The Polar Cap North (PCN) index

The Polar Cap (PC) index consists of the Polar Cap North (PCN) and the Polar Cap South (PCS) indices, which are derived from magnetic measurements taken at the geomagnetic observatories in Qaanaaq (THL, Greenland, +85o magnetic latitude) and Vostok (VOS, Antarctica, -83o magnetic latitude), respectively.
These indices are used to estimate the intensity of anti-sunward plasma convection in the polar caps. This convection is associated with electric Hall currents and consequent magnetic field variations perpendicular to the anti-sunward plasma flow (and related Hall current). These magnetic field variations are monitored at the Qaanaaq and Vostok magnetic observatories.
The PC index aids in monitoring the energy input from solar wind to the magnetosphere (loading activity). The index is constructed in such a way that it has a linear relationship with the merging Electric Field at the magnetopause; consequently, the PC index is given in units of mV/m as for the electric field.