Numerical simulations of spicules driven by weakly-damped Alfven waves - I. WKB approach

We present results of time-dependent 1.5 dimensional numerical simulations of the effects that upward travelling Alfven waves, damped by ion-neutral collisions, have on the chromospheric plasma in a vertical magnetic flux tube. Assuming a rigid flux tube, we use a combination of hydrodynamic equations and a transport equation for the wavelength-averaged wave action density (using the WKB assumption). We find that the damping of a continuous train of upward travelling Alfven waves with a frequency of 0.5 Hz causes enough upward momentum transfer and heating nf thp plasma to form structures that are similar to chromospheric spicules in many aspects. We use a non-LTE approximative formula for the hydrogen ionization and assume optically thin radiative losses in the spicular environment. We find that the formed structure reaches a maximum height of 6000 km, temperatures between 8000 to 12000 K, electron number densities of the order 10(17) m(-3) and maximal velocities of about 20 km s(-1). The lifetime of our structure depends on the lifetime of the wave source and can be brought into accordance with observed spicular lifetimes.