Example of plot from

The Kiruna ground magnetometer is located in northern Sweden. During the nighttime hours, it lies under the auroral oval in a good location to measure the magnetic field perturbations associated with ionospheric and field-aligned currents generated during substorms.

The black line shows the northward component of the field, red the eastwards component, and green the downward component (the dominant direction of the magnetic field).

A typical substorm begins with a gradual decrease in the H (northward) component of the magnetic field over 30 to 60 minutes as magnetic field lines within the magnetotail stretch antisunward.

At substorm onset, magnetotail magnetic field lines rapidly (~several minutes) collapse to dipolar (bar magnet) configurations, and the duskward currents that had existed in the magnetotail are diverted to flow duskward through the high latitude ionosphere. Ground stations under these currents observe southward (negative perturbations) and intense fluctuations.

Kiruna Magnetogram High Latitude Magnetic Field Measurements