What causes the high apparent speeds in chromospheric and transition region spicules on the Sun?

Spicules are the most ubuiquitous type of jets in the solar atmosphere. The advent of high-resolution imaging and spectroscopy from the Interface Region Imaging Spectrograph (IRIS) and ground-based observatories has revealed the presence of very high apparent motions of order 100-300 km s(-1) in spicules, as measured in the plane of the sky. However, line of sight measurements of such high speeds have been difficult to obtain, with values deduced from Doppler shifts in spectral lines typically of order 30-70 km s(-1). In this work, we resolve this long-standing discrepancy using recent 2.5D radiative MHD simulations. This simulation has revealed a novel driving mechanism for spicules in which ambipolar diffusion resulting from ion-neutral interactions plays a key role. In our simulation, we often see that the upward propagation of magnetic waves and electrical currents from the low chromosphere into already existing spicules can lead to rapid heating when the currents are rapidly dissipated by ambipolar diffusion. The combination of rapid heating and the propagation of these currents at Alfvenic speeds in excess of 100 km s(-1) leads to the very rapid apparent motions, and often wholesale appearance, of spicules at chromospheric and transition region temperatures. In our simulation, the observed fast apparent motions in such jets are actually a signature of a heating front, and much higher than the mass flows, which are of order 30-70 km s(-1). Our results can explain the behavior of transition region "network jets" and the very high apparent speeds reported for some chromospheric spicules.