Detection of running penumbral waves in the photosphere of sunspots

Sunspot of NOAA11823 on August 21st 2013 at 15:00:06UTC in intensity (left) and peak period of wave power (right) at various photospheric and chromospheric wavelength positions. The periods are displayed from 2.5 min (blue) to 8 min (red). The black contours indicate the location of the umbra and penumbra in continuum intensity (bottom panel).

The most prominent wave phenomena in the atmosphere above sunspots are the so-called umbral flashes and running penumbral waves. In chromospheric layers they appear as recurrent vigorous brightenings flashing in sunspot umbrae and excited waves propagating radially along penumbrae. Visually these phenomena resemble the effect of a stone falling into water.

A novel high-resolution, multi-layer study concentrating on the characteristics of running penumbral waves finally proves their photospheric origin. A very regular sunspot in NOAA 11823 was observed with the Interferometric BIdimensional Spectro-polarimeter (IBIS/DST) for one hour in various photospheric and chromospheric wavelength. The detailed analysis of wave power and propagation gives strong evidence for the scenario of running penumbral waves being upward propagating slow-mode waves guided by the field lines of the sunspot magnetic field and intimately coupled to the spectacular umbral flashes phenomena.

Running penumbral waves are detected at photospheric layers. Those continuous oscillations occur preferably at periods between 4-6 min starting at the inner penumbral boundary of the sunspot. The photospheric oscillations have a slightly delayed, more prominent chromospheric counterpart with larger relative velocities, linked to preceding umbral flash events. In all layers the power of running penumbral waves follows a filamentary fine-structure and shows a typical ring-shaped power distribution increasing in radius for larger wave periods. The spatial shift of peak periods across the sunspot, as well as the decrease of apparent horizontal velocities for running penumbral waves from ≈ 50 km/s at photospheric layers to ≈ 37 km/s at chromospheric heights, reflects field-guided wave propagation and the dependence of the acoustic cut-off on the magnetic field inclination.

 

References

Löhner-Böttcher & Bello González (2015), arXiv:1503.09106

Bloomfield, D. S., Lagg, A., & Solanki, S. K. 2007, ApJ, 671, 1005

Jess, D. B., Reznikova, V. E., Van Doorsselaere, T., Keys, P. H., & Mackay, D. H., 2013, ApJ, 779, 168

 

Acknowledgements

The data were acquired in service mode operation within the transnational ACCESS program of SOLARNET, an EU-FP7 integrated activity project. The instruments IBIS and ROSA at the Dunn Solar Telescope (DST, NSO) were operated by INAF and QUB personnel.