[Paper Review] Solar H-alpha features with hot onsets. IV. Network fibrils

Robert J. Rutten, L. Rouppe van der Voort+1 more|arXiv (Cornell University)|Aug 25, 2019

Solar and Space Plasma Dynamics43 references4 citations

TL;DR

The paper proposes that long, dark Hα fibrils in the solar chromosphere are cooling aftermaths of intermittent, small-scale heating events—particularly type II spicules—where partial hydrogen ionization precedes recombination, creating enhanced opacity that makes the fibrils visible. High-resolution Hα observations reveal that these fibrils trace the paths of prior rapid blue- and red-shifted excursions (RBEs/RREs), indicating that chromospheric fibrils are not static structures but dynamic contrails from transient heating, with nonequilibrium ionization and opacity playing a key role in their visibility.

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 H-alpha line to suggest that many slender dark H-alpha 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 H-alpha fibrils enhanced by nonequilibrium overopacity. We suggest that the extraordinary fibrilar appearance of the H-alpha chromosphere around network results from intermittent, frequent small-scale prior heating.

Motivation & Objective

• To investigate the origin of long, slender dark Hα fibrils observed in the solar chromosphere around network regions.
• To determine whether these fibrils are remnants of prior heating events, particularly rapid blue- and red-shifted excursions (RBEs/RREs) associated with type II spicules.
• To assess the role of partial hydrogen ionization and nonequilibrium opacity in making these fibrils visible in Hα.
• To evaluate whether the fibrils represent cooling gas returning to the chromosphere after transient heating, rather than static magnetic structures.
• To challenge the assumption of statistical equilibrium in chromospheric modeling by demonstrating the importance of local gas history and dynamic ionization processes.

Proposed method

• Utilized high-resolution Hα imaging sequences from the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish 1-m Solar Telescope (SST), covering both blue and red wings of the Hα line.
• Analyzed temporal evolution of Hα fibrils by comparing their appearance with RBEs and RREs—rapidly moving, broadened features in the blue and red wings of Hα.
• Tracked the propagation of heating events via multi-wavelength observations from the Atmospheric Imaging Assembly (AIA) on SDO, identifying hot, accelerating jets (PHEs) in 94, 171, 193, and 304 Å channels.
• Identified dark Hα fibrils appearing minutes after PHEs, often aligned with the PHE trajectory and showing redshifted core reversals, indicating backflowing cooling gas.
• Used statistical analysis of fibril morphology and temporal correlation with RBE/RRE events to infer that fibrils are not static but dynamic afterglows of transient ionization.
• Evaluated Hα opacity using nonequilibrium ionization models, showing that recombination after partial ionization enhances opacity and creates dark fibrils.

Experimental results

Research questions

• RQ1 Are long, dark Hα fibrils in the chromosphere afterglows of transient heating events such as type II spicules?
• RQ2 To what extent do RBEs and RREs in the Hα wings represent the on-disk manifestation of these heating events?
• RQ3 What role does partial hydrogen ionization and nonequilibrium recombination play in the formation and visibility of Hα fibrils?
• RQ4 Why are fibrils particularly prominent in network regions despite the absence of strong magnetic activity?
• RQ5 How does the dynamic history of gas—especially prior ionization—determine the appearance of Hα fibrils, challenging static equilibrium models?

Key findings

• Long, dark Hα fibrils frequently appear minutes after propagating heating events (PHEs) and are aligned with the PHE trajectories, indicating they are cooling aftermaths of prior heating.
• The fibrils are most clearly visible when preceded by RBEs or RREs, which are on-disk signatures of type II spicules, suggesting a causal link between these events and fibril formation.
• The high opacity of the fibrils is attributed to nonequilibrium recombination following partial hydrogen ionization along the PHE path, not to static density enhancements.
• The fibrils exhibit redshifted Hα core reversals, indicating backflow of cooling gas, consistent with a cooling phase after transient heating.
• Statistical analysis shows that the finest fibrils correlate best with RBE/RRE events, implying that the most dynamic and small-scale heating events leave the clearest fibril traces.
• The results suggest that the Hα chromosphere is not in statistical equilibrium but is instead a highly dynamic, 3D, time-dependent domain where intermittent, small-scale heating drives the fibrilar morphology.