Solar p-modes: possible source of excitation and frequency splitting
Received 4 March 1998 / Accepted 27 August 1998
A new possible source of solar 5-minute oscillation is supposed. Weak periodic differential rotation (see Zaqarashvili 1997, hereafter paper I) generates exponentially growing slow magnetosonic waves with period of 22 yr in the radiative interior. The waves propagating with angles to the radial direction have maximal growth rates. The ideal MHD equations for slow and fast waves are linearly coupled due to the differential rotation. But linear coupling is inactive for the conditions in radiative interior. However preliminary non-linear analyses show energy resonant transformation from slow waves to the fast one. The period of resonant fast magnetosonic waves is shortest due to the large value of plasma . The period is 5 minute for . These fast magnetosonic waves transform into sound waves in unmagnetized convective envelope.
Frequencies of the sound waves are changed during propagation along the differentially rotating convective envelope. The frequency of the waves propagating in the direction of rotation is increased, but the frequency of the waves propagating in opposite direction of the rotation is decreased. It leads to a frequency splitting at the surface. The slight periodical variation of the differential rotation causes cyclic behaviour of the splitting. The rate and the phase of the splitting variation are in good agreement with the observations (Jimenez et al. 1994). Maximal splitting corresponds to the maximum of the solar activity.
Key words: MHD waves Sun: magnetic fields Sun: oscillations
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© European Southern Observatory (ESO) 1999
Online publication: December 4, 1998