NONLINEAR EXCITATION OF MAGNETIC UNDULAR INSTABILITY BY CONVECTIVE MOTION

The influence of convective motions on the evolution of the undular mode of magnetic buoyancy (the Parker instability) of an isolated horizontal flux sheet in the solar atmosphere is studied. The flux sheet is embedded in a two-temperature layer atmosphere (solar photosphere/chromosphere and its overlying much hotter corona) with a convection zone underneath. The atmosphere is assumed to be stratified under a constant gravitational acceleration. Convective motions considered are horizontal photospheric shear flows and vertical velocity fluctuations in the convectively unstable layer below the photosphere. The evolution is numerically studied in a two-dimensional space by using a two-and-one-half-dimensional code of ideal magnetohydrodynamics. Even if the initial magnetic flux sheet is stable to the Parker instability γ > γc, where γ is the gas constant, or not, the horizontal velocity shear causes destabilization and drives the expansion of magnetic flux into the corona. As the instability develops, the gas slides down the expanding loop, and the evacuated loop rises as a result of the enhanced magnetic buoyancy, which is similar to the nonlinear evolution of a flux loop that was originally linearly Parker unstable. Other signatures such as shock waves in the downflow region, self-similar loop expansion, etc., are also similar. Vertical velocity fluctuations in the underlying convection zone also lead to destabilization as long as the initial flux is localized within or just above the convectively unstable layer. If the initial flux is embedded in the higher layer, however, convective motions are not able to excite the Parker instability. Application to active region prominence is briefly discussed.