Galactic Cosmic-Ray Intensity to a Helicocentric Distance of 18 AU.
IOWA UNIV IOWA CITY DEPT OF PHYSICS AND ASTRONOMY
Pagination or Media Count:
An updated report is given of observations of galactic cosmic ray intensity to heliocentric radial distances of 8.6 and 18.4 AU with Pioneer 11 and Pioneer 10, respectively. Solar activity via the magnetic structure of the interplanetary medium continues to modulate the intensity out to the greatest distance reached thus far. During the period 1972-1979, aperiodic temporal variations of intensity by about a factor of two and on a time scale of the order of a year are observed as are quasi-persistent cyclic variations of 26-day period and amplitude of a few percent. The latter are associated with fast-slow solar wind streams, not with toward-away magnetic field sectors. For protons of energy E sub P 80 MeV, there is a fairly consistent heliocentric radial gradient of 2.1 or - 0.3 percent per AU in intensity until April-May 1978, at which time a substantial disruption of the distribution of cosmic rays in the heliosphere occurred. The radius of the heliosphere is estimated to be of the order of 80 AU. Author