Oscillations and waves in a sunspot

Abstract

Observations have been made in Hα of the vertical velocity distribution in a sunspot. Over the umbra the pattern consists of structures of scale-size 2–3″. The velocity distribution undergoes oscillations with a period of about 165 s and typical amplitude ±3 km s⁻¹, but the pattern breaks down after one or two cycles because the period of oscillation varies typically by ±20 s from place to place. Transverse waves develop in the outer 0.1 of the umbral radius and propagate outwards with a velocity of about 20 km s⁻¹, becoming gradually invisible by or before the outer penumbral boundary; the amplitude is about ±1 km s⁻¹ at the umbra-penumbra border.

The penumbral waves are believed to be basically of the Alfvén type, with ρ ≈ 3 × 10⁻⁸ g cm⁻³. The umbral oscillations presumably represent gravity waves. In both cases the fluxes are inadequate by two orders of magnitude to account for the sunspot energy deficit.