Solar wind interaction with interstellar helium

Solar wind interaction with neutral interstellar helium focused by the Sun's gravity in the downwind solar cavity is discussed in a hydrodynamical approach. Upon ionization the helium atoms “picked up” by the (single fluid) solar wind plasma cause a slight decrease in the wind speed and a corresponding marked temperature increase. For neutral helium density outside the cavity n_He^∞ = 0.01 atoms cm^?3 and for interstellar kinetic temperature T_He= 10,000 K, the reduction is speed of the solar wind on the downwind axis at 10 AU from the Sun amounts to about 2kms^?1; the solar wind temperature excess attains 7000 K. The resulting pressure excess leads to a non-radial flow of the order of 0.25 km s^?1. The possibility of experimental confirmation is discussed.