New observations of the quiet-time energy spectrum of 0.16 to 2 MeV electrons have been made with the Caltech Electron/Isotope Spectrometer which was launched on IMP-7 in September 1972. Earlier measurements of quiet-time electrons in this energy range by other groups have resulted in spectra differing by more than an order of magnitude in intensity. We find a minimum quiet-time flux level somewhat lower than the lowest previously reported spectra and consistent with an extrapolation of the spectrum measured at higher energies. A galactic secondary source of knock-on electrons is consistent with our results and with independent studies of the interstellar spectra of cosmic-ray nuclei provided that solar modulation does not suppress the 0.16-2 MeV electron flux by more than a factor of 3. Although not required, other recently suggested sources may also contribute to the observed fluxes. Subject heading: cosmic rays
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Hurford et al. (1974) studied this question.