Wave phenomena in sunspots

The highly dynamic magnetised solar atmosphere exhibits a wealth of oscillatory magneto-hydrodynamic (MHD) waves. These MHD waves are thought to play an important role in the transport of energy to the solar chromosphere and corona since they are channeled by the magnetic fields.

Circular sunspot recorded on August 21, 2013 with the IBIS & ROSA instruments (DST) and HMI (SDO) in various wavelengths, i.e. in different atmospheric layers.

Oscillatory phenomena present in sunspots are investigated from an observational point of view by means of a 1-hour time series acquired with the ROSA and IBIS instruments at the Dunn Solar Telescope in Sacramento Peak, USA. The data consist of high-resolution observations taken simultaneously at different wavelengths in the solar spectrum, i.e., at different layers in the solar atmosphere.

Powerful brightenings shocking the darkest part of the sunspot —umbral flashes—, as well as waves propagating along the filamentary penumbra —running penumbral waves— appear continuously with a periodicity of 2-3 minutes (see animation). Against the former belief that umbral flashes and running penumbral waves are of pure chromospheric nature, we find that their first imprints can be tracked down to high photospheric layers (at around 400km above the solar surface).

The analysis of wave energy sampled at different atmospheric heights suggests that

(1) both phenomena are realisations of the same upward propagating shock along the magnetic field of the sunspot and

(2) the magnetic field geometry in sunspots can be inferred from the frequency distribution of running penumbral waves.

The comprehensive dataset used for this analysis was acquired under the SOLARNET access program and processed within the framework of the CASSDA project.