  We examine the structure of the turbulence boundary of a temporal plane jet
at $Re=5000$ using statistics conditioned on the enstrophy. The data is
obtained by direct numerical simulation and threshold values span 24 orders of
magnitude, ranging from essentially irrotational fluid outside the jet to fully
turbulent fluid in the jet core. We use two independent estimators for the
local entrainment velocity $v_n$ based on the enstrophy budget. The data show
clear evidence for the existence of a viscous superlayer (VSL) that envelopes
the turbulence. The VSL is a nearly one-dimensional layer with low surface
curvature. We find that both its area and viscous transport velocity adjust to
the imposed rate so that the integral entrainment flux is independent of
threshold, although low-Reynolds-number effects play a role for the case under
consideration. This threshold independence is consistent with the inviscid
nature of the integral rate of entrainment. A theoretical model of the VSL is
developed that is in reasonably good agreement with the data and predicts that
the contribution of viscous transport and dissipation to interface propagation
have magnitude $2 v_n$ and $-v_n$, respectively. We further identify a
turbulent core region (TC) which is a region that shrinks over time and a
buffer region connecting the VSL and the TC that grows in time and where the
inviscid enstrophy production is important. The BR shows many similarities with
the turbulent-nonturbulent interface (TNTI), although the TNTI seems to extend
into the TC. The average distance between the TC and the VSL, i.e.\ the BR
thickness is about 10 Kolmogorov length scales or half a Taylor length scale,
indicating that intense turbulent flow regions and viscosity-dominated regions
are in close proximity.
