Coronal Dimmings and the Early Phase of a CME Observed with STEREO and Hinode/EIS

TL;DR

This study uses STEREO and Hinode/EIS observations to analyze the early phase of a 2009 CME, revealing synchronized coronal dimmings linked to flux rope eruption and arcade stretching, with mass loss during peak acceleration.

Contribution

First simultaneous comparison of on-disk and above-limb EUV dimmings during a CME, linking dimming motion to flux rope eruption and arcade dynamics.

Findings

Dimmings are synchronized with CME acceleration.

Mass loss occurs mainly during peak CME acceleration.

Dimming regions show migration and rotation consistent with flux rope eruption.

Abstract

We investigate the early phase of the 13 February 2009 coronal mass ejection (CME). Observations with the twin STEREO spacecraft in quadrature allow us to compare for the first time in one and the same event the temporal evolution of coronal EUV dimmings, observed simultaneously on-disk and above the limb. We find that these dimmings are synchronized and appear during the impulsive acceleration phase of the CME, with the highest EUV intensity drop occurring a few minutes after the maximum CME acceleration. During the propagation phase two confined, bipolar dimming regions, appearing near the footpoints of a pre-flare sigmoid structure, show an apparent migration away from the site of the CME-associated flare. Additionally, they rotate around the 'center' of the flare site, i.e., the configuration of the dimmings exhibits the same 'sheared-to-potential' evolution as the postflare loops. We conclude that the motion pattern of the twin dimmings reflects not only the eruption of the flux rope, but also the ensuing stretching of the overlying arcade. Finally, we find that: (1) the global-scale dimmings, expanding from the source region of the eruption, propagate with a speed similar to that of the leaving CME front; (2) the mass loss occurs mainly during the period of strongest CME acceleration. Two hours after the eruption Hinode/EIS observations show no substantial plasma outflow, originating from the 'open' field twin dimming regions.