The February 5, 1965, solar proton event: 1. Time history and spectrums observed at 1100 kilometers

The solar proton event of February 5, 1965, was observed in the polar regions with detectors aboard 1963 38C. The initial detection of protons occurred during the pass at 1914 UT, February 5, and the maximum intensity of 115 cm−2 sec−1 for Ep ≥ 25 Mev was seen during the pass at 2103 UT, February 5. The time history of the low-energy protons (Ep ≥ 1–2 Mev) was considerably more complex than that of the higher energy particles. The maximum observed intensity (Ep ≥ 2 Mev) was equal to 775 cm−2 sec−1 at 0232 UT, February 6, and a second maximum was observed following the onset of the magnetic storm on February 6. The detailed discussion of the low-energy proton behavior is taken up in the companion paper. For the higher energies the particle flux decayed approximately exponentially in time with the shorter ‘lifetimes’ associated with the higher energies. An attempt to fit the time history with diffusion theory using a diffusion coefficient of the form D = Mrβ was moderately successful for Ep ≥ 25 Mev. The spectrum showed a (nearly) monotonic softening with time, and a power law generally provided a better approximation to the spectrum than did an exponential form.