A search for the Rieger-type periodicity on the Sun- like stars

von Eka Gurgenashvili
Dissertation
Datum der mündl. Prüfung:2023-04-12
Erschienen:2023-10-06
Betreuer:Prof. Dr. Ansgar Reiners
Gutachter:Prof. Dr. Jens Niemeyer
Gutachter:Prof. Dr. Jose Luis Ballester
Gutachter:Prof. Dr. Stefan Dreizler
Gutachter:Dr. Timo Reinhold
Gutachter:Prof. Dr. Teimuraz Zaqarashvili
Zum Verlinken/Zitieren: http://dx.doi.org/10.53846/goediss-10101

 

 

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Englisch

Solar activity undergoes variations over many different timescales. The main periodicity is 11 years, called Schwabe cycle. In addition to the 11-year cycle, shorter periodic variations known as Rieger cycles have been found in solar activity. Observations over several decades showed that the periodicity always appears near Schwabe cycle maxima and permanently changes its value from cycle to cycle. Observations also revealed that the Rieger periods are shorter during stronger cycles and vice versa. The Rieger periodicity is probably connected to the magnetic Rossby waves in the internal dynamo layer, therefore its cycle to cycle variation can be explained by different dynamo mag- netic field strengths in individual cycles. Then the dynamo field strength in each cycle can be estimated by the observed periods and the theory of magnetic Rossby waves. This method can be a very useful tool also for Sun-like stars. Recent space missions (Kepler, CoRoT, TESS) have collected a large stellar database, which allows us to look for Rieger-type cycles in other Sun-like stars. Before looking for the stellar cycles, we examined the total solar irradiation (TSI) in cycles 23-24 from SOHO/VIRGO data. TSI, which includes the combined contribution of sunspots and faculae, is analogous to stellar light curves and hence permits to study the Sun as a star. We found strong peaks around 180 and 115 days in cycle 23, and 170 and 145 days in cycle 24. Using magnetic Rossby wave theory, we estimated the magnetic field strength in the solar dynamo layer as 10-15 kG. The Rieger period on the Sun is around 5-7 times longer than the rotation period. Therefore we searched for the solar-type stars, that fulfill the same condition. We found a Sun-like star KIC 2852336 from Kepler data, which showed a clear peak near Rieger time scale in addition to the rotation period, making it a potential Rieger-type candidate. We computed the Lomb-Scargle periodogram and a wavelet power spectrum from the light curve. Periods were found at 9.5 days and 60 days. The 60-day periodicity in the stellar light curve is entirely consistent with the solar case. We used several methods to rule out the possible instrumental appearance of the period and confirmed that it is related with the stellar activity. Then, we estimated the strength of the dynamo magnetic field inside the star as 40 kG. It is found that the ratio of the rotation period and dynamo field strength has the same value for the star and the Sun.
Keywords: sun; sun magnetic fields; sun activity; star activity; stars magnetic fields; stars solar-type
 
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