Magnetograms recording quiet times

Figure 5. Magnetograms recording quiet times. (a) Data from five observatories show the horizontal intensity H for 8–12 October 2003. The magnetograms are arranged by geomagnetic latitude, which is measured relative to the axis of Earth’s tilted dipole. Diurnal variation is prominent, espe- cially for the Huancayo (HUA) observatory located near the magnetic equator. Note the change in polarity of the varia- tion at high latitudes. (b) Observatory data can be used to map the electric currents in the ionosphere that sustain quiet- time variation. Shown here is a map for noon, Greenwich Mean Time. Contour lines indicate 10-kA increments; more

densely packed contour lines indicate higher local current density. Red contours indicate clockwise-circulating current; blue contours, counterclockwise current.