Dark off-limb gap: manifestation of temperature minimum and dynamic nature of the chromosphere

TL;DR

This study investigates the off-limb emission features of the solar chromosphere, revealing that the observed dark gap in Hβ line wings indicates the temperature minimum and dynamic behavior of the chromosphere, supported by synthetic modeling.

Contribution

It demonstrates that the dark off-limb gap in Hβ lines maps the temperature minimum and dynamic chromospheric processes, with improved modeling including microturbulence to match observations.

Findings

The Hβ line wing gap corresponds to the temperature minimum region.

Synthetic spectra agree well with observed off-limb emissions.

Enhanced microturbulence is needed to reproduce the observed gap.

Abstract

We study off-limb emission of the lower solar atmosphere using high-resolution imaging spectroscopy in the H$\beta$ and Ca II 8542 \r{A} lines obtained with the CHROMospheric Imaging Spectrometer (CHROMIS) and the CRisp Imaging SpectroPolarimeter (CRISP) on the Swedish 1-m Solar Telescope. The H$\beta$ line wing images show the dark intensity gap between the photospheric limb and chromosphere which is absent in the Ca II images. We calculate synthetic spectra of the off-limb emissions with the RH code in the one-dimension spherical geometry and find good agreement with the observations. The analysis of synthetic line profiles shows that the gap in the H$\beta$ line wing images maps the temperature minimum region between the photosphere and chromosphere due to the well known opacity and emissivity gap of Balmer lines in this layer. However, observed gap is detected farther from the line core in the outer line wing positions than in the synthetic profiles. We found that an increased microturbulence in the model chromosphere is needed to reproduce the dark gap in the outer line wing, suggesting that observed H$\beta$ gap is the manifestation of the temperature minimum and the dynamic nature of the solar chromosphere. The temperature minimum produces a small enhancement in synthetic Ca II line-wing intensities. Observed off-limb Ca II line-wing emissions show similar enhancement below temperature minimum layer near the edge of the photospheric limb.