Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004a%26a...424..691g&link_type=abstract
Astronomy and Astrophysics, v.424, p.691-712 (2004)
Astronomy and Astrophysics
Astrophysics
16
Sun: Chromosphere, Sun: Photosphere, Magnetohydrodynamics (Mhd), Sun: Magnetic Fields, Stars: Chromospheres, Magnetic Fields
Scientific paper
A mechanism that creates the chromospheres of solar type stars everywhere outside of flaring regions is proposed. The identification of the mechanism is based on previous work and on the results of a model presented here that computes the electric current, its driving electric field, the heating rate due to resistive dissipation, and the flow velocity in a specified class of horizontally localized, two dimensional magnetic structures in the steady state approximation. The model is applied to the Sun over the height range from the photosphere to the upper chromosphere. Although the model does not contain time explicitly, it contains information about the dynamics of the atmosphere through inputs from the FAL CM solar atmosphere model, which is based on time averages of spectroscopic data. The model is proposed to describe the time averaged properties of the heating mechanism that creates the chromosphere. The model magnetic structure is horizontally localized, but describes heating of the global chromosphere in the following way. Recent observations indicate that kilogauss strength magnetic structures exist in the photospheric internetwork with a filling factor f˜ 2%, and characteristic diameters < 180 km. Assuming f = 2 % and a maximum field strength of 103 G for the model magnetic structure, and assuming that the chromospheric heating rate predicted by FAL CM represents a horizontal spatial average over such magnetic structures, it is found that the model magnetic structures that best reproduce the FAL CM heating rate as a function of height have characteristic diameters in the range of 98 - 161 km, consistent with the upper bound inferred from observation. Based on model solutions and previous work it is proposed that essentially all chromospheric heating occurs in magnetic structures with sub-resolution horizontal spatial scales (⪉ 150 ; km), that the heating is due to dissipation of Pedersen currents driven by a convection electric field, and that it is the increase in the magnetization of particles with height in a magnetic structure from values ≪1 in the lower photosphere to values ⪆1 near the height of the temperature minimum in the magnetic structure that causes the Pedersen current dissipation rate to increase to a value large enough to cause a temperature inversion. The magnetization of a particle is the ratio of its cyclotron frequency to its total collision frequency with unlike particle species.
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