A source for field-aligned currents at auroral latitudes

Energetic plasma drifting toward the earth from the plasma sheet in the earth's magnetospheric tail is terminated by an inner boundary (the Alfvén layer). Sheets of field-aligned current (Birkeland currents) flow down to the ionosphere from this plasma boundary. The mechanism for generating the current is essentially that originally proposed by Alfvén in 1939. A certain amount of charge separation develops in the Alfvén layer because the geocentric drift paths of ions and electrons are not coincident within the layer. The charge separation produces an electric field that is perpendicular to B and is transmitted along lines of force to the ionosphere. Currents flow across magnetic field lines in the ionosphere and along the magnetic field lines to the Alfvén layer. The electric field is reduced by these currents, which flow to neutralize the charge separation. The position of the Alfvén layer is such as to produce a current sheet that enters the ionosphere at auroral latitudes. The Birkeland current necessary to neutralize the Alfvén layer is calculated to be about 10⁶ amps, which is sufficient to produce the transverse magnetic disturbances of about 100 γ that have been observed by satellites in auroral latitudes; the thickness of the Alfvén layer corresponds to the width of the zone of these transverse magnetic disturbances. Finally, the Birkeland current is related to a larger current system that includes the DS current system.