Convective instability of a model chromosphere

Abstract

The convective stability of a simple model chromosphere is investigated. The model chromosphere consists of protons, electrons, and hydrogen atoms in the ground state; ionization is collisional and recombination is radiative. The analysis indicates stability when the kinetic temperature (T) is less than 17 500K (assuming T increases with height). However, for T > 17 500K, the model chromosphere is overstable in the absence of magnetic fields provided the temperature inversion is sufficiently steep. For smaller values of the temperature gradient, field-free regions are stable if the density is small and monotonically unstable if it is large. In the presence of a magnetic field, the model chromosphere is monotonically unstable for T > 17 500K, regardless of the temperature gradient.

The convective instability of the model chromosphere results from the fact that the plasma is thermally unstable for T > 17 500K. Thermally unstable regions of the solar atmosphere, although not represented in detail by the model, should behave in a similar fashion.

Field-free regions of the solar chromosphere are probably not monotonically unstable, but overstability is possible and may explain the origin of chromospheric oscillations with periods less than 200 sec. It is suggested that spicules result from the monotonic instability of magnetic regions. A similar instability in the corona may be responsible for the large Doppler spreading of radar echoes.

Elementary considerations of thermal balance predict that the temperature gradient should diverge at levels of marginal stability. The chromospheric region of spicule formation and the corona should therefore both be bounded below by abrupt temperature jumps.