The constant main field caused by internal currents in the Earth and the variable external field dominated by currents in the ionosphere. Based upon this model magnetometer analysis can be reduced to three basic steps:

1. Remove the background magnetic field. To do this, find a day close to the interval of interest when the magnetic field is very steady. This is known as the quiet-time field. Subtract the quiet-time field from the magnetic field measured on the day of interest to yield the external magnetic perturbations caused by ionospheric currents.

   Note that the quiet time field comprises not only the constant main field but also the external field due to ever-present ionospheric current, ring current, and magnetopause current. We assume that the ionospheric currents of interest (i.e., those driven by processes out in Space) are inherently time-varying.

2. Calculate the equivalent current. Take the horizontal magnetic field perturbation vector and rotate 90^o clockwise to yield the equivalent ionospheric current.

3. Examine the vertical component. If d_Z = 0 then the current is overhead or spatially uniform. If d_Z does not equal 0 then use the line current model to infer where the current is concentrated relative to the magnetometer.