Effects of Electron Precipitation on E-Region Instabilities: Theoretical Analysis

During periods of strong geomagnetic activity, intense currents and electric fields originating in the magnetosphere inundate the high-latitude E-region ionosphere. These strong electric fields drive plasma instabilities, including the Farley-Buneman instability (FBI). These instabilities give rise to small-scale plasma turbulence that modifies the large-scale ionospheric conductance that, in turn, affects the evolution of the magnetosphere-ionosphere-thermosphere system. Also, during geomagnetic storms, high-energy precipitating electrons, greater than or similar to 5 keV, frequently penetrate down to the same regions where the intense currents and electric fields exist. This research examines the effects of precipitating electrons on the generation of the FBI and shows that, under many common conditions, it can easily suppress the FBI in a predictable manner. We demonstrate that the plasma pressure of superthermal electrons may significantly exceed the regular plasma pressure of the cold ionospheric plasma. This effect will increase the FBI threshold and suppress the instability in auroral regions. However, our detailed theoretical analysis shows that the effect of the superthermal precipitating electrons on the FBI threshold is much stronger than the pressure effect. The energy dependence of the electron-N-2 collision frequency can greatly enhance the effect of this additional pressure, further suppressing the FBI, even at a moderate precipitation level. Therefore, we expect that precipitation will exert an additional significant feedback on the magnetosphere by preventing the elevated conductivity caused by FBI driven turbulence. Both the turbulence-enhanced conductivities and this suppression should be taken into account in global modeling of the magnetosphere-ionosphere coupling.