Annual variation of the interplanetary He+velocity distribution at 1 AU

The interplanetary velocity distribution of He+ ions of interstellar origin is calculated for various ecliptic longitudes Φ at 1 AU and for various temperatures T of the local interstellar medium. For concreteness we assume a relative streaming velocity of the local interstellar medium with respect to the sun of 〈V∞〉 = 10 km/sec oriented in the ecliptic plane, nHe(r = ∞) = 0.005 cm−3, and we neglect interaction with magnetic irregularities convected by the solar wind. We find that these He+ fluxes should be observable at 1 AU with present observation techniques and that, if T ≃ 100 K, they should exhibit sharp peaks in velocity space that vary systematically with Φ. If T ≃ 5000 K, He+ fluxes should appear almost structureless except for a well-defined high-energy cutoff. The annual variation of He+ flux at this cutoff should exhibit a maximum in the anti-apex direction, with maximum to minimum ratio of 6:1, and ΔΦFWHM = 70° for T = 5000 K (FWHM is full width at half maximum).