E.1 Overview of the effects of the geomagnetic field

Spacecraft motion across the geomagnetic field results in a motionally-induced e.m.f. given by in the spacecraft_. If a current path can be completed, a current flows through the spacecraft and the surrounding plasma_. These phenomena can lead to generation of a few volts potential differences on large spacecraft in LEO_. The effect is also used or studied in tethered satellite missions where the length of the tether perpendicular to B can lead to large currents and potentials.

Interaction between the magnetic field and an onboard magnetic moment m gives rise to a force:

(E-1)

which can be used for attitude control where an onboard magnetic torquer provides m.

Dynamic fluctuations in plasma populations, electric fields and geomagnetic fields are intimatly connected and plasma disturbances can be monitored via observations of the geomagnetic field_. These are quantified by the familiar K_p, A_p, D_st and other geomagnetic indices (see clause 6 for more detail on geomagnetic activity indices.