The K indices show the local disturbance of the earth's magnetic
field. They are displayed for an 81-day period and are updated on
workdays. 81 days correspond to three (synodic) rotations of the
sun. During strong disturbances, a 27-day repetition tendency is
The K index is a quasi-logarithmic measure of the maximum
disturbance in steps of 0 to 9 for three hours (UTC) each; 500 nT
is the lower limit for K = 9 at Wingst. Large indices point to a
correspondingly higher penetration of corpuscular radiation.