Relationship of southward-drifting auroral arcs to the magnetospheric electric field and substorm activity

The magnitude of the westward-directed auroral zone electric field is inferred from the measurement of the southward drift of moderately bright auroral arcs. In a sample of 43 events recorded over a period of 15 months at Fort Smith (68.1°N corrected geomagnetic) it was found that southward-drifting arcs were predominantly associated with the decay phase of polar magnetic substorms, and no evidence was found for the association of southward-drifting arcs with the so-called growth phase of substorms. It is suggested that the southward-drifting arcs are generated through the precipitation of energetic plasma affected by the limit on stably trapped particle fluxes proposed by Kennel and Petschek (1966).