Large-scale quasi-stationary X-ray coronal structures associated with eruptive solar flares

Abstract

About ten years ago the Hard X-Ray Imaging Spectrometer aboard the Solar Maximum Mission detected for the first time large, faint coronal structures associated with dynamic flares. These have come to be known as “giant arches”. Notwithstanding their extreme faintness, the energy content of these giant structures is of the order of 1–10% of the total energy released by large flares (≅ 10³² erg), thus representing a non-negligible term in the global flare energy balance. Analogously, their mass content (− 10¹⁵ g) is quite similar to that inferred for post-flare loops and coronal mass ejections.

In spite of their obvious relevance, little is known about these giant features. Only about ten such arches have been identified to date, and the observations have both insufficient spatial resolution and inadequate time coverage. As a consequence, it is hard even to define their “typical” behavior and to ascertain their basic characteristics.

Because of these difficulties, we still lack a generally accepted flare scenario which accounts for the presence of such structures. After reviewing the observational properties of giant arches we will describe the hypotheses advanced thus far to explain their origin, evolution and energy supply. Uncertainties in the interpretations will be emphasized and alternative models proposed, in an attempt to define future observations which may allow us to understand the role of arches in the flare process and to make full use of the new information they provide.