Origin of the cusp Birkeland currents

The authors argues that the cusp-region field-aligned (Birkeland) currents, first reported by Iijima and Potemra (1976b), arise from the motion of reconnected open field lines shortening over the dayside magnetopause. Initially, field tension is dominant so the open field lines have a significant east/west component to their motion. Later, as the field lines shorten, the magnetosheath flow becomes increasingly important, and the field lines are pulled antisunward into the polar cap. Where the field lines change their direction of movement from east/west to antisunward, the magnetic field must tilt and this is the site of the cusp Birkeland current. All the major observational features of the cusp Birkeland currents fit qualitatively well with our model.

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