Solar wind-driven disturbances in the magnetosphere cause the strength of the Earth's surface magnetic field to vary. (This is because large-scale currents in the magnetosphere and ionosphere contribute to the surface field.) These variations are measured at magnetic observatories around the world. From these measurements, indices of geomagnetic activity are produced that tell us about the level of magnetospheric disturbance. Two standard indices are the "Dst index" and the "Ap index."

• Real-time Dst Estimate Web Page The Dst or disturbance storm time index is a measure of geomagnetic activity used to assess the severity of magnetic storms. (from the Space Sciences Laboratory at the University of California, Berkeley)

• Geomagnetic Disturbances Index A plot of the Ap index, prepared from USAF data by Northwest Research Associates, Space Weather Services, Inc. The Ap index is a daily average of global geomagnetic activity.

• Preliminary Report and Forecast of Solar Geophysical Data by the NOAA SEC Choose "Daily Magnetic Data" from the menu. First read "About the Indices, Events, and Regions Data Menu."