The Acceleration of Electrons at Perpendicular Shocks and its Implication for Solar Energetic Particle events

TL;DR

This study investigates how turbulent magnetic fields at perpendicular shocks accelerate electrons efficiently, with implications for understanding solar energetic particle events, using hybrid and test-particle simulations to reveal the acceleration mechanisms.

Contribution

It demonstrates that large-scale magnetic fluctuations and shock ripples significantly enhance electron acceleration at perpendicular shocks, a novel insight into SEP origins.

Findings

Large-scale magnetic fluctuations lead to rapid electron energization.

Shock ripples contribute to electron acceleration via mirroring effects.

Perpendicular shocks are crucial in electron acceleration related to SEPs.

Abstract

We present a study of the acceleration of electrons at a perpendicular shock that propagates through a turbulent magnetic field. The energization process of electrons is investigated by utilizing a combination of hybrid (kinetic ions and fluid electron) simulations and test-particle electron simulations. In this method, the motions of the test-particle electrons are numerically integrated in the time-dependent electric and magnetic fields generated by two-dimensional hybrid simulations. We show that large-scale magnetic fluctuations effect electrons in a number of ways and lead to efficient and rapid energization at the shock front. Since the electrons mainly follow along magnetic lines of force, the large-scale braiding of field lines in space allows the fast-moving electrons to interact with the shock front and get accelerated multiple times. Ripples in the shock front occurring at various scales will also contribute to the acceleration by mirroring the electrons. Our calculation shows that this process favors electron acceleration at perpendicular shocks. The acceleration efficiency is critically dependent on the turbulence amplitude and coherence length. We also discuss the implication of this study for solar energetic particles (SEPs) by comparing the acceleration of electrons with that of protons. Their correlation indicates that perpendicular shocks play an important role in SEP events.