Hydromagnetic waves in solar wind flow-structures

Abstract

Propagating magnetosonic waves in the observed fine structures of high-speed solar wind streams are theoretically examined, taking into account the non-parallel propagation of such waves. It is found that non-parallel propagation introduces new magnetoacoustic wavemodes, such as the forward-propagating slow surface modes and the backward-propagating fast surface modes that propagate outward in a direction away from the Sun; and the oppositely-propagating (slow and fast) body and surface modes that propagate towards the Sun in a frame co-moving with the ambient flow external to the solar wind flow-sheet. Such a wide variety of wavemodes is not available in a case where the propagation is purely parallel to the axis of the solar wind fine structure. It is argued here that the frequencies (in the satellite frame) of the outwardly propagating waves, that may transport photospheric/chromospheric oscillations into the heliospheric space, possibly lie in the range 6–100 mHz. Such a high-frequency band has not so far been explored for the detection of magnetosonic waves in the high-speed solar wind. Particularly interesting in the present study is the appearance of the sunward-propagating magnetosonic fast body waves . When viewed in a satellite (or solar) frame, such waves propagate towards the Sun provided that their angle of propagation exceeds a certain critical value, so that their propagation-vector is nearly normal to the solar equatorial plane. More high-resolution, multi-point in-situ observations are necessary to detect such sunward-propagating magnetoacoustic fast body waves in the fine flow structures of the solar wind at a distance of 1 AU from the Sun.