A solar source of Alfv\'enic magnetic field switchbacks: {\em in situ} remnants of magnetic funnels on supergranulation scales

TL;DR

This paper links Alfvénic magnetic switchbacks observed by Parker Solar Probe to magnetic funnels on supergranulation scales, suggesting they originate from interchange reconnection events near the solar transition region.

Contribution

It provides in situ evidence connecting switchbacks to solar magnetic structures and proposes a novel origin involving interchange reconnection and turbulent outflow exhausts.

Findings

Switchbacks are localized within plasma structures originating at the solar corona.

Structures show pressure balance and are consistent with overexpanded flux tubes.

Presence of suprathermal ions suggests energetic tail of solar wind alpha particles.

Abstract

One of the striking observations from the Parker Solar Probe (PSP) spacecraft is the prevalence in the inner heliosphere of large amplitude, Alfv\'enic magnetic field reversals termed 'switchbacks'. These $\delta B_R/B \sim \mathcal{O}(1$) fluctuations occur on a range of timescales and in {\em patches} separated by intervals of quiet, radial magnetic field. We use measurements from PSP to demonstrate that patches of switchbacks are localized within the extensions of plasma structures originating at the base of the corona. These structures are characterized by an increase in alpha particle abundance, Mach number, plasma $\beta$ and pressure, and by depletions in the magnetic field magnitude and electron temperature. These intervals are in pressure-balance, implying stationary spatial structure, and the field depressions are consistent with overexpanded flux tubes. The structures are asymmetric in Carrington longitude with a steeper leading edge and a small ($\sim$1$^\circ$) edge of hotter plasma and enhanced magnetic field fluctuations. Some structures contain suprathermal ions to $\sim$85 keV that we argue are the energetic tail of the solar wind alpha population. The structures are separated in longitude by angular scales associated with supergranulation. This suggests that these switchbacks originate near the leading edge of the diverging magnetic field funnels associated with the network magnetic field - the primary wind sources. We propose an origin of the magnetic field switchbacks, hot plasma and suprathermals, alpha particles in interchange reconnection events just above the solar transition region and our measurements represent the extended regions of a turbulent outflow exhaust.