Suprathermal electron acceleration by a quasi-perpendicular shock: simulations and observations

TL;DR

This study combines simulations and observations to demonstrate that shock drift acceleration (SDA) plays a crucial role in energizing suprathermal electrons at quasi-perpendicular shocks driven by ICMEs in the solar wind.

Contribution

It provides a detailed simulation-based analysis confirming SDA's significance in electron acceleration, aligning with observational data and advancing understanding of shock-related particle energization.

Findings

Downstream to upstream intensity ratio peaks at 90° pitch angle

Electron energy spectral index exceeds diffusive shock acceleration predictions

Drift length scales with energy, drift time is energy independent

Abstract

The acceleration of suprathermal electrons in the solar wind is mainly associated with shocks driven by interplanetary coronal mass ejections (ICMEs). It is well known that the acceleration of electrons is much more efficient at quasi-perpendicular shocks than at quasi-parallel ones. Yang et al. (2018, ApJ, 853, 89) (hereafter YEA2018) studied the acceleration of suprathermal electrons at a quasi-perpendicular ICME-driven shock event to claim the important role of shock drift acceleration (SDA). Here, we perform test-particle simulations to study the acceleration of electrons in this event, by calculating the downstream electron intensity distribution for all energy channels assuming an initial distribution based on the averaged upstream intensities. We obtain simulation results similar to the observations from YEA2018 as follows. It is shown that the ratio of downstream to upstream intensities peaks at about 90$^\circ$ pitch angle. In addition, in each pitch angle direction the downstream electron energy spectral index is much larger than the theoretical index of diffusive shock acceleration. Furthermore, considering SDA, the estimated drift length is proportional to the electron energy but the drift time is almost energy independent. Finally, we use a theoretical model based on SDA to describe the drift length and time, especially, to explain their energy dependence. These results indicate the importance of SDA in the acceleration of electrons by quasi-perpendicular shocks.