Solar wind-driven thermospheric winds over the Venus North Polar region

Abstract

Data from the neutral particle imager (NPI) on Venus Express reveals the presence of a persistent +Y (Venus Solar-Oriented) flow of Energetic Neutral Atoms (ENAs) in the Venus polar thermosphere. The overall flow morphology and altitude distribution of ENAs, solar wind H⁺ (SWH⁺), and ionospheric O⁺ imply that SWH⁺&O⁺ Charge Exchange (CE) with thermosphere neutrals causes the ENA flow. A dusk-dawn and noon-midnight ENA flux asymmetry is consistent with an asymmetric CE-interaction with higher ENA fluxes in the dayside and dawn (+Y). Model ENA fluxes derived from CE between SWH⁺&O⁺ and a model oxygen thermosphere is consistent with NPI data. Adding energy-corrected NPI fluxes and ion fluxes gives essentially constant fluxes of ≈10¹² part/(m²s) in the altitude range of 200-1200 km. The energy and momentum of the 1-10 km/s ion and ENA flow, induced by solar wind forcing, should have implications for wind patterns in the polar thermosphere above 200 km.