Auroral perturbation experiments

Neutral gas releases from sounding rockets into the auroral ionosphere result in a number of perturbation effects: those directly related to the release as such, including excitation of plasma waves and the enhancement or reduction of local plasma density; and those resulting from ionospheric-magnetospheric responses to the release, including stimulated particle acceleration, generation of large-amplitude electric field (current) pulse, and propagation of field-aligned current pulse in an Alfven wave mode associated with the release. A number of early barium thermite and shaped charge experiments provided valuable insights into the latter effects, despite the fact that they were not anticipated and consequently the observations were often incomplete and their interpretations tentative. These early insights and a number of theoretical models for auroral are formation suggested an important role for ionospheric conductivity and field-aligned currents in the formation of auroral arcs, and laid the foundations for subsequent auroral perturbation experiments (Project Trigger and Project Waterhole), in which the perturbation to the auroral ionosphere was intentional and used as a tool to probe the auroral acceleration mechanism. The local ionospheric plasma density was enhanced by 1–2 decades in Trigger and was depleted by that amount in Waterhole. In Trigger, energetic particle precipitation was stimulated. In Waterhole, it was impeded in return current regions (2 cases) but was enhanced in the upward current region (1 case). Together, the two experiments strongly suggest that (1) field-aligned currents are capable of driving instabilities in the topside ionosphere and thereby stimulating particle acceleration; (2) ionospheric conductivity plays an important role in auroral arc formation; and (3) increased conductivity inside the auroral arcs provides a positive feedback to the precipitating particles resulting in increased particle precipitation.