Climax Neutron Monitor Research Articles

Solar modulation of the low-energy galactic helium spectrum as observed on the Imp 1 satellite

The time variations in the low-energy cosmic-ray helium of galactic origin were studied over a continuous period of 5.5 months near the minimum of the present solar activity cycle. We find that during this time the helium flux in the 30- to 90-Mev per nucleon energy interval increased by (65±22) % while the flux in the 250- to 500-Mev per nucleon energy range increased by (30±8)%. The increase in the flux of primaries > 3 Gv in rigidity, as derived from the Climax neutron monitor intensity, was only ∼5% in this time interval. It is shown from these observed increases that the modulation of the galactic cosmic radiation at these low energies is dependent on the velocity of the galactic particle rather than on its rigidity and that it is well described by the theory of Parker. The nature of the spectrum outside the solar system is discussed. The time-averaged modulated galactic helium spectrum in early 1964 which has now been extended down to 7 Mev per nucleon can be approximated by dJ/dE = 3 × 10−6 E1.65 helium/m2 sec ster-Mev, with total kinetic energy E measured in Mev, over the energy range 7 to 90 Mev per nucleon. We report the evidence for helium of solar system origin below ∼50 Mev per nucleon in the ‘27-day’ recurrent regions which are already known to contain large fluxes of low-energy protons. The measurements were made on the Imp 1 (Explorer 18) satellite over the continuous period from November 27, 1963, to May 15, 1964. The instrument used was a four-element charged particle telescope, capable of measuring the energy loss and the total energy of low-energy proton and helium nuclei.