Electron-impact ionization of interstellar hydrogen and helium at interplanetary shocks

Abstract

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