In Situ Mechanisms are Necessary for Switchback Formation

Abstract

Magnetic switchbacks are reversals (deflection angle θ _B > 90°) in the radial magnetic field B _r , accompanied by proton bulk flow v _p enhancement. The switchback generation mechanism(s) is still debated, though switchback evolution is linked to coronal heating. In this study, we provide an exhaustive investigation of magnetic fluctuations in the first 14 Parker Solar Probe encounters. Our results show (1) enhanced magnetic fluctuations ( θ _B ≤ 90°) and (2) clear absence of B _r reversals θ _B > 90° in sub-Alfvénic solar corona. It is concluded that switchbacks are either generated locally in super-Alfvénic solar wind (in situ) or that a subpopulation of magnetic fluctuations generated in sub-Alfvénic solar corona (ex situ) evolves into switchbacks once in super-Alfvénic solar wind flow. In other words, in situ mechanisms are necessary for switchback formation. It is then hypothesized that interchange reconnection in sub-Alfvénic solar corona can provide both the necessary ex situ trigger and in situ evolution mechanisms for switchback formation, though other trigger mechanisms cannot be ruled out.

Keywords

• Solar corona
• Alfven waves
• Magnetic fields