The Dynamic Formation of Pseudostreamers

Roger B Scott,C Richard Devore,Spiro K Antiochos,David I Pontin,Peter F Wyper

doi:10.3847/1538-4357/abec4f

Abstract

Abstract Streamers and pseudostreamers structure the corona at the largest scales, as seen in both eclipse and coronagraph white-light images. Their inverted-goblet appearance encloses broad coronal loops at the Sun and tapers to a narrow radial stalk away from the star. The streamer associated with the global solar dipole magnetic field is long-lived, predominantly contains a single arcade of nested loops within it, and separates opposite-polarity interplanetary magnetic fields with the heliospheric current sheet (HCS) anchored at its apex. Pseudostreamers, on the other hand, are transient, enclose double arcades of nested loops, and separate like-polarity fields with a dense plasma sheet. We use numerical magnetohydrodynamic simulations to calculate, for the first time, the formation of pseudostreamers in response to photospheric magnetic-field evolution. Convective transport of a minority-polarity flux concentration, initially positioned under one side of a streamer, through the streamer boundary into the adjacent preexisting coronal hole forms the pseudostreamer. Interchange magnetic reconnection at the overlying coronal null point(s) governs the development of the pseudostreamer above—and of a new satellite coronal hole behind—the moving minority polarity. The reconnection dynamics liberate coronal-loop plasma that can escape into the heliosphere along so-called separatrix-web (“S-Web”) arcs, which reach far from the HCS and the solar equatorial plane, and can explain the origin of high-latitude slow solar wind. We describe the implications of our results for in situ and remote-sensing observations of the corona and heliosphere as obtained, most recently, by Parker Solar Probe and Solar Orbiter.

Highlights

Understanding the origins of the solar wind and interplanetary magnetic field, which dictate the composition of our heliosphere, remains one of the most enduring and important problems in solar system science
The closed boundary condition at the source surface prevents the apex of the helmet streamer (HS) from rising through the surface, so the identification of the HS boundary using field lines emanating from the sourcesurface polarity inversion line is self-consistent throughout the simulations
Following the formation of electric currents near the nulls, the closed flux between the HS boundary and the leading edge of the domes erodes until each eventually emerges, in full or in part, through the flank of the HS. The stages of this emergence are discussed in Section 4.2; here we focus on the reconnection dynamics

Summary

Introduction

Understanding the origins of the solar wind and interplanetary magnetic field, which dictate the composition of our heliosphere, remains one of the most enduring and important problems in solar system science. The field is observed to form a complex and ever-changing pattern of positive and negative magnetic polarities, and while the coronal magnetic field is more difficult to measure, its structure can be inferred from extrapolations and observations of bright coronal threads These show that in many locations, the magnetic field lines arc up into the solar atmosphere, connecting two opposite polarities as “closed” loops, while in other parts of the atmosphere, the dynamic pressure of the solar wind stretches the magnetic field out to great distances, forming “open” magnetic flux tubes, where coronal plasma can escape freely into the heliosphere.

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