Bulletin of Taras Shevchenko National University of Kyiv. Astronomy, no. 60, p. 15-22 (2019)
Spectral investigations of magnetic fields and thermodynamic conditions in active regions on the Sun
V. Lozitsky, Dr Hab.
Taras Shevchenko National University of Kyiv
Abstract
The main results of studies of magnetic fields and thermodynamic conditions in active regions in the Sun obtained at the Astronomical Observatory of the Taras Shevchenko National University of Kyiv (AO KNU) during the last 10 years (2010–2019) are presented. The true diameter of the extremely small(spatially unresolved) magnetic flux tubes was estimated on a base of data from Hinode Space Observatory; this diameter was found to be within the range of 15–20 km, which is much smaller than the spatial resolution limit of the largest solar telescopes to date (≈ 60 km). According to the observations made at AO KNU, it was shown that there are three types of magnetic fields in a solar faculae: strong (kilogauss range) fields in small-scale flux tubes, areas of weak and moderate background fields of regular polarity, and areas of sub-telescopic fields of mixed polarity. The magnetic flux of mixed polarity exceeds the absolute flux of the entire field of regular polarity by at least 2 times. In the sunspot umbra observed at GST of AO KNU and ATsU-5 of GAO NASU, thin spectral effects of Zeeman splitting were found, which indicate presence of very strong magnetic fields at least of 5,8 kG. The polarity of the magnetic field in sub-telescopic structures with such very strong fields is found to be the same as in the background field, and the Doppler velocity is about 2 km/s (plasma lifting). Super- strong magnetic fields in the range of 5–5,7 kG were also detected from observations of the Big Bear Observatory (BBSO), USA. A unique observational material also obtained on AO KNU concerning the area of a seismic source of extremely powerful solar flare on October 28, 2003 of X17.2 / 4B class was analyzed. This solar flare had a Balmer decrement with a record intensity ratio I (Hβ) / I (Hα) = 1,68 of the Hβ and Hα lines, which is unprecedented for all flares observed. In this flare, indications of the existence of particularly strong magnetic fields in the range of several tens of kilogauss were found. The semi-empirical model of this flare has an interesting feature, namely three discrete layers with high plasma concentration and temperature, including a very thick and thin layer in the chromosphere with the following parameters: hydrogen concentration nH = 1018 cm–3, thickness Δh = 3–5 km and height h ≈ 1200 km above the level of the photosphere. In active solar prominences, rather strong magnetic fields (up to 4 kG) were detected at altitudes of 3–14 Mm. The modeling of spectral line profiles within the two-component model showed that the true magnitude of local magnetic fields can be even greater by at least 3 times. A theoretical MHD model is proposed, according to which such high strengths occur in force-free magnetic ropes with a characteristic scale of 300 km.
Key words
Sun, solar activity, spectral-polarization observations, magnetic fields, thermodynamic conditions, solar faculae, spots, flares,prominences, semi-empirical models
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