When the GOES satellites are on the nightside of the Earth, the extent to which the Earth's magnetic field has been stretched from its natural dipole inclination into a long magnetotial is measured by the GOES magnetometers. The more stretched the Earth's magnetic field, the more active is the state of the magnetosphere. GOES magnetometer data is available real-time from NOAA's space data center.

The equations linked to below were developed under UPOS to convert the GOES nightside magnetometer readings into a nowcast Kp and b2i.