Cross sections of electron impact ionization for H, He, Ne, and O (Lotz 1967). The accuracies for H, He, and O are 10%, for Ne 15%. The vertical bars mark the energies corresponding to the temperatures of the core and halo solar wind electron populations in the slow solar wind at 1 AU from the Sun. The noble gases can only be ionized by a high-energy wing of the core population and mostly by the halo population.