Coronal Acceleration of Large and Acute SEP Events

Abstract:

Solar energetic particle (SEP) events pose a serious threat to spacecraft and astronauts throughout the heliosphere. On Earth, strong events can harm aircraft avionics, communication, and navigation. In space, energetic particles can be hazardous for crews of Low Earth Orbit spacecraft and the International Space Station, especially when engaged in extravehicular activity. One important goal when studying energetic particles in the heliosphere is providing a meaningful estimate of their flux at a the location of a particular observer. At Earth, good estimates of both the energetic particle flux and the expected intra-event variability can significantly improve our ability to protect space-based assets without incurring unnecessary operational delays. The largest SEP events typically arise in conjunction with X-class flares and very fast coronal mass ejections (CMEs). One probable mechanism for accelerating energetic particles that propagate to Earth is the shock wave or compression that forms low in the corona during the passage of a CME. After the shock wave or compression forms, it propagates outward and accelerates particles over a finite space for a finite time. These energetic particles can travel at a significant fraction of the speed of light and reach an observer soon after an eruptive event if the observer and the acceleration site are magnetically connected. This connectivity depends, in turn, on both the structure of the coronal and heliospheric magnetic field and the local shock properties. Furthermore, intra-event variability depends on local properties (e.g., mean free path and rigidity) along the field line where the shock wave or compression accelerates particles, and the finite scales over which the shock wave or compression operates. This work presents results from a simulation of the extreme SEP event on 14 July (Bastille Day) 2000, using the Energetic Particle Radiation Environment Model (EPREM) coupled to the Magnetohydrodynamic Algorithm outside a Sphere (MAS) code from Predictive Science Incorporated (PSI). We show how coronal variability in acceleration rate and compression strength maps to variability at 1 au, with a focus on field lines connected to near-Earth observers, and how this intra-event variability compares to observed inter-event variability in GOES data.