Changes in the distribution of low-energy trapped protons associated with the April 17, 1965, magnetic storm

Abstract

The absolute intensity of geomagnetically trapped protons in the energy ranges 0.52 ≤ Ep ≤ 4.0 Mev and 0.90 ≤ Ep ≤ 1.8 Mev has been measured with the solid-state proton detector on the University of Iowa low-altitude (initial apogee 2502 km, perigee 527 km) high-latitude (inclination 81°) satellite Injun 4 for the period from March 1 to May 31, 1965. A study of the temporal variations of these fluxes associated with the April 17, 1965, magnetic storm (sc 1313 UT April 17, main phase onset ∼0200 UT April 18) shows a general redistribution of these protons for L ≳ 2.5 and all sampled |B| ranges, which persisted for at least 36 days after the storm. The effect of the sudden commencement was a general depression in the intensities and a hardening of the energy spectrums, although the intensities recovered to their prestorm level during the initial phase. The major redistribution was apparently initiated by the polar substorm that began at ∼0620 UT on April 18 and continued long after (to 1100 UT) the substorm had subsided (∼0800 UT). During the recovery phase, a secondary peak developed in the intensity profile at L ∼ 3.5 for 0.52-Mev protons that had no counterpart at this energy at the equator (L. R. Davis, private communication, 1971). No such peak was observed for 0.9-Mev protons. The over-all effect of the storm on the steady-state distribution was a nonadiabatic one with an increase in intensities at L ≲ 3 and a decrease at L ≳ 3. The loss and/or gain of particles appears to be fractionally the same at all |B| values sampled here. The post-storm dependence of the spectral parameter E0 on L generally follows the relation E0 ∝ L−3, although the prestorm dependence cannot be described by a simple function at all L values. The appearance of a secondary peak and the behavior of the prestorm and post-storm spectrums are in qualitative agreement with the predictions of the bimodal diffusion model of Theodoridis et al. [1969], whereby the solar wind is the source of the observed protons.