The Temperature-Dependent Nature of Coronal Dimmings

TL;DR

This paper investigates the temperature-dependent behavior of coronal dimmings during solar eruptions, revealing that cooler plasma remains bound while hotter plasma is ejected, and introduces the concept of a propagating heat wave during flux reconnection.

Contribution

It demonstrates that coronal dimmings are more pronounced at higher temperatures and introduces the observation of a propagating heat wave caused by flux reconnection, not density depletion.

Findings

Cooler plasma remains bound during eruptions.

Dimmings are more pronounced at 19.5 nm than at 17.1 nm.

A propagating heat wave occurs during flux reconnection.

Abstract

The opening-up of the magnetic field during solar eruptive events is often accompanied by a dimming of the local coronal emission. From observations of filament eruptions recorded with the Extreme-Ultraviolet Imager on STEREO during 2008-2009, it is evident that these dimmings are much more pronounced in 19.5 nm than in the lower-temperature line 17.1 nm, as viewed either on the disk or above the limb. We conclude that most of the cooler coronal plasma is not ejected but remains gravitationally bound when the loops open up. This result is consistent with Doppler measurements by Imada and coworkers, who found that the upflow speeds in a transient coronal hole increased dramatically above a temperature of 1 MK; it is also consistent with the quasistatic behavior of polar plumes, as compared with the hotter interplume regions that are the main source of the fast solar wind. When the open flux reconnects and closes down again, the trapped plasma is initially heated to such high temperatures that it is no longer visible at Fe IX 17.1 nm. Correspondingly, 17.1 nm images show a dark ribbon or ``heat wave'' propagating away from the polarity inversion line and coinciding with the brightened Fe XV 28.4 nm and Fe XII 19.5 nm post-eruptive loops and their footpoint areas. Such dark ribbons provide a clear example of dimmings that are not caused by a density depletion. The propagation of the ``heat wave'' is driven by the closing-down, not the opening-up, of flux and can be observed both off-limb and on-disk.