Anomalous Resistivity in the Auroral Plasma

One of the key problems to the understanding of auroral arcs is the resistivity that they present to the flow of fieldaligned electric currents. Since collisional resistivity cannot account for the observations, one is faced with the question of what kind of collisionless process can account for the enhanced resistance. Previous theories [Kindel and Kennel, 1971] attempted to associate the anomalous resistivity with electron-ion instabilities driven by the electron current. Such instabilities are excited only when the current exceeds a certain threshold. Field-aligned measurements of currents [Reasoner and Chappel, 1973] show that the thresholds predicted by Kindel and Kennel [1971] occur only at altitudes greater than 1000 km. It is the purpose of this note to demonstrate that one can have large values of anomalous resistivity even when the auroral plasma is stable with respect to current driven instabilities. The underlying physical notion of the proposed model is that during times of high-energy electron precipitation the auroral plasma is in a state of stable but highly nonthermal collisionless equilibrium. The details of the processes involved and the possible stationary state can be found in Papadopoulos and Coffey [1974]. The model proposed there for the explanation of various nonthermal features of the auroral plasma has the following aspects: 1. One can view the high-energy precipitating electrons (• 10 keV) as a spread out beam in velocity space [(AVo/ • 1/3]. Unless the beam is completely flat in velocity [(A • 1], it will act as a source of high-frequency (COo • COe) very long wavelength [koXo • (V e/V o) •] plasma waves. Although the spectrum of these waves is broad in k space [(/Xk/ko) (Ix Vol V0)], it will be seen by the ambient plasma as having a very narrow bandwidth /xCO due to the low group velocity of these waves