Magnetic switchback formation: a review of proposed mechanisms

TL;DR

This review discusses various proposed mechanisms for magnetic switchback formation in the solar wind, emphasizing their origins in the solar atmosphere or in situ processes and their implications for solar wind dynamics.

Contribution

It categorizes and evaluates different mechanisms for switchback formation, highlighting the interplay between solar atmospheric seeds and in situ evolution.

Findings

Lower solar atmosphere mechanisms provide seed perturbations but do not form reversals alone.

In situ mechanisms can generate magnetic reversals from seed perturbations.

Switchback features likely contain imprints of their coronal source and local evolution.

Abstract

Magnetic switchbacks are large amplitude deflections of the magnetic field within the solar wind. They are Alfv\'enic in character and so are associated with a spike in velocity and a generally small variation in local plasma density. Early orbits of Parker Solar Probe revealed that the solar wind near the Sun is dominated by these structures, and therefore, they may be playing an important role in the energy budget and acceleration of the young solar wind. In this review, we present an overview of different mechanisms that have been proposed for how switchbacks could be formed. We group the mechanisms by whether they predominantly act in the low solar atmosphere or within the solar wind (in situ). We focus on mechanisms that can create reversals of the ambient magnetic field direction and, thus, account for the most extreme perturbations. The general consensus is that mechanisms in the lower solar atmosphere do not form such reversals on their own but provide the seed perturbations, flows, or particle beams necessary for in situ mechanisms to create switchbacks within the solar wind. Switchback observations thus likely contain an imprint of the coronal source of the seed perturbation or flow, which is evolved further locally by one of several plausible in situ mechanisms. We discuss the strengths and weaknesses of each mechanism and outline future observational and theoretical tests that could help differentiate between them.