Current flow in auroral loops and surges inferred from ground-based magnetic observations

Recent studies of polar magnetic substorms have revealed that after the initial onset of the substorm expansive phase and the subsequent poleward expansion the ensuing activity is usually confined to the northern border of the auroral electrojet. It is shown that the loops and surges, which quasi-periodically distort the northern border of the electrojet, have associated with them regions of current flow from north to south. Various models of the current flow are presented to account for the observed magnetic perturbation pattern. It is found that the D component perturbation is critical for the study of distorted auroral forms; however, knowledge of all three components is necessary to distinguish between the possible current configurations associated with loops and surges.