Articles | Volume 35, issue 6
Ann. Geophys., 35, 1275–1291, 2017
Ann. Geophys., 35, 1275–1291, 2017

Regular paper 01 Dec 2017

Regular paper | 01 Dec 2017

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

Allan R. Macneil1, Christopher J. Owen1, and Robert T. Wicks1,2 Allan R. Macneil et al.
  • 1Mullard Space Science Laboratory, University College London, Surrey, UK
  • 2Institute for Risk and Disaster Reduction, University College London, London, UK

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 1 AU. 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 1 AU. It cannot yet be confirmed whether this is due to the effects of coronal conditions on the establishment of this relationship or due to the altering of the electron distributions by processing during transport in the solar wind en route to 1 AU. 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.

Short summary
We aim to understand the link between the Sun's atmosphere, the corona, and the constant stream of plasma which escapes it, the solar wind. To do so we test how similar energetic electrons in the solar wind are to their earlier state in the corona, using oxygen ionisation states as a proxy. We find only a very weak link which varies with the type of solar wind stream and the 11-year solar cycle. We find minor evidence to suggest that this is due to solar wind processing during its outward flow.