NUMERICAL SIMULATIONS OF THE REBOUND SHOCK MODEL FOR SOLAR SPICULES

Using time-dependent numerical simulations, we have examined the proposed rebound shock mechanism for spicules. The calculations include heat conduction and an approximate treatment of radiation. At temperatures above a critical value, Tc, the radiation is characteristic of the conditions in the optically thin corona and near optically thin transition region. When T < Tc, the atmosphere has a radiative cooling time, τrad, characteristic of the chromosphere. We initiate the spicule with a quasi-impulsive force in the low chromosphere, which drives a train of upward propagating rebound shocks along the rigid magnetic flux tube. These shocks then move the transition region upward. The material below the displaced transition region has temperatures and densities similar to those of spicules when Tc. ≥ 20,000 K and τrad = > 500 s, but not when Tc = 10,000 K, and probably not when τrad = 100 s. For all the cases we have studied where the cross sectional area diverges rapidly with height, the upward velocity of the transition region is less than that of spicules. Moreover, the maximum height is less than that of average spicules. Taller (maximum height ≃ 10,000 km), higher velocity (≃ 23 km s-1) spicules result when the magnetic field cross sectional area is constant. In all cases, the rebound shock mechanism produces substantial motions and temperature and density variations in chromospheric and transition region material. We suggest that this may be a partial explanation for the con-tinuous dynamic state of the lower solar atmosphere.