Ulysses measurements of variations in the solar wind-interstellar hydrogen charge exchange rate

Charge exchange with solar wind protons is the primary ionization process for interstellar hydrogen traveling through the heliosphere. In this study we use Ulysses solar wind observations to examine and quantify variations in charge exchange. We find that these variations are largest within stream interaction regions where the charge exchange rate is tens of times larger in compressional, corotating interaction regions (CIRs) than in rarefactions. Similarly, we find enhanced charge exchange in compression regions ahead of fast CMEs. As a consequence, much of the interstellar hydrogen at low latitudes is picked up in pulses as CIRs and fast CMEs sweep past. Observations over Ulysses' first full polar orbit indicate that the charge exchange rate is higher at low than at high latitudes, and that this rate drops off more slowly than the inverse square of heliocentric distance. We believe that this is due to the growth of CIRs as they propagate out through the inner heliosphere. The variability of hydrogen charge exchange has implications for the analysis of scattered Lyman-a observations and produces variations in the pickup rates of newly ionized interstellar hydrogen, which is a seed population for anomalous cosmic rays.