Propagating MHD Waves

8.4 Summary

Propagating MHD waves have moving nodes, in contrast to standing modes with fixed nodes. Propagating MHD waves result mainly when disturbances are generated impulsively, on time scales faster than the Alfvén or acoustic travel time across a structure.

Propagating slow-mode MHD waves (with acoustic speed) have been recently detected in coronal loops using TRACE and EIT, usually being launched with 3-minute periods near sunspots, or with 5-minute periods away from sunspots. These acoustic waves propagate upward from a loop footpoint and are quickly damped, never being detected in downward direction at the opposite loop side. Propagating fast-mode MHD waves (with Alfvén speeds) have recently been discovered in a loop in optical (SECIS eclipse) data, as well as in radio images (from Nobeyama data).

Besides coronal loops, slow-mode MHD waves have also been detected in plumes in open field regions in coronal holes, while fast-mode MHD waves have not yet been detected in open field structures. However, spectroscopic observations of line broadening in coronal holes provide strong support for the detection of Alfvén waves, based on the agreement with the theoretically predicted scaling between line broadening and density, Δv(h)αn_e(h)^−1/4.

The largest manifestation of propagating MHD waves in the solar corona are global waves that spherically propagate after a flare and/or CME over the entire solar surface. These global waves were discovered earlier in Hα, called Moreton waves, and recently in EUV, called EIT waves, usually accompanied with a coronal dimming behind the wave front, suggesting evacuation of coronal plasma by the CME. The speed of Moreton waves is about three times faster than that of EIT waves, which still challenges dynamic MHD models of CMEs.