The radial gradient of interplanetary radiation measured by Mariners 4 and 5

A detector package on the Mariner 4 spacecraft, which was launched on a Mars‐bound trajectory on November 28, 1964, included a shielded GM tube (detector C) whose threshold is about 50 to 55 Mev / nucleon for proton and alpha particles. By comparing the counting rate of detector C to that of a similar earth‐orbiting GM tube on the IMP‐OGO series of spacecraft, we find the following: ( a ) As the difference in heliocentric longitude between the two spacecraft increases, there appear differences in the time and/or space development of a Forbush decrease. We estimate that the Forbush decrease may be confined to a region of dimensions as small as 0.5 × 0.7 AU. ( b ) The gradient of particles in the interplanetary medium with energies above the detector energy threshold is (−14.4 ± 2) % /AU (i.e., directed toward the sun), during the solar minimum period of 1964–1965. ( c ) Additional data from the Venus‐bound Mariner 5 spacecraft and the Mariner 4 and IMP‐OGO spacecraft during the latter part of 1967 are consistent with the 1965 results and show that the gradient may have increased in magnitude. (d) Based on the above results, it is shown that the interplanetary flux of particles of E ≳ 50 Mev/nucleon must in part be of solar origin. ( e ) It is argued that the gradient of galactic cosmic radiation of E ∼ 50 Mev/nucleon cannot be measured even at the time of solar minimum, in the range 1.0 to 1.6 AU. (ƒ) The astrophysical consequences derived by O'Gallagher (1967) regarding the ( R β) −1 dependence of the gradient and the size of the modulation region are now open to question.