Electric Current System in the Undisturbed Magnetospheric Tail

Satellite measurements of the magnetic field B in the earth's tail indicate the existence of electric currents flowing inside the tail but outside the so‐called ‘Current’ or ‘neutral’ sheet. The Imp 3 data indicate that the density of these currents, although not insignificant, is smaller than the current density inside the ‘neutral’ sheet. Preliminary discussion of the effect of the polarity of the interplanetary magnetic field Bi on the tail geometry is presented. The ‘neutral’ sheet is systematically displaced northward if the polarity of Bi is positive (i.e., if Bi has a component directed away from the sun), but its average position is close to the plane Z = 0 if the polarity of Bi is negative. ((X, Y, Z) is the standard solar magnetospheric coordinate system.) The tail structure outside the ‘neutral’ sheet is not symmetric with respect to the plane Z = 0, even if the geomagnetic latitude of subsolar point (χ) is close to zero; i.e., the magnetic field in the part of the magnetotail where the X component of B is antiparallel to the X component of Bi is smaller than in the other part of the tail.