Issue |
A&A
Volume 424, Number 2, September III 2004
|
|
---|---|---|
Page(s) | 691 - 712 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20040310 | |
Published online | 23 August 2004 |
On the creation of the chromospheres of solar type stars
Institute for Scientific Research, 2500 Fairmont Avenue – Suite 734, Fairmont, WV 26555-2720, USA e-mail: mgoodman@isr.us
Received:
20
February
2004
Accepted:
19
April
2004
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 ,
and characteristic diameters <180 km. Assuming
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
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
, 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.
Key words: Sun: chromosphere / Sun: photosphere / magnetohydrodynamics (MHD) / Sun: magnetic fields / stars: chromospheres / magnetic fields
© ESO, 2004
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