Generation of ion‐conic distribution by upgoing ionospheric electrons

Abstract

Downward currents in auroral regions are commonly measured with amplitudes of 1–5 µ A/m². Such currents are likely the result of upgoing thermal ionospheric electrons falling through a field‐aligned potential drop on the order of their thermal energy. Similar distributions of upgoing ionospheric electrons may also occur in regions of diffuse auroral electron precipitation to preserve current continuity in the presence of the loss of the precipitating electrons to the ionosphere. The drift velocity of the upgoing electrons is sufficient to excite electrostatic ion‐cyclotron waves. In addition to being in Landau resonance with the upgoing electrons, these waves cyclotron resonate with the upgoing thermal ions with a parallel energy of several eV. Calculated quasi‐linear diffusion rates using measured wave spectra indicate the resonant ions can be heated to perpendicular energies on the order of 100 times the initial ion thermal energy. Comparison with auroral particle observations, both at low altitude (∼1500 km) and higher altitude (∼6000 km) shows that ion‐conic distributions can readily be explained by such quasi‐linear diffusion. The results imply that conic distributions should not occur simultaneously with the keV electron precipitation associated with discrete arcs in regions of upward current but should occur in regions of upgoing ionospheric electrons which may include regions near the edges of auroral arcs and regions of diffuse auroral electron precipitation.