 0.1-1 MeV. This is known as the electron injection problem and a lot of research is currently spent on studying this pre-acceleration phase of electron acceleration. We use spacecraft data from the Magnetospheric Multiscale mission to study an electron acceleration mechanism able to accelerate electrons from typical solar wind energies of 20 eV up to around 100 keV. One of the most promising theories for explaining electron acceleration is the so-called stochastic drift acceleration theory, which involves electron interaction with plasma waves forming within a shock. We provide additional observational evidence supporting this theory. Using Magnetospheric Multiscale data to observe energetic electron events at Earth's collisionless bow shock Electron acceleration at all crossings is well described by the Stochastic Shock Drift Acceleration Theory Pitch angle diffusion rate depends on Alfvenic Mach number and shock angle (theta Bn)