Evolving Solar Wind Flow Properties of Magnetic Inversions Observed by Helios

TL;DR

This study analyzes magnetic inversions in the solar wind observed by Helios and Parker Solar Probe, revealing their Alfvénic properties, evolution with distance, and potential formation mechanisms within different solar wind streams.

Contribution

It expands previous work by examining plasma property changes and Alfvénic characteristics of magnetic inversions across a range of heliocentric distances using combined Helios and PSP data.

Findings

Inversions are often Alfvénic but vary widely in properties.

Departure from ideal Alfvénicity increases with distance from the Sun.

No evidence that different properties imply different generation mechanisms.

Abstract

In its first encounter at solar distances as close as r = 0.16AU, Parker Solar Probe (PSP) observed numerous local reversals, or inversions, in the heliospheric magnetic field (HMF), which were accompanied by large spikes in solar wind speed. Both solar and in situ mechanisms have been suggested to explain the existence of HMF inversions in general. Previous work using Helios 1, covering 0.3-1AU, observed inverted HMF to become more common with increasing r, suggesting that some heliospheric driving process creates or amplifies inversions. This study expands upon these findings, by analysing inversion-associated changes in plasma properties for the same large data set, facilitated by observations of 'strahl' electrons to identify the unperturbed magnetic polarity. We find that many inversions exhibit anti-correlated field and velocity perturbations, and are thus characteristically Alfv\'enic, but many also depart strongly from this relationship over an apparent continuum of properties. Inversions depart further from the 'ideal' Alfv\'enic case with increasing r, as more energy is partitioned in the field, rather than the plasma, component of the perturbation. This departure is greatest for inversions with larger density and magnetic field strength changes, and characteristic slow solar wind properties. We find no evidence that inversions which stray further from 'ideal' Alfv\'enicity have different generation processes from those which are more Alfv\'enic. Instead, different inversion properties could be imprinted based on transport or formation within different solar wind streams.