Identification of moving magnetic field lines

Magnetic field line motion and its relationship with particle E × B drifts are considered. It is shown that a unique and significant field line identification can be made whenever all field lines in a region pierce a conducting, but arbitrarily shaped surface. If the region in question is free space, particle electric drifts, in general, bear no relation with field line motion identified in this fashion. However, with a conducting plasma filling the region and allowing no parallel component of electric field ( E·B = 0), particles drift so as to always remain on the same magnetic line of force. These points are specifically illustrated for a time-dependent model of the earth's magnetosphere.