ИСТИНА

During the years 2014 – 2017 we made spectroscopical observations of several prominences in the MgI lines. These lines occurring in visible part of the solar spectrum play an important diagnostic role, complementing the UV Mg II resonance lines currently regularly observed by the IRIS satellite. Our preliminary analysis of the observations shows that a rather weak Mg I emission, e.g., in the 5172.6 Å line is detected, and this correlates well with the presence of extended bright areas on the solar disk that are visible in the 1600 Å SDO / AIA band (Heinzel P., et_al, 2016). However, in many cases the radiation in the 5172.6 Å line is very low, even not detectable, although a bright prominence above the limb or flare on the solar disc is observed. In order to explain satisfactorily nature of this phenomenon we have calculated several spectra of optically thin plasma with T = 5-24 000 К, N = 10^8 - 10^14 cm^- 3 using the Cloudy photoionization code (Ferland, 2017).