| Abstract: | A model treating a solar coronal hole as an axially symmetrical magnetic formation that is in equilibrium with the surrounding
medium is proposed. The model is applicable in the lower corona (to heights of the order of several hundreds of Mm), where
the influence of the solar-wind outflow on the state of the system can still be neglected. The magnetic field of the coronal
hole is comprised of a relatively weak open flux that varies with height, which extends into interplanetary space, and a closed
field, whose flux closes at the chromosphere near the coronal hole. Simple analytical formulas are obtained, which demonstrate
for a given equilibrium configuration of the plasma and field the main effect of interest—the lowering of the temperature
and density of the gas in the coronal hole compared to their values in the corona at the same geometric height. In particular,
it is shown that, at heights of several tens of Mm, the temperature and density of the plasma in the coronal hole are roughly
half the corresponding values at the same height in the corona, if the cross-sectional radius of the hole exceeds the scale
height in the corona by roughly a factor of 1.5: R
h
≈ 1.5H(T
0). In the special case when R
h
≈ H(T
0), the plasma temperature in the hole is equal to the coronal temperature, and the darkening of the coronal hole is due only
to an appreciable reduction of the plasma density in the hole, compared to the coronal density. An analogy of the properties
of coronal holes and sunspots is discussed, based on the similarity of the magnetic structures of these formations. In spite
of the fundamental difference in the mechanisms for energy transport in coronal holes and sunspots, the equilibrium distributions
of the plasma parameters in these formations are determined only by the magnetic and gravitational forces, giving rise to
a number of common properties, due to their similar magnetic structures. |
