Energetic particle dynamics in a simplified model of a solar wind magnetic switchback

TL;DR

This study uses a simplified magnetic switchback model to investigate how energetic particles are affected by magnetic reversals near the Sun, revealing complex dynamics including scattering and reflection that influence particle transport.

Contribution

The paper introduces a numerical model of magnetic switchbacks with constant magnetic magnitude to analyze their impact on energetic particle propagation, highlighting the role of particle gyroradius and initial conditions.

Findings

Particle dynamics are highly sensitive to the ratio of gyroradius to switchback size.

Multiple pitch-angle scatterings occur when gyroradius is comparable to switchback size.

A significant fraction of particles can be reflected upstream due to interactions with the switchback.

Abstract

Context. Recent spacecraft observations in the inner heliosphere have revealed the presence of local Alfvenic reversals of the magnetic field, while the field magnitude remains almost constant. They are called magnetic switchbacks and are very common in the plasma environment close to the Sun explored by the Parker Solar Probe satellite. Aims. A simple numerical model of a magnetic field reversal with constant magnitude is used in order to explore the influence of switchbacks on the propagation of energetic particles, within a range of energy typical of solar energetic particles. Methods. We model the reversal as a region of space of adjustable size bounded by two rotational discontinuities. By means of test particle simulations, beams of mono-energetic particles can be injected upstream of the switchback with various initial pitch- and gyro-phase angles. In each simulation, the particle energy may also be changed. Results. Particle dynamics is highly affected by the ratio between the particle gyroradius and the size of the switchback, with multiple pitch-angle scatterings when the particle gyroradius is of the order of the switchback size. Further, particle motion is extremely sensitive to the initial conditions implying a transition to chaos; for some parameters of the system, a large share of particles is reflected backwards upstream as they interact with the switchback. These results can have a profound impact on the solar energetic particle transport in the inner heliosphere, thus possible comparisons with in-situ spacecraft data are discussed.