IONIZATION PROCESSES IN THE HELIOSPHERE - RATES AND METHODS OF THEIR DETERMINATION

The rates of the most important ionization processes acting in interpl anetary space on interstellar H, He, C, O, Ne and Ar atoms are critica lly reviewed in the paper. Their long-term modulations in the period 1 974 - 1994 are reexamined using updated information on relevant cross- sections as well as direct or indirect data on variations of the solar wind/solar EUV fluxes based on IMP 8 measurements and monitoring of t he solar 10.7 cm radio emission. It is shown that solar cycle related variations are pronounced (factor of similar to 3 between maximum and minimum) especially for species such as He, Ne, C for which photoioniz ation is the dominant loss process. Species sensitive primarily to the charge-exchange (as H) show only moderate fluctuations similar to 20% around average. It is also demonstrated that new techniques that make use of simultaneous observations of neutral He atoms on direct and in direct orbits, or simultaneous measurements of He+ and He++ pickup ion s and solar wind particles can be useful tools for narrowing the uncer tainties of the He photoionization rate caused by insufficient knowled ge of the solar EUV flux and its variations.