Rossby‐Type Wave–Induced Periodicities in Flare Activities and Sunspot Areas or Groups during Solar Maxima

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Abstract

The Gamma-Ray Spectrometer (GRS) aboard the Solar Maximum Mission (SMM) discovered a 154 day periodicity in solar flare rates. Subsequently, periodicities in various solar flare activities and in sunspot areas or groups during a few years around solar maxima have been extensively monitored using different diagnostics and at many electromagnetic wavelengths. Notable periods are ~154, 128, 102, 78, and 51 days during maxima of different solar cycles from various data sets. The origin of such quasi periodicities particularly prominent during solar maxima has remained a mystery for nearly two decades. For slow and large-scale photospheric motions, the shallow magnetofluid approximation may be invoked when the Rossby number 0 ≡ U/(2 Ω☉L) is small, where U (≲103 cm s-1) and L (≳R☉) are typical horizontal velocity and spatial scales. Physical properties of equatorially trapped Kelvin waves, Poincaré waves, Rossby waves, and mixed Rossby-Poincaré waves are examined. For typical solar parameters, period estimates of Rossby and mixed Rossby-Poincaré waves are ~151-155, 126-127, 101-102, 76-78, and 51-54 days, in good agreement with observed periodicities. The effect of large-scale subsurface magnetic fields is estimated. Two methods of directly detecting solar Rossby-type waves are discussed. It would be of interest to examine whether large-scale coronal mass ejections also carry similar periodicities.

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