Changes of the outer to inner heliospheric intensity ratios of >70 MeV galactic cosmic rays as a function of solar magnetic polarity

Abstract

We have examined the temporal history of the outer to inner heliospheric intensity ratios of >70 MeV cosmic rays using the Voyager, Pioneer, and IMP spacecraft during the time period from 1984 to 2001. This ratio effectively measures the changes in the integral radial cosmic ray gradient between the outer heliosphere and the Earth. In the time period from 1984 to 1990, when the solar magnetic polarity was negative, this outer to inner heliospheric ratio out to ∼50 AU changed temporally in such a way that an average radial gradient ∼2%/AU near the ecliptic plane was maintained throughout this entire period. After 1990, when the solar magnetic polarity became positive, this average ratio began to slowly decrease, as measured at all three spacecraft throughout the heliosphere out to 50–70 AU, reaching a minimum value corresponding to an average 0.5%/AU radial gradient in 1996. In 1997–1998, in conjunction with the first large transient decreases in the new 11‐year solar cycle, the intensity ratios between the outer heliosphere spacecraft and the Earth began to increase rapidly. By the time the Bastille Day transient in 2000 at the Earth had propagated to V2 and V1, which were now between 65 and 82 AU, the outer to inner heliospheric intensity ratios had increased from a value ∼1.4 in 1997 to between 3.0 and 4.0, respectively, at V2 and V1 in 2001. Therefore this time period represents a transition back to a negative solar magnetic polarity and to the reestablishment of a ∼2%/AU average radial gradient similar to that of the previous negative polarity cycle in the heliosphere, but now observed out to ∼80 AU in 2001. This particularly simple pattern for the temporal changes in the average radial gradient in alternate solar magnetic polarity cycles can thus be followed through almost a complete 22‐year solar magnetic cycle.