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

Abstract

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