The radial gradient of cosmic radiation measured by Mariners 2 and 4

Abstract

The ionization rate and omnidirectional flux of radiation in interplanetary space were measured by similar ion chambers and GM tubes on Mariners 2 and 4 during September-December 1962, and November 1964 to March 1965, respectively. The energy threshold (determined by 0.2 g/cm² of wall material) was 10 Mev for protons. Average measured values were 681 ion pairs/ sec cm³ STP air and 2.89 particles/cm² sec in September-October 1962, and 984 and 4.29 in December 1964 to January 1965. Daily averages of ionization when the spacecraft was near earth correlate with the Ottawa neutron monitor with a coefficient of 0.6–0.8, and with the IMP/OGO Geiger monitor (available for Mariner 4 only) with a coefficient of 0.94. Using these monitors to remove temporal variations, we calculate the following radial gradients: Mariner 2, 1962 Ottawa Monitor 5.6–12.7%/AU 1.0–0.7 AU Mariner 4, 1964–1965 Ottawa Monitor 14–22 1.0–1.28 AU IMP/OGO Monitor 5.2–8.4 The most accurate result is derived using the IMP/OGO monitor for a reference. If data from it were available in 1962, we estimate that a ‘true’ gradient of 12–27%/AU would be derived.