Proton and alpha particle precipitation onto the upper atmosphere of Venus

Abstract We study the precipitation of protons and alpha-particles onto the upper atmosphere of Venus, using particle data recorded by the Venus Express spacecraft inside the induced magnetosphere. Our investigations are limited to the dayside close to the terminator. We observe on average a net downward flux of protons, which originate partly from the planetary atmosphere and partly from the solar wind. We present median energy spectra of the precipitating protons divided into two energy ranges, 10-100 eV and 100 eV-30 keV. The total dayside precipitation of solar wind protons is estimated to be 3×10²² s^-1, assuming only protons with energies above 500 eV will reach the exobase. Downgoing protons are frequently observed but only in 3% of the available data records we see He2⁺. These observations are made close to the induced magnetosphere boundary and we argue that at lower altitude the countrates for alpha-particles fall below detection limits. We estimate the precipitation of He2⁺ onto the dayside exobase to be 1×10²¹ s^-1, which is not enough enough to replace the helium escaping from the planet.