Tearing Instability and Periodic Density Perturbations in the Slow Solar Wind

Victor Réville,Alexis P Rouillard,Antoine Strugarek,Chen Shi,Benoit Lavraud,Anna Tenerani,Marco Velli

doi:10.3847/2041-8213/ab911d

Abstract

Abstract In contrast with the fast solar wind, which originates in coronal holes, the source of the slow solar wind is still debated. Often intermittent and enriched with low first ionization potential elements—akin to what is observed in closed coronal loops—the slow wind could form in bursty events nearby helmet streamers. Slow winds also exhibit density perturbations that have been shown to be periodic and could be associated with flux ropes ejected from the tip of helmet streamers, as shown recently by the WISPR white-light imager on board Parker Solar Probe (PSP). In this work, we propose that the main mechanism controlling the release of flux ropes is a flow-modified tearing mode at the heliospheric current sheet (HCS). We use magnetohydrodynamic simulations of the solar wind and corona to reproduce realistic configurations and outflows surrounding the HCS. We find that this process is able to explain long (∼10–20 hr) and short (∼1–2 hr) timescales of density structures observed in the slow solar wind. This study also sheds new light on the structure, topology, and composition of the slow solar wind, and could be, in the near future, compared with white light and in situ PSP observations.

Highlights

In the recent years, the usual dichotomy between a fast (> 600 km/s) and a slow (∼ 350 − 400 km/s) wind has been challenged as the main discriminating factor to identify the source of the solar wind plasma
Slow winds exhibit density perturbations which have been shown to be periodic and could be associated with flux ropes ejected from the tip of helmet streamers, as shown recently by the WISPR white light imager onboard Parker Solar Probe (PSP)
We propose that the main mechanism controlling the release of flux ropes is a flow-modified tearing mode at the heliospheric current sheet (HCS)

Summary

INTRODUCTION

The usual dichotomy between a fast (> 600 km/s) and a slow (∼ 350 − 400 km/s) wind has been challenged as the main discriminating factor to identify the source of the solar wind plasma. Endeve et al (2003, 2004), while studying simple dipolar configurations, unveiled an instability that leads to magnetic reconnection and the periodic release of flux ropes from the tips of the streamers These works identified thermal processes as the origin of the streamer’s instability, the thermal conduction coefficient being the critical parameter to make the instability vanish. The model was further developed by Rappazzo et al (2005), who showed how the diamagnetic plasmoid expulsion in a spherical geometry would lead to a rapid plasmoid acceleration profile These works started with a finite thickness current sheet and did not take into account either the specific geometry of the helmet streamer cusp, nor the natural thinning of the sheet arising from the converging plasma flow, they did comment on the role such convergence might play in the evolution. Using the ideal tearing scalings to very high S typical of the solar corona, we show that these periodicities are fully compatible with the timescales of density structures observed in the slow solar wind

MHD MODEL AND SOLAR WIND PARAMETERS

ONSET OF A TEARING INSTABILITY IN THE HCS

RECONNECTION AND PERIODIC DENSITY PERTURBATIONS

DISCUSSIONS