  Observations of coronal jets increasingly suggest that local fragmentation
and intermittency play an important role in the dynamics of these events. In
this work we investigate this fragmentation in high-resolution simulations of
jets in the closed-field corona. We study two realizations of the
embedded-bipole model, whereby impulsive helical outflows are driven by
reconnection between twisted and untwisted field across the domed fan plane of
a magnetic null. We find that the reconnection region fragments following the
onset of a tearing-like instability, producing multiple magnetic null points
and flux-rope structures within the current layer. The flux ropes formed within
the weak-field region in the center of the current layer are associated with
"blobs" of density enhancement that become filamentary threads as the flux
ropes are ejected from the layer, whereupon new flux ropes form behind them.
This repeated formation and ejection of flux ropes provides a natural
explanation for the intermittent outflows, bright blobs of emission, and
filamentary structure observed in some jets. Additional observational
signatures of this process are discussed. Essentially all jet models invoke
reconnection between regions of locally closed and locally open field as the
jet-generation mechanism. Therefore, we suggest that this repeated tearing
process should occur at the separatrix surface between the two flux systems in
all jets. A schematic picture of tearing-mediated jet reconnection in three
dimensions is outlined.
