Electric currents and voltage drops along auroral field lines

The current state of knowledge concerning Birkeland currents (j_∥) and parallel electric field (E_∥) is briefly reviewed. Four types of j_∥ are discussed-the primary ‘region 1’ sheets, the ‘region 2’ sheets which parallel them and which seem to close in the partial ring current, the cusp currents which appear to correlate with interplanetary B_y, and the ‘Harang filament’. The energy required by E_∥ and by the associated particle acceleration processes seems to be derived from j_∥. Much of the evidence for e_∥ comes from particles, from ‘inverted V’ spectra, rising ion beams and expanded loss cones, while ‘conies’ may signify acceleration by Electrostatic Ion Cyclotron (EIC) waves, associated with beams accelerated by E_∥. Different theoretical studies predict for E_∥ a smooth, disordered or abrupt structure, and evidence for all 3 types can be deduced from S3-3 electric field probe observations.