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Y-.Q. Lou (The University of Chicago)
The Gamma-Ray Spectrometer (GRS) aboard the Solar Maximum Mission (SMM) first revealed a 154-day periodicity in solar flare rates. Subsequently, periodicities in various solar flare activities and 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 long-time periodicities particularly prominent during solar maxima remains a mystery for nearly two decades. For slow and large-scale photospheric dynamics, the shallow magnetofluid approximation can be invoked when the Rossby number {\cal R}0\equiv U/(2\Omega\odot L) is small, where U (\lesssim 103cm s-1) and L (\gtrsim R\odot) 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 inferred periodicities. The effect of large-scale subsurface magnetic fields is estimated. Two methods (i.e., surface velocity correlations and precise diameter measurements) of directly detecting equatorially trapped solar Rossby-type waves are emphasized. Plausible connections among the large-scale photospheric wave dynamics, Rossby radius of deformation, sunspot zones, subsurface magnetic fields, magnetic flux emergence, active regions and solar flare activities are discussed. The scenario of a dynamic feedback cycle is advanced for excitations of these low-frequency Rossby-type waves during solar maxima. It would be of considerable interest to examine whether large-scale coronal mass ejections also carry the similar periodicities in the current solar maximum using SOHO observations.
This work was supported in part by grants of NSF (ATM-9320357 and AST-9731623) to the U. of Chicago and of the Institute of Astronomy and Astrophysics, Academia Sinica (NSC-88-2816-M-001-0010-6).