UK Solar System Data Centre
Copyright WDC C1 for STP at RAL.
It is derived from the horizontal magnetic perturbation in the central polar cap, measured at a single station located very near to the pole. The PC index is obtained by normalization with respect to the solar wind parameter, and a natural unit for PC would therefore be "equivalent" mV/m. It should be stressed, however, that PC is primarily an index of geomagnetic activity, and only in a statistical sense a measure of the electric field.