QUASI-BIENNIAL OSCILLATIONS IN THE SOLAR TACHOCLINE CAUSED BY MAGNETIC ROSSBY WAVE INSTABILITIES

Abstract

Quasi-biennial oscillations (QBO) are frequently observed in the solar\nactivity indices. However, no clear physical mechanism for the observed\nvariations has been suggested so far. Here we study the stability of magnetic\nRossby waves in the solar tachocline using the shallow water\nmagnetohydrodynamic approximation. Our analysis shows that the combination of\ntypical differential rotation and a toroidal magnetic field with a strength >\n10^5 G triggers the instability of the m=1 magnetic Rossby wave harmonic with a\nperiod of 2 years. This harmonic is antisymmetric with respect to the equator\nand its period (and growth rate) depends on the differential rotation\nparameters and the magnetic field strength. The oscillations may cause a\nperiodic magnetic flux emergence at the solar surface and consequently may lead\nto the observed QBO in the solar activity features. The period of QBO may\nchange throughout the cycle, and from cycle to cycle, due to variations of the\nmean magnetic field and differential rotation in the tachocline.\n