Solar Hα features with hot onsets

IV. Network fibrils^⋆

¹ Lingezicht Astrophysics, ’t Oosteneind 9, 4158 CA Deil, The Netherlands
e-mail: R.J.Rutten@uu.nl
² Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern 0315, Oslo, Norway
³ Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029, Blindern 0315, Oslo, Norway
⁴ Lockheed-Martin Solar and Astrophysics Laboratory, 3251 Hanover Street, Palo Alto, CA 94304, USA

Received: 16 June 2019
Accepted: 19 August 2019

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.