Oscillations and waves in a sunspot

Abstract

Observations have been made in Ha 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 • 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 -j, 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 Alfv6n type, with 0 ~ 3 • 10 -s g cm-L 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.

Dynamics of the solar atmosphere above a pore with a light bridge

“…Later, umbral oscillations with dominant periods in the five-minute band (3−4 mHz) in the photosphere (Bhatnagar et al 1972;Lites 1992) and in the three-minute band (5−6 mHz) in the chromosphere (Beckers & Schultz 1972;Brynildsen et al 1999) were found and have been extensively studied since (see Bogdan & Judge 2006, for a review and more references). Running penumbral waves (Giovanelli 1972;Zirin & Stein 1972) are disturbances propagating radially in the form of dark and bright arcs from the inner to the outer penumbral boundaries. Their phase speed is around 10−15 km s −1 and they have a frequency in the five-minute band (Christopoulou et al 2000;Tziotziou et al 2006).…”

Observational characteristics and association of umbral oscillations and running penumbral waves

“…Three years later, intensity and/or velocity in various spectral-line observations revealed the existence of the 5-min oscillations in the umbral photosphere (Bhatnagar et al 1972) and of the 3-min oscillations in the umbral chromosphere (Beckers & Schultz 1972;Giovanelli 1972). Chromospheric umbral oscillations are present in almost every sunspot umbra, whereas umbral flashes are localized phenomena that are observed occasionally when the velocity amplitudes exceed a certain value depending on the line.…”

“…Chromospheric umbral oscillations are present in almost every sunspot umbra, whereas umbral flashes are localized phenomena that are observed occasionally when the velocity amplitudes exceed a certain value depending on the line. Zirin & Stein (1972) and Giovanelli (1972) were the first to realize that the penumbra oscillates as well. Using filtergrams obtained in the wings of Hα they detected concentric rings moving from the inner to the outer penumbra; these are called running penumbral (RP) waves.…”

Oscillations on the Sun in regions with a vertical magnetic field

“…In the beginning of the 70s the discovery of sunspot umbral oscillations Giovanelli 1972;Beckers & Schultz 1972;Bhatnagar & Tanaka 1972) attracted attention of many theorists, as it seemed that the umbral oscillations could provide unique information about the sub-photospheric structure of sunspots.…”