The Effect of Horizontal Density-Inhomogeneity on Spicules Driven by Vertical Velocity Pulses

Observations have shown that the chromosphere networks are rich in features with density higher than the ambient atmosphere. To investigate the effect of horizontal density-inhomogeneity on spicules, here we carry out two-dimensional magnetohydrodynamic (MHD) simulations based on the shock scenario. In a gravitationally stratified solar atmosphere, we insert a vertical preexisting density structure (PeDS) that has higher density than the ambient regions, and then we drive a spicule by a velocity pulse at the bottom of the chromosphere. We find a horizontal flow of 2 km s-1 caused by a rarefaction wave that may have a certain material supplement effect for the spicules and a V-shaped shock front in the chromosphere. An interesting feature found in our experiment is that the existence of PeDS leads to the formation of multiple threads in spicules. Their formation results from the larger density, lower transition region, and higher speeds of magnetoacoustic waves in the PeDS than at its outer boundaries. Parameter studies show that multiple threads of a spicule can be more pronounced in cases with wider velocity pulses and a larger internal/external density ratio in the PeDS. Our study shows that the horizontal density-inhomogeneity in the solar atmosphere is an important factor that is responsible for the complexity of a spicule.