  The solar chromosphere and transition region are highly structured and
complex regimes. A recent breakthrough has been the identification of dynamic
fibrils observed in H alpha as caused by field-aligned magnetoacoustic shocks.
We seek to find whether such dynamic fibrils are also observed in Ly alpha. We
used a brief sequence of four high-resolution Ly alpha images of the solar limb
taken by the Very high Angular resolution ULtraviolet Telescope (VAULT), which
displays many extending and retracting Ly alpha jets. We measured their top
trajectories and fitted parabolas to the 30 best-defined ones. Most jet tops
move supersonically. Half of them decelerate, sometimes superballistically, the
others accelerate. This bifurcation may arise from incomplete sampling of
recurrent jets. The similarities between dynamic Ly alpha jets and H alpha
fibrils suggest that the magnetoacoustic shocks causing dynamic H alpha fibrils
also affect dynamic Ly alpha jets.
