MI coupling impact of superthermal electrons on the diffuse aurora precipitation and the ionospheric conductance: the missing piece in the global MHD models

Ionospheric conductance is important because it couples the Earth's magnetosphere and ionosphere via current continuity equation. Diffuse electron aurora that provides 60% of total aurora power is a strong contributor to the ionospheric conductance. Global magnetosphere - ionosphere (MI) MHD models can provide a physics-based calculation of auroral precipitation and thus a realistic ionospheric conductance pattern. However, these models have limitation. They calculate mean energy and energy flux of auroral electrons using ion parameters at the inner magnetospheric boundary (2-3RE). Then, they precipitate the same auroral information to the ionosphere along dipole field lines assuming no change between the inner boundary and the ionosphere. In reality, however, the MI coupling processes of superthermal electrons (1eV - 30keV) can significantly change the electron energy distribution and thus modify the mean energy and energy flux of precipitating electrons. Due to collision with upper atmospheric neutrals, auroral electrons lose their energies and create secondary electrons. Both auroral and secondary electrons can escape back to the magnetosphere, experience multiple reflection between the two conjugate ionospheres, and continuously ionize the thermosphere. In this study, we investigate the MI coupling impact of superthermal electrons on diffuse auroral precipitation and height-integrated ionospheric conductance, using a superthermal electron transport (STET) code that solves a kinetic Boltzmann-Landau equation. Just like the MHD models, we assume Maxwellian energy distribution of the initial auroral precipitation with 1mW/m2of total energy flux, and investigate how this distribution changes during a journey between the magnetosphere and ionosphere. Our results show that the MI coupling processes produce stronger auroral energy flux than the initial energy flux and thus increase the ionospheric conductance up to 40-70%. This implies that the MHD models may underestimate ionospheric conductance by omitting the MI coupling processes and thus inaccurately increase the ionospheric potentials via current continuity equation, which partially contributes to a well-known problem of stand-alone MHD models, a stronger transpolar potential than observations.