Cosmic-ray observations in 1964–65 with Mariner IV

The University of Iowa equipment on Mariner IV, which was launched towards Mars on 28 November 1964, consists in part of two thin-window G-M tubes sensitive to electrons of [Formula: see text] (detector B) and [Formula: see text] (detector A), and a shielded G-M tube (detector C) sensitive to electrons of E e > 150 keV and having a threshold of ~55 MeV for omnidirectional protons. Observations over the period 28 November 1964 to 30 September 1965 have shown the following: (a) Electrons in the energy range [Formula: see text] were present in the interplanetary medium prior to day 12, 1965 with intensities of ~0.5 (cm 2 s sr) −1 . (b) The interplanetary cosmic-ray gradient for protons of [Formula: see text] was less than 3% per AU and the data are consistent with zero gradient. (c) There are large changes in the intensities of protons of [Formula: see text], but the direction and magnitude of the gradient for such protons were uncertain during solar minimum. The implications of these results are discussed.