Abstract
Abstract Two states of the slow solar wind are identified from in situ measurements by the Parker Solar Probe (PSP) inside 50 solar radii from the Sun. At such distances the wind measured by PSP has not yet undergone significant transformation related to the expansion and propagation of the wind. We focus in this study on the properties of the quiet solar wind with no magnetic switchbacks. The two states differ by their plasma beta, flux, and magnetic pressure. PSP’s magnetic connectivity established with potential field source surface reconstructions, tested against extreme ultraviolet and white-light imaging, reveals the two states correspond to a transition from a streamer to an equatorial coronal hole. The expansion factors of magnetic field lines in the streamer are 20 times greater than those rooted near the center of the coronal hole. The very different expansion rates of the magnetic field result in different magnetic pressures measured by PSP in the two plasma states. Solar wind simulations run along these differing flux tubes reproduce the slower and denser wind measured in the streamer and the more tenuous wind measured in the coronal hole. Plasma heating is more intense at the base of the streamer field lines rooted near the boundary of the equatorial hole than those rooted closer to the center of the hole. This results in a higher wind flux driven inside the streamer than deeper inside the equatorial hole.
Highlights
The origin of slow solar wind is still debated
The individual streams are guided along individual magnetic flux tubes, which are obtained from the optimised Potential Field Source Surface (PFSS) model: we manually identify the magnetic field tube A as representing the edge of streamer tubes connected to Parker Solar Probe (PSP) from March 31 to April 3 2019, and the magnetic field tube B for the egde of the equatorial coronal hole
Our results suggest that the two states present very different properties which are characteristic of two different magnetic field configurations in the vicinity of the Sun: state 1 originates from open magnetic field lines rooted on the boundary of the equatorial coronal hole and that expand significantly over streamer loops before reaching the upper corona
Summary
The origin of slow solar wind is still debated. Sources of slow solar wind could be the boundaries of coronal holes, along magnetic flux tubes expanding superradially (Wang & Sheeley 1990; Strachan et al 2002; Antonucci et al 2005; Stakhiv et al 2015; Arge et al 2003). During E2, PSP crossed a boundary between two different plasma states on April 3, 2019 at 07:55 UT. For a few days before April 3, 2019, 07:55 UT, PSP was sampling a dense, slow and highly variable solar wind (state 1). PSP’s first encounter with the Sun revealed that the slow solar wind close to the Sun is highly variable consisting of a profusion of magnetic field ’switchbacks’ (Bale et al 2019) and plasma jets (Kasper et al 2019). We focus our analysis on a subset of time intervals of very quiet solar wind that are not perturbed by switchbacks. We found those intervals in both plasma states. We searched for the origin of the sharp transition in wind properties measured on April 3, 2019, 07:55 UT
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