Measurement of the potential drop across the earth's collisionless bow shock

The normal component of the dc electric field measured on ISEE-1 ordinarily exhibits an enhancement of a few mV/m over both upstream and downstream values at the earth's bow shock. Using the measured relative velocity between the shock and the spacecraft (from the ISEE-1/2 time delay in the magnetometer data), it is possible to transform the observed E enhancement to a potential drop (delta phi). For a subcritical shock the potential drop is found to be very close to the measured change of particle kinetic energy (delta phi, approximately 280 V on day 330, 1977), whereas for a supercritical shock the potential drop is only a fraction of the measured change of kinetic energy (delta phi, approximately 140 V on day 324, 1977).