Helium pickup ion focusing cone as an indicator of the interstellar flow direction

The spatial distribution of pickup He+ in the helium focusing cone during the most recent very quiet solar minimum is studied numerically by solving the transport equation for the anisotropic velocity distribution function of the ions, which takes into account pitch-angle scattering, adiabatic cooling, and focusing in the interplanetary magnetic field. It is shown that the pickup He+ focusing cone observed with the STEREO spacecraft can be shifted in ecliptic longitude relative to the neutral helium cone, and thus can be considered as an indicator of the interstellar flow direction only in the case if all relevant transport effects are taken into account. The value of the shift depends on the mean free path of pickup ions. The larger is the mean free path, the larger is the shift. Under unusually quiet solar wind conditions during the recent solar minimum the shift can reach 5 degrees.