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Sunspots and the physics of magnetic flux tubes. IX - Umbral dots and longitudinal overstability
Parker, E. N.
AA(Chicago, University, Chicago, Ill.)
Astrophysical Journal, Part 1, vol. 234, Nov. 15, 1979, p. 333-347. (ApJ Homepage)
Publication Date:
Solar Physics
NASA/STI Keywords:
Magnetic Flux, Magnetohydrodynamic Stability, Sunspots, Umbras, Dynamic Stability, High Temperature Gases, Longitudinal Stability, Magnetohydrodynamic Waves, Perturbation Theory, Solar Magnetic Field
Bibliographic Code:


The dynamical properties of the sunspot field and of a column of hot gas confined by such a vertical magnetic field are examined in order to understand the umbral dot within the context of the magnetic sunspot structure. Attention is given to the conditions necessary for gas intrusion, longitudinal as well as convective overstability, the growing modes, and the even mode. With the hypothesis that the subsurface magnetic field of a sunspot splits into many separate flux tubes with field-free gas between, it is suggested that the field-free columns occasionally punch their way up through the overlying magnetic field to the surface, appearing there as the bright, field-free umbral dots. Effects fostering the phenomenon are also discussed, that is, the enhanced temperature of a column of rising gas, the strongly reduced overhead magnetic pressure, and the initiated upward intrusion; these effects are illustrated with examples.

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