Investigation of the Cooling Behavior of Interstellar HE+ Pickup Ions in the Inner Heliosphere

Interstellar neutral gas is streaming though the inner heliosphere where it is ionized and picked up by solar wind via interaction with the interplanetary magnetic field (IMF). The dominant ionization process is solar EUV radiation, augmented slightly by charge exchange with the solar wind and electron impact ionization. The resulting pickup ion (PUI) distributions broaden as they are transported radially outward with the solar wind and are cooled in this process. The ionization rate, which controls the radial profile of the interstellar neutrals, and the cooling process together determine the slope of the observed PUI distributions. Thus far, a cooling index of 3/2 for the PUI velocity distributions as a function of radial distance from the Sun (Vasyliunas & Siscoe, JGR, 81,1247, 1976) has been used in almost all studies. Here, we use the observations of PUI distributions from ACE SWICS over one month in the upwind direction of the interstellar flow and of the ionization rate at 1AU from solar maximum to solar minimum to deduce the actual PUI cooling index. We start with an isotropic model for interstellar He+ PUIs that is valid for the upwind region and then compare the modeled phase space density spectrum after integration over the instrument fields-of-view and energy steps with ACE SWICS observations. To account for the influence of PUI transport processes we sort the PUI distributions for IMF orientation and solar wind speed and start with distributions for nearly perpendicular IMF. The comparison between the simulated and the time averaged He+ PUI phase space density contains the cooling index as free parameter because the ionization rate is obtained simultaneously from observations. We find noticeable differences of the cooling index from the previously used value of 3/2 and will discuss potential implications on PUI expansion and transport.