A HEMISPHERICAL MODEL OF ANISOTROPIC INTERSTELLAR PICKUP IONS

We present an analytical model for the distribution function of inters tellar pickup ions in a radially directed heliospheric magnetic field which naturally results in anisotropic particle distributions. The mod el includes the effects of convection and spatial transport in the sol ar wind, adiabatic deceleration in the radial flow, adiabatic focusing in the radial field, and pitch angle scattering toward isotropy in th e frame of the solar wind. The pitch angle scattering is approximated by the hemispherical assumption: we take the scattering to be very eff icient within each pitch angle range mu greater than or less than 0 (w here mu is the cosine of the pitch angle) but inhibited between the he mispheres separated by mu = 0. The analytical solution is obtained for the case where the scatter ing rate across mu = 0 scales as the parti cle speed divided by the radial position of the fluid parcel. The mode l distribution functions can be used to interpret recent observations of anisotropic pickup ions.