Bulletin of Taras Shevchenko National University of Kyiv. Astronomy, no. 71, p. 22-29 (2025)
FEATURES OF THE SMALL-SCALE STRUCTURE OF THE MAGNETIC FIELD IN A SUNSPOT BASED ON DATA IN THE SPECTRAL LINES OF TITANIUM, IRON AND CALCIUM
Vsevolod LOZITSKY, DSc (Phys. & Math.)
Taras Shevchenko National University of Kyiv, Kyiv, Ukraine
Valentina SHEMINOVA, DSc (Phys. & Math.)
Main Astronomical Observatory of National Academy of Sciences, Kyiv, Ukraine
Ivan YAKOVKIN, PhD
Taras Shevchenko National University of Kyiv, Kyiv, Ukraine
Institute of Physics, National Academy of Sciences of Ukraine
Maxym HROMOV, Student
Taras Shevchenko National University of Kyiv, Kyiv, Ukraine
Abstract
Background. The most reliable data on magnetic fields in sunspots are obtained based on the study of the Zeeman effect in the lines of neutral iron Fe I with large Lande factors, within 2.5–3.0. Since sunspots are very heterogeneous formations, with an extremely fine (spatially unresolved) structure of the magnetic field and the distribution of thermodynamic parameters, it is of considerable interest to compare the measured magnetic fields in the lines of other chemical elements, in particular the lines of titanium and calcium. These lines are significantly enhanced in sunspots and more closely reflect the physical conditions in the coldest areas of sunspots, where particularly strong magnetic fields can exist. That is why the purpose of this work is to study the magnetic fields in the sunspot by the spectral lines of titanium, calcium and iron.
Methods. The spectral-polarization method of measuring magnetic fields was used to process observations made on the Echelle spectrograph of the horizontal solar telescope of the Astronomical Observatory of the Taras Shevchenko National University of Kyiv. The Zeeman spectrogram of the sunspot on July 17, 2023 was scanned using an Epson Perfection V 550 scanner and digitized taking into account the nonlinearity of the characteristic curves of both the photographic material and the scanner itself. Estimates of the longitudinal and local magnetic fields in the studied spot were obtained based on the study of the splitting of the bisectors of the I ± V profiles of the Ti I 6554.238 and 6556.066 Ǻ, Fe I 6569.224 Ǻ and Ca I 6572.795 Ǻ lines.
Results. Since the above spectral lines have incomplete spectral splitting due to low Lande factors (from 1.08 to 1.5), estimates of only the longitudinal component of the magnetic field BLOS, and not the intensity modulus, were obtained from direct measurements. This parameter in some places of the spot significantly differs between the lines of different elements and has the highest values (up to 2400 G) in the Ti I 6556.066 Ǻ line. The data for both titanium umbral lines correlate well with each other, but the Ti I 6556.066 Ǻ line shows significantly higher fields everywhere than the other titanium line. It is interesting to note that the other umbral line, namely Ca I 6572.795 Ǻ, shows the lowest measured fields, up to 1700 G. In the latter line, the bisectors of the I ± V profiles have a maximum splitting at a distance of about 0.32 Ǻ from its center, which may indicate particularly strong local fields with an intensity of about 10.5 kG. C
Conclusions. In the studied sunspot, significant inhomogeneity of the magnetic field was observed both on the surface of the Sun and at a height in its atmosphere. Comparison of the measured magnetic field strengths with the heights of the line formations shows that there was a significant altitudinal gradient of the magnetic field in the spot, at the level of about 10 G / km in absolute value. It is important to note that the sign of this gradient was different at different heights in the spot: above the level of 305 km it was negative, and at lower altitudes it was positive. This may indicate that the sunspot is a relatively shallow formation, resembling a thin pancake, rather than a deep force tube. The obtained data suggest that the magnetic field in this spot had a spatially unresolved structure, the local intensities in which were significantly higher than those obtained from direct observations and probably reached 10.5 kG. This is most convincingly shown by the data on the calcium line. It is the latter line that has a noticeable slope of the bisectors in the I ± V profiles, which indicates an altitudinal velocity gradient in the photosphere.
Key words
Sun, solar activity, sunspots, magnetic fields, bisectors of the Ti I, Fe I and Ca I line profiles, 10-kilogauss magnetic fields.
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