  We use the Cluster EFW experiment to measure the cross-shock electric field
at ten low $\beta$, quasi-perpendicular supercritical bow shock crossings on
March 31, 2001. The electric field data are Lorentz-tranformed to a Normal
Incidence frame (NIF), in which the incoming solar wind velocity is aligned
with the shock normal. In a boundary normal coordinate system, the cross-shock
(normal) electric field is integrated to obtain the cross shock potential.
Using this technique, we measure the cross-shock potential at each of the four
Cluster satellites and using an electric field profile averaged between the
four satellites. Typical values are in the range 500-2500 volts. The
cross-shock potential measurements are compared with the ion kinetic energy
change across the shock. The cross-shock potential is measured to be from 23 to
236% of the ion energy change, with large variations between the four Cluster
spacecraft at the same shock. These results indicate that solar wind flow
through the shock is likely to be variable in time and space and resulting
structure of the shock is therefore nonstationary.
