MAGNETIC ROSSBY WAVES IN THE SOLAR TACHOCLINE AND RIEGER-TYPE PERIODICITIES

Abstract

Apart from the 11-year solar cycle, another periodicity around 155-160 days\nwas discovered during solar cycle 21 in high energy solar flares, and its\npresence in sunspot areas and strong magnetic flux has been also reported. This\nperiodicity has an elusive and enigmatic character, since it usually appears\nonly near the maxima of solar cycles, and seems to be related with a periodic\nemergence of strong magnetic flux at the solar surface. Therefore, it is\nprobably connected with the tachocline, a thin layer located near the base of\nthe solar convection zone, where strong dynamo magnetic field is stored. We\nstudy the dynamics of Rossby waves in the tachocline in the presence of a\ntoroidal magnetic field and latitudinal differential rotation. Our analysis\nshows that the magnetic Rossby waves are generally unstable and that the growth\nrates are sensitive to the magnetic field strength and to the latitudinal\ndifferential rotation parameters. Variation of the differential rotation and\nthe magnetic field strength throughout the solar cycle enhance the growth rate\nof a particular harmonic in the upper part of the tachocline around the maximum\nof the solar cycle. This harmonic is symmetric with respect to the equator and\nhas a period of 155-160 days. A rapid increase of the wave amplitude could give\nplace to a magnetic flux emergence leading to observed periodicities in solar\nactivity indicators related with magnetic flux.\n