Computer Science – Sound
Scientific paper
Jan 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990apj...348..346e&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 348, Jan. 1, 1990, p. 346-356.
Computer Science
Sound
19
Magnetic Flux, Perturbation Theory, Solar Oscillations, Sunspots, Gravitational Effects, Magnetohydrodynamic Waves, Solar Atmosphere
Scientific paper
The modes of oscillation of an isolated magnetic flux tube in the absence of gravity is examined, with parameters chosen to mimic a sunspot. Gravitational stratification of the umbral atmosphere leads to consider two cases, distinguished primarily by the ordering of the Alfven speed and the external sound speed. The transition between these two regimes occurs at about the level where the optical depth, tau(c), is equal to 1 in the umbra. The modes given by the model, taken together with the observations, suggest that 3 minute oscillations are slow-body modes (driven by overstable convection) and that a sunspot consists of a bundle of pore-sized flux tubes rather than a single 'monolithic' one. Fast-body modes are identified in the tube with the observed 5 minute oscillations of the umbral photosphere and below. The excitation of these modes propagating up or down the tube may explain the recent observation that sunspots act as sinks for p-modes propagating in their environment. Running penumbral waves are associated with fast- and slow-surface modes. The fast-surface wave could arise from fast-body modes driven below the level where tau(c) = 1; the slow-surface waves may arise from granular buffeting or overstable convection.
Evans David J.
Roberts Barney B.
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