Solar Energetic Particle Acceleration at a Spherical Shock with the Shock Normal Angle $\theta_{B_n}$ Evolving in Space and Time

TL;DR

This study models how the evolving geometry of a spherical shock driven by a coronal mass ejection influences solar energetic particle acceleration, revealing spatial variations in high-energy particle intensities and spectra.

Contribution

Introduces a 2D kinematic model accounting for evolving shock geometry and magnetic field angles, enhancing understanding of SEP acceleration at CME-driven shocks.

Findings

High-energy SEP intensity varies along the shock front.

West flank accelerates particles more efficiently than east flank.

Double power-law energy spectra emerge from local acceleration and transport.

Abstract

We present a 2D kinematic model to study the acceleration of solar energetic particles (SEPs) at a shock driven by a coronal mass ejection. The shock is assumed to be spherical about an origin that is offset from the center of the Sun. This leads to a spatial and temporal evolution of the angle between the magnetic field and shock normal direction ($\theta_{Bn}$) as it propagates through the Parker spiral magnetic field from the lower corona to 1 AU. We find that the high-energy SEP intensity varies significantly along the shock front due to the evolution of $\theta_{Bn}$. Generally, the west flank of the shock preferentially accelerates particles to high energies compared to the east flank and shock nose. This can be understood in terms of the rate of acceleration, which is higher at the west flank. Double power-law energy spectra are reproduced in our model as a consequence of the local acceleration and transport effects. These results will help better understand the evolution of SEP acceleration and provide new insights into large SEP events observed by multi-spacecraft, especially those close to the Sun, such as Parker Solar Probe and Solar Orbiter.