There are two most often used classifications of the geomagnetic storm intensity: via the Dst index and Kp index (see tables on the right). The both indices assess the intensity and duration of geomagnetic storms. 

  The Disturbance Storm Time (Dst) measures variations in the horizontal component of the terrestrial magnetic field near the magnetic equator. During a geomagnetic storm,  an increased magnetospheric ring current activity causes a decrease in the Dst index below -30 nT. The primary Dst index is calculated hourly by the World Data Center for Geomagnetism in Kyoto, Japan, through averaging data from four ground observation stations at low latitudes (see real time data above).

   The planetary K index (Kp-index) is a measure of global geomagnetic activity, calculated using data from magnetometers worldwide. Each station reports a K-index based on the local geomagnetic activity measured, calibrated for its location. The K-index reflects three hours of geomagnetic activity compared to a calm day. A magnetometer measures the maximum magnetic field deviation horizontally and reports it. The global Kp-index is determined by combining reported K-values from all stations, ranging from 0 (minimal activity) to 9 (extreme geomagnetic storm). Real time Kp index estimations from GFZ can be found below.