Sunspots and the physics of magnetic flux tubes. IX. Umbral dots and longitudinal overstability

Journal Article · Thu Nov 15 04:00:00 EST 1979 · Astrophys. J.; (United States)

DOI:https://doi.org/10.1086/157501· OSTI ID:5543377

Parker, E N

Observations of the transient umbral dots that commonly appear in sunspots suggest that the dots are tiny inslands of gas near normal photospheric temperatures. Each dot appears to be an upward intrusion of gas from somewhere beneath the sunspot. With the hypothesis that the subsurface magnetic field of a sunspot splits into many separate flux tubes, with field-free gas between, we suggest that the field-free columns occasionally punch their way up through the overliying magnetic field to the surface, where they appear as the bright, field-free umbral dots. The phenomenon is fostered by several effects. First of all, the temperature of a column of rising gas is enhanced relative to the background by the strong superadiabatic temperature of a column of rising gas is enhanced relative to the background by the strong superadiabatic temperature gradient along which it is moving. Second, the overhead magnetic pressure, through which the column must penetrate to reach the surface, is strongly reduced by the curvature of the lines of force of the sunspot field. Third, the upward intrusion may be initiated by the strong vertical oscillations to which the field-free columns are subject (convective overstability). These effects are illustrated with formal examples. They seem to be sufficient to produce the umbral dots.

• Research Organization: Department of Physics and Department of Astronomy and Astrophysics, University of Chicago
• OSTI ID: 5543377
• Journal Information: Astrophys. J.; (United States), Journal Name: Astrophys. J.; (United States) Vol. 233:4; ISSN ASJOA
• Country of Publication: United States
• Language: English

Related Subjects

640104* -- Astrophysics & Cosmology-- Solar Phenomena
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
ALFVEN WAVES
CONVECTION
DIFFERENTIAL EQUATIONS
EQUATIONS
EQUATIONS OF MOTION
FLUID MECHANICS
HYDRODYNAMICS
HYDROMAGNETIC WAVES
MAGNETIC FIELDS
MAGNETIC FLUX
MAGNETOHYDRODYNAMICS
MECHANICS
OSCILLATION MODES
OSCILLATIONS
SOLAR ACTIVITY
STABILITY
SUNSPOTS