A number of independent and diverse ionospheric electric field, current, particle, and plasma wave observations can be interpreted in terms of intense parallel current density bursts that would require parallel electric fields of the order of up to a few millivolts per meter to exist at ionospheric altitudes as low as 140 km at times. While such observations are rare, they indicate that the upper limits on the values of parallel current densities and electric fields could be extremely high. In this paper, we theoretically explore the conditions under which the ionosphere could possibly sustain very large parallel fields and current densities. Since this is a first attempt at this question, we look for the simplest possible requirements; that is, we study the possibility for the presence of a quasi-static electric field on timescales greater than the electron collision time and the Alfven transit time. Our initial focus is on two-dimensional linear situations, which should apply if the parallel fields are less than about 0.1 mV/m. Nonlinear effects are invoked in the event that stronger parallel fields would be implied by the observations or when very strong horizontal gradients in conductivities are required. We link the problem to a magnetospheric source through the introduction of intense fluxes of medium to strong energy (hundreds of electron volts) electrons with and without an abrupt latitudinal cutoff.

When considering the presence of an abrupt latitudinal change in the precipitating energy spectrum, we also require the presence of a ''background'' perpendicular electric field which is assumed to be uniform on latitudinal scales 1 km or greater. One important conclusion is that for the generation of parallel fields intense enough to generate ion-acoustic waves along the geomagnetic field as well as for the generation of shears of the order of meters per second per meter, 100-m horizontal gradient scales are required. Another result is that for larger horizontal gradient scales, a substantial fraction of the returning currents carried by thermal electrons will flow along the same magnetic field lines that precipitating electrons are coming from; this leads to a substantial reduction in the net parallel current densities when compared to the currents borne by each type of carriers. ¿ American Geophysical Union 1996