Evolution of Plasma Composition in an Eruptive Flux Rope

TL;DR

This study uses Hinode EIS observations to analyze plasma composition changes in an active region's flux rope formation and eruption, providing evidence for flux rope formation via photospheric flux cancellation.

Contribution

It offers the first detailed plasma composition evolution analysis during flux rope formation and eruption, supporting models involving photospheric flux cancellation.

Findings

Photospheric plasma observed near polarity inversion line during flux cancellation.

Plasma composition in the flux rope evolved from coronal to photospheric during formation.

Flux rope contained mainly photospheric plasma during and after a failed eruption.

Abstract

Magnetic flux ropes are bundles of twisted magnetic field enveloping a central axis. They harbor free magnetic energy and can be progenitors of coronal mass ejections (CMEs), but identifying flux ropes on the Sun can be challenging. One of the key coronal observables that has been shown to indicate the presence of a flux rope is a peculiar bright coronal structure called a sigmoid. In this work, we show Hinode EUV Imaging Spectrometer (EIS) observations of sigmoidal active region 10977. We analyze the coronal plasma composition in the active region and its evolution as the sigmoid (flux rope) forms and erupts as a CME. Plasma with photospheric composition was observed in coronal loops close to the main polarity inversion line during episodes of significant flux cancellation, suggestive of the injection of photospheric plasma into these loops driven by photospheric flux cancellation. Concurrently, the increasingly sheared core field contained plasma with coronal composition. As flux cancellation decreased and the sigmoid/flux rope formed, the plasma evolved to an intermediate composition in between photospheric and typical active region coronal compositions. Finally, the flux rope contained predominantly photospheric plasma during and after a failed eruption preceding the CME. The Hence, plasma composition observations of active region 10977 strongly support models of flux rope formation by photospheric flux cancellation forcing magnetic reconnection first at the photospheric level then at the coronal level.