The Associations of Solar Wind Transients with Large Scale Structures

Abstract:

The solar wind consists of different types of plasma and field structures whose spatial distribution depends on the state of the solar magnetic field and corona at the time. Around solar minimum, the solar wind is from a roughly dipolar coronal source, with magnetic pole-to-equator velocity gradients resulting from the polar open field regions, and related corotating compressions when the magnetic dipole axis was tilted with respect to the solar rotation axis. The solar wind at active phases of the solar cycle significantly departed from this ideal, instead suggesting a relatively disorganized global wind with many transient features related to the more complex and rapidly evolving solar and coronal fields. In this study, we consider the relationships between the large scale solar wind structure and common solar wind transient phenomena observed in the ecliptic at 1AU: Interplanetary coronal mass ejections (ICMEs), magnetic field switchbacks, counter streaming suprathermal electrons (CSEs), and electron heat flux drop-out events (HFDs). We take advantage of the nearly continuous record of STEREO-A suprathermal electron, magnetic field and plasma observations that were obtained in the period from the beginning off its heliocentric prime mission to the present (early 2007 to late 2019), using the nearly 4pi pitch angle distributions of several 100 eV electrons from the IMPACT SWEA electron analyzer and magnetometer to identify the transients. Synoptic plots of the transient occurrences compared to those made from the PLASTIC plasma observations and the magnetometer bring out the relative coincidences and displacements of the different transient types in the context of the solar wind structure, including their relationships to magnetic field sector boundaries, high speed streams, and interstream compressions. They also highlight the STEREO-era solar cycle evolution of the solar wind structure, and the differences between the previous weak solar minimum and the current weak solar minimum that is now nearing its end.