Stability of cool flux tubes in the solar chromosphere Linear analysis

Abstract

The linear stability of cool flux tubes in the solar chromosphere which are initially in radiative equilibrium is examined. Owing to the presence of carbon monoxide, there exists a narrow region near the temperature minimum where the temperature gradient becomes steep enough to drive a convective instability. The thin flux tube equations are used and in a simple manner radiative heat exchange with the ambient medium are included. Initial states of constant beta (where beta is the ratio of gas to magnetic pressure) are considered. Results for various values of beta are presented. It is found that for beta less than 5.7 the tube is overstable with periods in the range 300-600 s. At beta = 5.7 a bifurcation occurs into two purely growing modes. Growth rates, eigenvectors of the fundamental modes are calculated and phase relationships are examined. It is suggested that overstable oscillations should invariably be associated with cool flux tubes. These oscillations transport energy and can thus change the thermodynamic structure of flux tubes. It is conjectured that the CO overstability may be responsible for spicules.