Magnetic Field is measured in three different directions: the H-component is towards magnetic north; the D-component is along magnetic east-west; and the Z-component is perpendicular to the ground. The graph shows the H-component of the field as a black line, with a typical quiet day shown in blue. The difference between the current field and a quiet day is plotted as a colour-coded bar chart: green for quiet, orange for active and red for stormy.
The different stages of magnetic activity will all show up in the H-component. As a CME hits the magnetosphere, the compression of the magnetic field produces an increase in the field measured at ground level. This gives a characteristic rise in the field and is known as a Sudden Storm Commencement (SSC). Not all SSCs are followed by auroral activity, but it is a good early indicator. If the solar wind speed and IMF direction are favourable, the energy in the magnetosphere will gradually build up into a large storm, with the magnetic field decreasing and changing rapidly.
