The chart below is a running 36 hour plot of the readings taken from the fluxgate magnetometer, built by the University of Alaska, Geophysical Institute, operating at the HAARP ionospheric observatory in Gakona, Alaska. The three traces represent mutually orthogonal components of the earth's magnetic field as follows:
Geomagnetic storminess is usually indicated in oscillatory variations in the earth's magnetic field. Additional detail concerning the nature and severity of the ionospheric disturbance can be found through analysis of the three components of the field.
Data from a chain of magnetometers are also available.