Return current and heat transport into zero-order SAR arcs

Zero-order SAR arcs are defined here as those stable auroral red arcs having zero net field-aligned current in the same tube of magnetic force. At 880 km altitude they are invariably accompanied by precipitation of supra-thermal electrons of energy in the range (5– 10 eV ) sufficient for at least one-third, and possibly more, of their power requirements. We use thermoelectric relations to demonstrate the significant change in the temperature profile caused by the localised return current (counter streaming of electrons) induced by such precipitation. This effect is analysed in the altitude range of absorption of the supra-thermals in the topside ionosphere (about 500– 1500 km altitude), and is a new factor in the theory of heat transport into such SAR arcs. Having established the effect on the basis of existing observations, we apply it to assess predictions of hypothetical models.