A study of the effects of Forbush decreases and the 11-year variation of cosmic rays out to approximately 16 AU

In this study the onset of the new solar modulation cycle in late 1977 has been observed out to 16 AU. 27-day average cosmic-ray data (E greater than 60 MeV) from Pioneers 10, 11 and IMP 8 for the period for 1973-1978, normalized to the ground-based neutron monitor rates, have been used. Observations indicate that the long-term modulation effects propagate outward radially at 200-300 Km/sec. Forbush decreases, in contrast, are local phenomena and the associated time delays between their occurrence at earth and out to a few AU are much shorter. The data yield a radial gradient of 2-3% per AU.