Upper limit on the low energy interstellar cosmic-ray flux

DETERMINATIONS of the flux of low energy, non-solar cosmic rays extend at present down to 5 to 10 MeV for observations above the Earth¹. But the interstellar flux of cosmic rays below 1 GeV is highly uncertain because of solar modulation effects. The interstellar differential energy spectrum in the range 10 MeV to 1 GeV is usually deduced from theoretical models, by choosing more or less plausible interplanetary diffusion coefficients that will give a fit to the data. The state of sophistication of these theories is, however, still far from satisfactory, since predicted (low energy) values of the demodulated flux can vary by orders of magnitude among different authors, no experimental data on the interstellar flux having been available until now to help in selecting among different models.