Alternate Definitions for geomagnetic storm

(1) A worldwide disturbance of the Earth’s magnetic field, distinct from regular diurnal variations. A storm is precisely defined as occurring when the daily Ap index exceeds 29, or (2)  NOAA Space Weather Scale (G) for geomagnetic storm disturbances (see Appendix A).

A geomagnetic storm is a temporary disturbance of the Earths magnetosphere caused by a disturbance in space weather. Associated with solar coronal mass ejections (CME), coronal holes, or solar flares, a geomagnetic storm is caused by a solar wind shock wave which typically strikes the Earths magnetic field 24 to 36 hours after the event. This only happens if the shock wave travels in a direction toward Earth. The solar wind pressure on the magnetosphere will increase or decrease depending on the Sun's activity. These solar wind pressure changes modify the electric currents in the ionosphere.