Compressive waves, often identified from the perturbations in intensity, are a common sight in sunspot observations. Dr. Krishna Prasad, from Queen's University Belfast (UK), explains how they propagate.
Credit: Observations taken with the Dunn Solar Telescope and IRIS - The Interface Region Imaging Spectrograph.
Sunspots, the dark spots on the Sun, harbour a multitude of waves and oscillations. Compressive waves, often identified from the perturbations in intensity, are a common sight in sunspot observations. The strong magnetic fields of sunspots enable the propagation of the compressive waves from the visible surface of the Sun - the photosphere - where the waves are believed to be generated, all the way up to the outermost layer of the solar atmosphere - the corona - where the waves eventually get dissipated.
The movie shows the behaviour of the compressive waves observed in a sunspot across different layers of the solar atmosphere from near the photosphere to the transition region, a thin layer beneath the corona. The left panel displays the image sequences across multiple layers depicting the changes in the structure of the sunspot and the oscillatory pattern within the umbra (outlined by a white contour), whereas the right panel shows the respective normalised power spectra.
Notably, the oscillatory power within the 3-20 mHz range (grey region) increases up to a certain height, represented by Ca II K, and decreases at greater heights. In contrast, the corresponding energy flux was found to decrease right from the photosphere, even in the layers where the oscillation power has displayed increased levels. The reduction in the energy flux, nearly by a million units across those layers, implies a significant loss in the wave energy which, therefore, may be crucial for the solar atmospheric heating. The energy loss could be due to both dissipative (e.g., radiative losses, shock dissipation) and non-dissipative (e.g., transfer to other wave modes) processes, the contributions of which we are yet to decipher. The multi-wavelength and ultra-high-resolution capabilities of the European Solar Telescope will be extremely useful in resolving such issues.
For further details, see Krishna Prasad et al., 2017, ApJ, 847