Magneto-Rossby waves in the solar tachocline and the annual variations in solar activity

Abstract

<p><em>[eng] Annual oscillations have been detected in many indices of solar activity during many cycles. Recent multispacecraft</em></p><p><em>observations of coronal bright points revealed slow retrograde toroidal phase drift (with the speed of</em></p><p><em>∼3 m s−1) of 1 yr oscillations, which naturally suggested their connection with Rossby-type waves in the interior.</em></p><p><em>We have studied, from a theoretical point of view, the dynamics of global magneto-Kelvin and magneto-Rossby</em></p><p><em>waves in the solar tachocline with toroidal magnetic field. Using spherical coordinates, the dispersion relations of</em></p><p><em>the waves and latitudinal structure of solutions were obtained analytically. We have also obtained the spectrum of</em></p><p><em>unstable magneto-Rossby wave harmonics in the presence of the latitudinal differential rotation. Estimated periods</em></p><p><em>and phase speeds show that the magneto-Rossby waves rather than the Kelvin waves match with the observations</em></p><p><em>of 1 yr oscillations. On the other hand, Morlet wavelet analysis of Greenwich Royal Observatory sunspot areas for</em></p><p><em>the solar cycle 23 has revealed multiple periodicities with periods of 450–460, 370–380, 310–320, 240–270, and</em></p><p><em>150–175 days in hemispheric and full disk data. Comparison of theoretical results with the observations allow us to</em></p><p><em>conclude that the global magneto-Kelvin waves in the upper overshoot tachocline may be responsible for the</em></p><p><em>periodicity of 450–460 days (∼1.3 yr), while the remaining periods can be connected with different harmonics of</em></p><p><em>global fast magneto-Rossby waves.</em></p>