  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 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. 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 Alfv\'enic speeds in excess of 100 km/s 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. Our results can explain the behavior of transition region "network jets"
and the very high apparent speeds reported for some chromospheric spicules.
