ELECTRON-IMPACT IONIZATION OF INTERSTELLAR HYDROGEN AND HELIUM AT INTERPLANETARY SHOCKS

We investigate the ionization of interstellar hydrogen and helium due to electron impact by shock-heated electrons. Taking the electron dist ributions measured at four interplanetary shocks at 1 AU, we show that the electrons in the downstream region of strong shocks can ionize in terstellar atoms at rates matching or exceeding the nominal photoioniz ation or charge-exchange rates. We suggest that this process may expla in some puzzling observations of interstellar pickup ions made by the Ulysses spacecraft.