Tests for coronal electron temperature signatures in suprathermal electron populations at 1 AU

TL;DR

This study investigates whether suprathermal electron populations at 1 AU retain signatures of the coronal electron temperature, finding only weak correlations and suggesting transport effects may obscure initial coronal signatures.

Contribution

It re-examines the correlation between suprathermal electron properties and coronal temperature proxies, providing new insights into their relationship at 1 AU.

Findings

Weak correlation between suprathermal electron energy and O7+/O6+ ratio.

Transport effects may obscure initial coronal temperature signatures.

Potential for stronger correlations closer to the Sun, testable with Solar Orbiter data.

Abstract

The development of knowledge of how the coronal origin of the solar wind affects its in situ properties is one of the keys to understanding the relationship between the Sun and the heliosphere. In this paper, we analyse ACE/SWICS and WIND/3DP data spanning >12 years, and test properties of solar wind suprathermal electron distributions for the presence of signatures of the coronal temperature at their origin which may remain at 1AU. In particular we re-examine a previous suggestion that these properties correlate with the oxygen charge state ratio O7+/O6+; an established proxy for coronal electron temperature. We find only a very weak but variable correlation between measures of suprathermal electron energy content and O7+/O6+. The weak nature of the correlation leads us to conclude, in contrast to earlier results, that an initial relationship with core electron temperature has the possibility to exist in the corona, but that in most cases no strong signatures remain in the suprathermal electron distributions at 1AU. It can not yet be confirmed whether this is due to the effects of coronal conditions on the establishment of this relationship, or to the altering of the electron distributions by processing during transport in the solar wind en route to 1AU. Contrasting results for the halo and strahl population favours the latter interpretation. Confirmation of this will be possible using Solar Orbiter data (cruise and nominal mission phase) to test whether the weakness of the relationship persists over a range of heliocentric distances. If the correlation is found to strengthen when closer to the Sun, then this would indicate an initial relationship which is being degraded, perhaps by wave-particle interactions, en route to the observer