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The pressure and energy balance of the cool corona over sunspots
Foukal, P. V.
AA(Smithsonian Astrophysical Observatory and Harvard College Observatory, Cambridge, Mass.)
Astrophysical Journal, vol. 210, Dec. 1, 1976, pt. 1, p. 575-581. (ApJ Homepage)
Publication Date:
Solar Physics
NASA/STI Keywords:
Energy Spectra, Far Ultraviolet Radiation, Plasma Temperature, Solar Corona, Spaceborne Astronomy, Sunspots, Apollo Telescope Mount, Emission Spectra, Energy Distribution, Magnetohydrodynamic Stability, Pressure Distribution, Radiant Flux Density, Radiative Transfer, Temperature Distribution, Thermal Instability, Ultraviolet Spectrometers
Bibliographic Code:


The 22 largest sunspots observed with the Skylab SO55 spectrometer are studied for a relation between their EUV radiation and their umbral size or magnetic classification. The ultimate goal is to determine why the coronal plasma is so cool over a sunspot and how this cool plasma manages to support itself against gravity. Based on the time behavior of the EUV emission, a steady-state model is developed for the pressure and energy balance of the cool coronal-plasma loops over the spots. Analysis of the temperature structure in a typical loop indicates that the loop is exceedingly well insulated from the outside corona, that its energy balance is determined purely by internal heating and cooling processes, and that a heat input of about 0.0001 erg/cu cm per sec is required along the full length of the loop. It is proposed that: (1) coronal material flows steadily across the field lines at the tops of the loops and falls downward along both sides under gravity; (2) the corona is heated by mechanical-energy transport across the very thin transition region immediately over network-cell interiors; and (3) strong magnetic fields tend to inhibit mechanical-energy dissipation in the corona.

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