Observations of protons in the magnetosphere with Mariner 4

On November 28, 1964, the Mariner 4 spacecraft was launched toward Mars and passed through the outer magnetosphere near the geomagnetic equatorial plane on the dawn side of the earth. The University of Iowa experiment on Mariner 4 consists in part of a thin (∼35 microns) surface barrier detector sensitive to protons in the energy ranges 0.50≤Ep≤11 Mev (D1) and 0.88≤Ep≤4 Mev (D2) and insensitive to electrons of any energy. The maximum proton intensities observed were ∼6×107 (cm2 sec ster)−1 in D1 for 2.97≤ L≤3.80 and 9.3×106 (cm2 sec ster)−1 in D2 for L = 3.14. The observed counting rates are consistent with a spectrum of the form dj/dE = KE−γ with γ increasing monotonically from 1.38 at L∼2.35 to ∼8 at L∼6.5. The counting rates are also consistent with a spectrum of the form dj/dE = Ke−E/E with Eo decreasing monotonically from 740 kev at L∼1.88 to ∼100 kev at L∼6.5. These results represent a new body of measurements of trapped proton intensities near the equatorial plane in the magnetosphere at radial distances by ≳2.5 RE and have been obtained by a different detection scheme than that employed by Davis and Williamson [1963]. Our results are consistent with those of Davis and Williamson. The significance of the observations with respect to proposed theoretical models is discussed.