Thermal instability and the convective stability of stellar chromospheres

Citation

Defouw, Richard John (1970) Thermal instability and the convective stability of stellar chromospheres. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/QDN8-FH95. https://resolver.caltech.edu/CaltechETD:etd-05012008-103453

Abstract

It is generally believed that stellar chromospheres are stable against convection since the Schwarzschild criterion indicates stability when temperature increases with height. It is shown, however, that the Schwarzschild criterion does not apply to chromospheres because it ignores the possibility of thermal instability. In the absence of a magnetic field, thermally unstable regions of a chromosphere will be overstable if the temperature inversion is sufficiently steep. This overstability may explain the origin of a certain class of oscillations in the solar chromosphere. Thermally unstable regions containing magnetic fields are monotonically unstable for all values of the temperature gradient. It is suggested that this monotonic instability of magnetic regions is responsible for spicule formation in the solar chromosphere. Elementary considerations of thermal balance predict that the temperature gradient should diverge at levels of marginal stability. The chromospheric region of spicule formation should therefore be bounded below by an abrupt temperature jump.

The above results are derived by analyzing the stability of a simple model chromosphere in which all neutral hydrogen atoms are assumed to be in the ground state. Although the model chromosphere emits only free-bound radiation, its thermal instability is caused by the same ionization effects which lead to instability in plasmas emitting line radiation. Thermally unstable regions of a stellar chromosphere, although not represented in detail by the model, should behave in a similar fashion.

Thesis Files