An Observationally Constrained Model of a Flux Rope that Formed in the Solar Corona

Alexander W James,Lucie M Green,Mark C M Cheung,Gherardo Valori,Yang Guo,Lidia Van Driel-Gesztelyi,Yang Liu

doi:10.3847/2041-8213/aab15d

Abstract

Coronal mass ejections (CMEs) are large-scale eruptions of plasma from the coronae of stars. Understanding the plasma processes involved in CME initiation has applications to space weather forecasting and laboratory plasma experiments. James et al. (Sol. Phys. 292, 71, 2017) used EUV observations to conclude that a magnetic flux rope formed in the solar corona above NOAA Active Region 11504 before it erupted on 14 June 2012 (SOL2012-06-14). In this work, we use data from the Solar Dynamics Observatory to model the coronal magnetic field of the active region one hour prior to eruption using a nonlinear force-free field extrapolation, and find a flux rope reaching a maximum height of 150 Mm above the photosphere. Estimations of the average twist of the strongly asymmetric extrapolated flux rope are between 1.35 and 1.88 turns, depending on the choice of axis, although the erupting structure was not observed to kink. The decay index near the apex of the axis of the extrapolated flux rope is comparable to typical critical values required for the onset of the torus instability, so we suggest that the torus instability drove the eruption.

Highlights

Coronal mass ejections (CMEs) are large-scale eruptions of plasma from the coronae of stars, and there is currently no consensus regarding their cause
We test the hypothesis of James et al (2017; Paper I) that a magnetic flux rope formed in the corona of NOAA Active Region 11504 before erupting as a CME
The decay index near the center of the flux rope is ≈1.8, which is comparable to the critical value for the torus instability onset determined in other works

Summary

Introduction

Coronal mass ejections (CMEs) are large-scale eruptions of plasma from the coronae of stars, and there is currently no consensus regarding their cause. The observed EUV sigmoid extended to the south of the NOAA Active Region 11504 before the CME occurred (see Figure 5(c) of Paper I), and so the field-of-view of the publicly available HMI SHARP series magnetogram (Bobra et al 2014) was too small to accurately reproduce the sigmoidal field in the extrapolation. The coronal magnetic field was extrapolated from the pre-processed magnetogram using the magnetofrictional NLFFF method detailed by Valori et al (2010), yielding a model of the active region with a force-free parameter σJ ≈ 25% (Wheatland et al 2000) and solenoidal error limited to 9% of the total energy (Valori et al 2013). Structures seen in the 193 Å channel of AIA, including field lines that fan out from the edges of the active region and a sheared arcade in the core of the active region (shown by the southern group of white field lines in Figure 2(a)) that matches the observed sheared arcade (see Section 3.3 and Figure 5 of Paper I for more details)

The Pre-eruptive Flux Rope

Twist and Writhe

Decay Index

Findings

Conclusions