Morphology of the dayside ionosphere of Mars: Implications for ion outflows

Abstract

Significant fluxes of tailward streaming ions have been detected in the Martian wake by instruments on spacecraft. Imposing outward fluxes at the top of a model will produce dayside ion density profiles that are characterized by smaller scale heights than those of diffusive equilibrium. We determine the maximum outward fluxes of ions, and those implied by radio occultation data, by constructing ∼180 models, with upward velocity boundary conditions in the range from 0 to (7–8) × 105 cm s−1 in small increments. As the upward velocity is increased, the topside ion or electron densities decrease until eventually the computed ion fluxes cease to increase, implying that this is the maximum outward flux that the ionosphere can sustain. By comparison to data, we derive a low solar activity upward flux of O2+ of ∼5 × 107 cm−2 s−1, and a maximum of ∼8 × 107 cm−2 s−1. For O+, the analogous fluxes are ∼4 × 106 cm−2 s−1 and ∼1.1 × 107 cm−2 s−1. We derive high solar activity upward fluxes of O2+ in the range ∼(1.2–1.6) × 108 cm−2 s−1, and a maximum of 2.4 × 108 cm−2 s−1. The O+ derived and maximum fluxes at high solar activity are ∼(1.5–2) × 107 and 5 × 107 cm−2 s−1, respectively. If these fluxes are averages over the dayside, we estimate total loss rates of O+ and O2+ of (2.8–11) × 1024 and (3.6–8.7) × 1025 s−1, respectively. Our computed escape rates of O+ are in substantial agreement with the models and data, but our O2+ escape rates are an order of magnitude larger. We discuss various mechanisms that would bring our O2+ escape rates or the O+/O2+ ratio into agreement with the measurements and models.