  During magnetic reconnection in collisionless space plasma, the electron
fluid decouples from the magnetic field within narrow current layers, and
theoretical models for this process can be distinguished in terms of their
predicted current layer widths. From theory, the off-diagonal stress in the
electron pressure tensor is related to thermal non-circular orbit motion of
electrons around the magnetic field lines. This stress becomes significant when
the width of the reconnecting current layer approaches the small characteristic
length scale of the electron motion. To aid in situ spacecraft and numerical
investigations of reconnection, the structure of the electron diffusion region
is here investigated using the Terrestrial Reconnection EXperiment (TREX). In
agreement with the closely matched kinetic simulations, laboratory observations
reveal the presence of electron-scale current layer widths. Although the layers
are modulated by a current-driven instability, 3D simulations demonstrate that
it is the off-diagonal stress that is responsible for breaking the frozen-in
condition of the electron fluid.
