Dark Off-limb Gap: Manifestation of a Temperature Minimum and the Dynamic Nature of the Chromosphere

We study off-limb emission of the lower solar atmosphere using high-resolution imaging spectroscopy in the H beta and Ca ii 8542 angstrom 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 one-dimensional 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, the 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 the 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 the temperature minimum layer near the edge of the photospheric limb.

Automated summary

In this study, off-limb emission of the lower solar atmosphere was investigated using high-resolution imaging spectroscopy. The results show that there is a dark intensity gap between the photospheric limb and chromosphere in the H beta line-wing images, which is absent in the Ca ii images. Synthetic spectra of the off-limb emissions were calculated and found to be in good agreement with the observations. The analysis of synthetic line profiles suggests that the observed gap in the H beta line-wing images maps the temperature minimum region between the photosphere and chromosphere. The observed gap was found to be located farther from the line core in the outer line-wing positions than in the synthetic profiles, indicating the need for an increased microturbulence in the model chromosphere to reproduce the dark gap. The temperature minimum also produces a small enhancement in synthetic Ca ii line-wing intensities, which is consistent with the observed enhancement below the temperature minimum layer in the off-limb Ca ii line-wing emissions.