Current Continuity in Auroral System Science: Defining Electron Precipitation

Auroral arc scale coupling of the magnetosphere and ionosphere involves complex system level science and is an open area of study (Wolf, 1975; Seyler, 1990; Cowley, 2000; Lotko, 2004). The auroral ionosphere plays a non-passive role in this coupling by keeping the closure of field-aligned currents (FAC) self-consistent with both magnetospheric convection patterns and the conductivity volume imparted by accelerated precipitation.

Eq. 6.12 in Kelley (2009) tells us that, given a 2D horizontal map of FAC (or perpendicular flow) and with knowledge of the ionosphere's conductances (i.e., height integrated conductivities), a solution can be found for the perpendicular flow (or FAC). This, however, ignores the neutral wind, induction, finite parallel resistivity in the lower E region, and, in fact, it integrates out all altitudinal dependencies. Closure currents which align to the electric field (Pedersen currents) peak at higher altitudes than those perpendicular to it (Hall currents). This is significant, not just from a morphological standpoint, but from an energetics point of view as well since Hall currents do not dissipate electromagnetic energy while Pedersen currents do.

Furthermore, the ionospheric conductivity is highly sensitive to the accelerated portion of the FAC by means of impact ionization (Fang, 2008, 2010). Though Eq. 6.12 is well posed for sheetlike auroral arc systems (those with no along-arc gradients), this accelerated charged particle precipitation often has significant spatiotemporal structure which directly impacts the 3D conductivity volume.

We aim to alleviate these aspects by providing a sample catalog of 3D building-block simulations provided by the Geospace Environment Model of Ion-Neutral Interactions (GEMINI) model (Zettergren and Semeter, 2012; Zettergren et al., 2015). This is a state-of-the-art, multi-fluid, quasi-electrostatic model that can be driven by 2D topside maps of characteristic precipitation energy, total precipitation energy flux, and either FAC or ExB flow by means of a potential map.

This work will focus on an important part of studying auroral arc systems: impact ionization imparted by the differential hemispherical electron flux. This flux can result from of a superposition of a primary accelerated Maxwellian population along with secondary low energy population of re-accelerated backscatter. We investigate the impact this has on auroral current closure by comparing unaccelerated vs. accelerated Maxwellian electron precipitation, both with and without the backscatter low-energy tail.