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

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) × 10 5 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 O 2 + of ∼5 × 10 7 cm −2 s −1 , and a maximum of ∼8 × 10 7 cm −2 s −1 . For O + , the analogous fluxes are ∼4 × 10 6 cm −2 s −1 and ∼1.1 × 10 7 cm −2 s −1 . We derive high solar activity upward fluxes of O 2 + in the range ∼(1.2–1.6) × 10 8 cm −2 s −1 , and a maximum of 2.4 × 10 8 cm −2 s −1 . The O + derived and maximum fluxes at high solar activity are ∼(1.5–2) × 10 7 and 5 × 10 7 cm −2 s −1 , respectively. If these fluxes are averages over the dayside, we estimate total loss rates of O + and O 2 + of (2.8–11) × 10 24 and (3.6–8.7) × 10 25 s −1 , respectively. Our computed escape rates of O + are in substantial agreement with the models and data, but our O 2 + escape rates are an order of magnitude larger. We discuss various mechanisms that would bring our O 2 + escape rates or the O + /O 2 + ratio into agreement with the measurements and models.