  Under sufficiently high electric field gradients, electron behaviour within
exactly perpendicular shocks is unstable to the so-called trajectory
instability. We extend previous work paying special attention to shortiscale,
high amplitude structures as observed within the electric field profile. Via
test particle simulations, we show that such structures can cause the electron
distribution to heat in a manner that violates conservation of the first
adiabatic invariant. This is the case even if the overall shock width is larger
than the upstream electron gyroradius. The spatial distance over which these
structures occur therefore constitutes a new scale length relevant to the shock
heating problem. Furthermore, we find that the spatial location of the
short-scale structure is important in determining the total effect of
non-adiabatic behaviour - a result that has not been previously noted.
