The particle acceleration at interplanetary shocks has been an active field of observational and theoretical investigation in the past five decades; however, the spatial distribution of energetic…

This is a $530,331.01 grant awarded by the National Aeronautics and Space Administration (NASA) to the University of Arizona, doing business as the Arizona Board of Regents, to investigate the spatial distribution of energetic particles across interplanetary shocks. The research aims to study the local intensity enhancements, or "spikes", of supra-thermal particles at the shock front, which are not well-explained by current theoretical models. The project will analyze high-resolution particle and magnetic field data from NASA's Advanced Composition Explorer (ACE) and Solar Terrestrial Relations Observatory (STEREO) spacecraft, and compare the observations with advanced numerical simulations. This work is intended to advance the understanding of the "injection problem" in the particle acceleration process at interplanetary shocks. The grant does not have a set-aside designation, and the performance location is in Tucson, Arizona. This award supports NASA's broader science mission to study space plasma physics and heliophysics through ground-based and space-based observations and modeling.

The particle acceleration at interplanetary shocks has been an active field of observational and theoretical investigation in the past five decades; however, the spatial distribution of energetic particles across the shock can not be reproduced by current theoretical models in several cases. We propose to investigate the local intensity enhancements at the shock (so-called "spikes") of supra-thermal particles, having a speed within one order of magnitude greater than the shock speed in the upstream fluid rest frame. This regime holds beyond the validity of the standard diffusive shock acceleration model, and has been poorly studied so far. We consider this work an overdue and innovative approach to the so-called "injection problem". We propose a data-guided study to combine: 1) analysis and interpretation of interplanetary shocks high time resolution data collected by energetic particle detectors and magnetometers at 1 AU onboard ACE (advanced composition explorer) and stereo (solar terrestrial relations observatory) a/b; 2) theory and advanced numerical simulations at shocks propagating in a turbulent medium to interpret the available data. An analysis recently conducted has suggested that the observed anisotropy at shock spikes results from bidirectional streaming of ions along field lines that cross the shock multiple times, at odds with a previous interpretation based on pile-up of reflected ions; we will compare such interpretations in detail. We have identified two cases of shock spike that we will analyze in detail and we will look for other cases from existing and upcoming spacecraft archives. We will compare those data with our multiple-approach simulations to investigate the origin of the shock spikes, their anisotropy and time-evolution.