CORONAL HOLES FROM He I AND Hα LINE OBSERVATIONS

Abstract

The physical state of matter at the base of the coronal hole at the level of the chromosphere was studied from observations in the Hα and He I 10830 Å lines. The polar coronal hole was observed in the northern hemisphere in 2015-2017 during 24 Carrington rotations. Boundaries of the coronal hole were determined from observations in the 193 Å line. Intensities in the lines were expressed in units of the unperturbed Sun. For the He I 10830 Å line, the relative intensity of coronal holes varied within 1.0046-1.0355, for the Hα line-within 0.9676-0.9753. Models of the chromosphere explaining the observed features are calculated. Models are built based on calculations of the He I and Hα line profiles in the non-LTE approximation. The difference in the atmospheres of coronal holes and the quiet Sun begins at the level of the upper photosphere at T > 410 3 K. Throughout the chromosphere, the temperature and density in the coronal hole is lower than in the undisturbed chromosphere.