Dst index is an estimate of the magnetic field change at the Earth's magnetic equator due to a ring of electric current at and just earthward of the geosynchronous orbit.[42] The index is based on data from four ground-based magnetic observatories between 21° and 33° magnetic latitude during a one-hour period. Stations closer to the magnetic equator are not used due to ionospheric effects. The Dst index is compiled and archived by the World Data Center for Geomagnetism, Kyoto.[43]