Solar Hα features with hot onsets

Abstract

Even in quiet areas underneath coronal holes the solar chromosphere contains ubiquitous heating events. They tend to be small scale and short lived, hence difficult to identify. Here we do not address their much-debated contribution to outer-atmosphere heating, but their aftermaths. We performed a statistical analysis of high-resolution observations in the Balmer Hα line to suggest that many slender dark Hα fibrils spreading out from network represent cooling gas that outlines tracks of preceding rapid type II spicule events or smaller similar but as yet unresolved heating agents in which the main gas constituent, hydrogen, ionizes at least partially. Subsequent recombination then causes dark Hα fibrils enhanced by nonequilibrium overopacity. We suggest that the extraordinary fibrilar appearance of the Hα chromosphere around network results from intermittent, frequent small-scale prior heating.