Letter from Thomas Gold to Mr J R Jokipli, Department of Physics, California Institute of Technology, Pasadena, California Read his paper with interest, especially as Gold had worried about the same problem. He is probably correct that Fermi acceleration does the job; however, the electrons are usually of low energy only, and Gold is not certain that in the presence of Fermi acceleration for the protons one could avoid giving a little energy to the electrons also. He would be interested in hearing Jokipi's comments on that point. 4 June 1971