Mode of propagation of penumbral waves

Abstract

THE solar atmosphere provides an interesting possibility for the study of low frequency waves both in magnetic and nonmagnetic regions, and the most spectacular result (see ref. 1) has been the discovery of waves propagating outwards over sun-spot penumbrae with periods of 180–240 s, horizontal wavelengths of 2,350–3,800 km, and horizontal phase velocities of 12–25 km s⁻¹. The theoretical interpretation of these waves has aroused considerable interest and, in particular, a number of authors have attempted to identify the precise mode of wave propagation involved. The modes proposed include Alfvén waves¹, acoustic waves², Lamb waves³, and magnetogravity waves of the ‘plus’ type vertically trapped at photospheric levels⁴. In the face of so many possibilities, further progress will probably depend on a more adequate observational knowledge of the actual propagation characteristics of the penumbral waves. Of special importance would be a determination of the direction of the particle velocity with respect to the wave propagation vector.