Bulletin of Taras Shevchenko National University of Kyiv. Astronomy, no. 58, p. 11-17 (2018)

Sunspot magnetic fields in 24 cycle of solar activity

N. Lozitska, PhD.

Astronomical Observatory of Taras Shevchenko National University of Kyiv, Kyiv


The data of visual measurements of the magnetic field in sunspots umbra, obtained in four observatories during 24 cycles of solar activity, are compared. Magnetic field strength modules with averaging over each observatory are analyzed. The measurements taken in the Astronomical Observatory of the University of Kyiv (KAO) were used as base, where 170 spots were measured on average 4 times each at intervals of one to two daysduring the group passing through the solar disk. Measurements of the Ural, Crimean and Mount Wilson Observatory, performed within 24 hours of observation in the KAO, are used in the study. It was found that the average value of the magnetic field per cycle, obtained from the data of three observatories UrAO, CrAO and KAO, was 26.0 ± 0.3, 25.0 ± 0.2 and 25.2 ± 0.2 cT, respectively. They differ by no more than 3 standard errors, while the Mount Wilson Observatory gives a significantly lower magnetic field – 23.8 ± 0.1 cT. This result is confirmed by the work of R. Rezaei et al., 2015, which compares the visual measurements in Mount Wilson with Tenerife Infrared Polarimeter (TIP) and Facility InfRared Spectropolarimeter (FIRS) for the 23rd and 24th cycle of solar activity. The average magnetic field value for observations of TIP and FIRS in 24 cycles is 26.0 cT, which is slightly higher than we obtained from the visual observations of three observatories, while MtW shows a few centitesla less. The mean square error of the average per cycle of the magnetic field, based on the data of TIP and FIRS is 0.2 cT, so the difference with the MtW is reliable. An understatement of the measured intensity of the magnetic field of the spots requires a revision of the measurement calibration in Mount Wilson. The data of the Crimean Astrophysical Observatory can be considered reference for subsequent comparisons with the results of observations of other observatories. For the calculation of the annual index Bsp of magnetic fields of sunspot size 22-44 Mm, the data of visual measurements of all 4 observatories are traditionally used. We have found that variations in the magnetic field of sunspots are the same as in previous cycles: the peak occurs in the second-third years after the maximum number of spots. The average for the 24 cycle Bsp index is 24.9 ± 0.3 сT, which allows us to consider this magnetic cycle as moderately strong. Reduction of the sunspots magnetic field in 2018 to 23 cT allows it to expect in 2019 the highest near-ground level of galactic cosmic rays in the last half century.

Key words
Solar activity, magnetic fields, sunspots, galactic cosmic rays

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DOI: https://doi.org/10.17721/BTSNUA.2018.58.11-17