Oscillations in sunspot umbrae due to trapped Alfvén waves excited by overstability.
Abstract
Oscillations detected in sunspot umbras are considered to be a vertical atmospheric motion caused by a standing Alfven wave trapped in the region between an overstable subphotospheric layer and the chromosphere-corona transition layer. It is suggested that the wave motion is excited by overstable convection at the base of the oscillating region. The fundamental eigenfrequency is found to correspond to a period of 140 to 180 sec for oscillations in the umbra of a typical sunspot, and it is shown that the eigenfrequencies depend primarily on the Alfven velocity at the base of the oscillating layer. Previous interpretations of this phenomenon are briefly discussed.
- Publication:
-
Publications of the Astronomical Society of Japan
- Pub Date:
- 1975
- Bibcode:
- 1975PASJ...27..259U
- Keywords:
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- Eigenvectors;
- Magnetohydrodynamic Stability;
- Magnetohydrodynamic Waves;
- Resonant Frequencies;
- Sunspots;
- Umbras;
- Chromosphere;
- Magnetic Flux;
- Oscillations;
- Photosphere;
- Standing Waves;
- Temperature Gradients;
- Transition Layers;
- Velocity Distribution;
- Vertical Motion;
- Wave Excitation;
- Solar Physics