PREDNASAJUCI / LECTURER :

Július Koza

(1) Astronomical Institute, Slovak Academy of Sciences,
059 60 Tatranská Lomnica

NAZOV / TITLE :

Temperature minimum region revealed

ABSTRAKT / ABSTRACT :

Temperature is the variable that most strongly controls the continuum and spectral line shapes and intensities 
yielded by the classical 1D semiempirical models of solar atmosphere. Their characteristic feature is a distinct 
temperature minimum separating the upper photosphere, dominated by the gas pressure, from the lower chromosphere 
where the magnetic field gains a dominance over the gas. High-resolution imaging spectroscopy taken off limb 
in the wing of the Hbeta line shows the dark intensity gap between the photospheric limb and chromosphere, which 
is absent in the Hbeta line-center images and also in the Ca II 854.2 nm images. We interpret the dark gap in the 
Hbeta line-wing images as a manifestation of the temperature minimum region between the photosphere and chromosphere. 
To support this hypothesis we calculate synthetic spectra of the off-limb Hbeta and Ca II 854.2 nm emissions with 
a dedicated non-LTE code based on Rybicki & Hummer's formalism in one-dimensional spherical geometry and find good 
agreement with the observations. The analysis of synthetic line profiles shows that the dark gap in the Hbeta line-wing 
images maps the temperature minimum region between the photosphere and chromosphere due to the opacity and emissivity 
gap of Balmer lines in this layer. 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 high-contrast Hbeta gap is the manifestation 
of the temperature minimum and the dynamic nature of the solar chromosphere. The work opens a new window for studying the 
temperature minimum region by other lines of the Balmer series and with a more realistic chromospheric model atmosphere, 
which includes various dynamic phenomena relevant to the chromosphere.