Origin of long-period Alfv{\'e}n waves in the solar wind

Abstract

We suggest that the observed long-period Alfv{\'e}n waves in the solar wind may be generated in the solar interior due to the pulsation of the Sun in the fundamental radial mode. The period of this pulsation is about 1 hour. The pulsation causes a periodical variation of density and large-scale magnetic field, this affecting the Alfv{\'e}n speed in the solar interior. Consequently the Alfv{\'e}n waves with the half frequency of pulsation (i.e. with the double period) can be parametrically amplified in the interior below the convection zone due to the recently suggested swing wave-wave interaction. Therefore the amplified Alfv{\'e}n waves have periods of several hours. The waves can propagate upwards through the convection zone to the solar atmosphere and cause the observed long-period Alfv{\'e}n oscillations in the solar wind.