The global distribution of magnetic helicity in the solar corona

TL;DR

This paper introduces a new way to measure magnetic helicity in the Sun's corona using field line helicity, revealing its distribution and dynamics during solar eruptions through simulations.

Contribution

It defines and applies a novel field line helicity measure to study magnetic helicity distribution and evolution in the solar corona, highlighting its concentration in flux ropes and during eruptions.

Findings

Helicity is largely carried away by the solar wind on open field lines.

Helicity accumulates in twisted magnetic flux ropes on closed field lines.

Flux rope eruptions cause sudden helicity output bursts.

Abstract

By defining an appropriate field line helicity, we apply the powerful concept of magnetic helicity to the problem of global magnetic field evolution in the Sun's corona. As an ideal-magnetohydrodynamic invariant, the field line helicity is a meaningful measure of how magnetic helicity is distributed within the coronal volume. It may be interpreted, for each magnetic field line, as a magnetic flux linking with that field line. Using magneto-frictional simulations, we investigate how field line helicity evolves in the non-potential corona as a result of shearing by large-scale motions on the solar surface. On open magnetic field lines, the helicity injected by the Sun is largely output to the solar wind, provided that the coronal relaxation is sufficiently fast. But on closed magnetic field lines, helicity is able to build up. We find that the field line helicity is non-uniformly distributed, and is highly concentrated in twisted magnetic flux ropes. Eruption of these flux ropes is shown to lead to sudden bursts of helicity output, in contrast to the steady flux along the open magnetic field lines.