Slow Solar Wind: Observations and Modeling

Abstract

While it is certain that the fast solar wind originates from coronal holes, where and how the slow solar wind (SSW) is formed remains an outstanding question in solar physics even in the post-SOHO era. The quest for the SSW origin forms a major objective for the planned future missions such as the Solar Orbiter and Solar Probe Plus. Nonetheless, results from spacecraft data, combined with theoretical modeling, have helped to investigate many aspects of the SSW. Fundamental physical properties of the coronal plasma have been derived from spectroscopic and imaging remote-sensing data and in situ data, and these results have provided crucial insights for a deeper understanding of the origin and acceleration of the SSW. Advanced models of the SSW in coronal streamers and other structures have been developed using 3D MHD and multi-fluid equations. However, the following questions remain open: What are the source regions and their contributions to the SSW? What is the role of the magnetic topology in the corona for the origin, acceleration and energy deposition of the SSW? What are the possible acceleration and heating mechanisms for the SSW? The aim of this review is to present insights on the SSW origin and formation gathered from the discussions at the International Space Science Institute (ISSI) by the Team entitled "Slow solar wind sources and acceleration mechanisms in the corona" held in Bern (Switzerland) in March 2014 and 2015.

Charge States and FIP Bias of the Solar Wind from Coronal Holes, Active Regions, and Quiet Sun

“…The distribution for all three regions together is similar to earlier studies (e.g. Abbo et al 2016, and the references therein). There are four important features concerning the relation between N F e /N O and the solar wind speeds.…”

“…Second, why does the charge state anti-correlate with the solar wind speed (Geiss et al 1995;von Steiger et al 2000;Gloeckler et al 2003;Wang & Sheeley 2003;Wang et al 2009)? Third, why is the FIP bias (FIP bias value range) higher (wider) in the slow solar wind than in the fast wind (Geiss et al 1995;von Steiger et al 2000;Abbo et al 2016)? Traditionally, solar wind is classified by its speeds. The speed, however, is not the only characteristic feature of the solar wind (Antiochos et al 2012;Abbo et al 2016).…”

“…Third, why is the FIP bias (FIP bias value range) higher (wider) in the slow solar wind than in the fast wind (Geiss et al 1995;von Steiger et al 2000;Abbo et al 2016)? Traditionally, solar wind is classified by its speeds. The speed, however, is not the only characteristic feature of the solar wind (Antiochos et al 2012;Abbo et al 2016). The plasma properties and magnetic-field structures can be significantly different depending on the solar regions, i.e.…”

“…We used the method suggested by Richardson & Cane (2004) where charge state O 7+ /O 6+ exceeding 6.008exp(−0.00578v) (v is the ICME speed) were discarded. In this study, the threshold for FSW and SSW is chosen as 500 km s −1 (Fu et al 2015;Abbo et al 2016). The two-step mapping procedure (Neugebauer et al 1998(Neugebauer et al , 2002 was applied to trace the solar wind parcels back to the solar surface.…”

Origin of the Wang–Sheeley–Arge solar wind model

“…Because this equatorial streamer-belt/currentsheet region was known to be associated with slow wind, it followed that the solar wind speed was inversely related to the amount of expansion that occurs in individual flux tubes of the coronal hole. However, the question of how much slow wind comes from rapidly expanding flux tubes at the edge of a coronal hole and how much comes from the tops of the stretched coronal loops in the streamer belt remains an active subject of study (Antiochos et al, 2011;Wang, 2012;Abbo et al, 2016).…”