Rossby-type wave-induced periodicities in flare activities and sunspot areas or groups during solar maxima

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 similar to 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 R(0) = U/(2 Omega. L) is small, where U (less than or similar to 10(3) cm s(-1)) and L (greater than or similar to R.) are typical horizontal velocity and spatial scales. Physical properties of equatorially trapped Kelvin waves, Poincare waves, Rossby waves, and mixed Rossby-Poincare waves are examined. For typical solar parameters, period estimates of Rossby and mixed Rossby-Poincare waves are similar to 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.