On the nature of homogeneous arcs of aurorae.

It is shown that the system of magnetospheric-ionospheric currents can be unstable in terms of stratification in regions where longitudinal currents flow out of the ionosphere. Here, the Lamor radius of hot ions near the equatorial plane must be larger than the inverse wavenumber of the instability, while the density of field-aligned currents in the ionosphere must exceed a certain critical value, determined by the geometry of the problem, ionospheric conductivity, and the temperature of hot ions, T sub i. The development of instability is facilitated if the velocity distribution function of hot ions has a maximum and the jump of potential between the magnetosphere and ionosphere is approximately (alpha)(T sub i)/e, where alpha = 0.5-1. A close connection between quiet auroral arcs and field-aligned currents that flow out of the ionosphere is indicated.