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Publication . Article . Preprint . 2016 . Embargo end date: 01 Jan 2016

Rieger-type periodicity during solar cycles 14-24: estimation of dynamo magnetic field strength in the solar interior

Eka Gurgenashvili; Teimuraz V. Zaqarashvili; V. Kukhianidze; Ramon Oliver; Jose Luis Ballester; Giorgi Ramishvili; Bidzina M. Shergelashvili; +2 Authors
Open Access

Solar activity undergoes a variation over time scales of several months known as Rieger-type periodicity, which usually occurs near maxima of sunspot cycles. An early analysis showed that the periodicity appears only in some cycles, and is absent in other cycles. But the appearance/absence during different cycles has not been explained. We performed a wavelet analysis of sunspot data from the Greenwich Royal Observatory and the Royal Observatory of Belgium during cycles 14-24. We found that the Rieger-type periods occur in all cycles, but they are cycle-dependent: shorter periods occur during stronger cycles. Our analysis revealed a periodicity of 185-195 days during the weak cycles 14-15 and 24, and a periodicity of 155-165 days during the stronger cycles 16-23. We derived the dispersion relation of the spherical harmonics of the magnetic Rossby waves in the presence of differential rotation and a toroidal magnetic field in the dynamo layer near the base of the convection zone. This showed that the harmonic of fast Rossby waves with m=1 and n=4, where m (n) indicate the toroidal (poloidal) wavenumbers, respectively, perfectly fit with the observed periodicity. The variation of the toroidal field strength from weaker to stronger cycles may lead to the different periods found in those cycles, which explains the observed enigmatic feature of the Rieger-type periodicity. Finally, we used the observed periodicity to estimate the dynamo field strength during cycles 14-24. Our estimations suggest a field strength of 40 kG for the stronger cycles, and 20 kG for the weaker cycles.

Comment: 23 pages, 4 figures, accepted in ApJ

Subjects by Vocabulary

Microsoft Academic Graph classification: Dynamo Field strength Convection zone Astrophysics Differential rotation Sunspot Physics Wavenumber Solar cycle Rossby wave


Solar and Stellar Astrophysics (astro-ph.SR), FOS: Physical sciences, Astrophysics - Solar and Stellar Astrophysics, Space and Planetary Science, Astronomy and Astrophysics, activity [sun], interior [sun], oscillations [sun]

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Funded by
Solar and Space Weather Network of Excellence
  • Funder: European Commission (EC)
  • Project Code: 269299
  • Funding stream: FP7 | SP3 | PEOPLE
FWF| Energy transport in the solar atmosphere: background flows
  • Funder: Austrian Science Fund (FWF) (FWF)
  • Project Code: P 25640
  • Funding stream: Einzelprojekte
FWF| Magnetic Rossby waves on the Sun
  • Funder: Austrian Science Fund (FWF) (FWF)
  • Project Code: P 26181
  • Funding stream: Einzelprojekte
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