Observatory

Measures of magnetic disturbance level

K-indices


Quasi-logarithmic scale from 0 (very quiet) through 3 (moderately disturbed) to 9 (very disturbed). One value for each 3-hour period. Local indices (K) are calculated at most observatories, mostly at higher mid-latitudes.

Ap index


A linear, daily index ranging from 0 (very quiet), through 15 (moderately disturbed) to 400 (very disturbed). Planetary average for the same observations as Kp.

AE index


Index of auroral activity. Derived from horizontal component of magnetic field variation at 10-12 auroral-zone observatories. Usually hourly values of the mean difference between the lowest and highest values of disturbance field measured at all the stations (in nT)

Dst index


Index of ring-current (and global magnetic storms). Derived from nearequatorial observatories. The mean hourly deviation of the horizontal component of the magnetic field from the quiet level (in nT).