Observing the dynamic Sun: MHD waves in the chromosphere

Mooroogen, Krishna (2018) Observing the dynamic Sun: MHD waves in the chromosphere. Doctoral thesis, Northumbria University.

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Abstract

Due to their ubiquity in the atmosphere magnetohydrodynamic waves have been theorised to play an important role in the coronal heating problem. The kink wave in particular, due to its Alfvenic qualities, is thought to be able to transport energy over the large distances of the stratified solar atmosphere. In the principal study, 'Dynamics of internetwork chromospheric fibrils: basic properties and MHD kink waves', we provide the first investigation on the nature and dynamics of elongated absorption features (fibrils) observed in H-α in the internetwork. The fibrils were found to support wave-like behaviour that we interpret as the MHD kink wave. Using the spectroscopic imaging capabilities of the Swedish Solar Telescope, multi-scale image processing, Fourier analysis and sub-pixel model fitting we provide a statistical study of these waves.

In the second study, 'Observational evidence of Kink waves in Ca II (8541 Å) superpenumbral fibrils'; we present the first observation of kink waves in a sunspot superpenumbra. By employing similar methodologies to the first study, we suggest that the kink waves may be the result of mode conversion of low-frequency magnetoacoustic waves propagating from the lower atmosphere.

Of both studies, we present the measured properties of the amplitudes and periods of the kink waves; we found similar behaviour to those previously studied in different atmospheric regions. However, we provide one of the few comprehensive measurements of the wave propagation speed, that serves as an approximation to the local Alfven speed. We find that relative to the Ca II data, the H-α kink waves have large phase speeds. Further, we exploit the wave properties via inversion to perform magnetoseismology. With this, we are able to infer the variation of plasma properties of the fibrils. Lastly, we develop average power spectra of the quiescent H-α and active Ca II chromosphere as a method of examining the local propagation of wave power. On comparison to previous results we observe amplification of power from the photosphere to chromosphere, supporting the notion of a stratified atmosphere.

The results of the measurements serve to constrain the properties of internetwork and super-penumbral fibrils to aid in the understanding of their role as wave guides. The discovery of kink waves in the Ca II super-penumbra may prove to be a source of Alfvenic waves in active regions in the corona.

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