Voyager and Pioneer spacecraft measurements of cosmic ray intensities in the outer heliosphere: Toward a new paradigm for understanding the global modulation process: 2. Maximum solar modulation (1990–1991)

Abstract

In this paper we present interplanetary radial intensity profiles of five particle energies/species of cosmic rays measured by the IMP, Voyager 1, Voyager 2, and Pioneer 10 spacecraft at the time of maximum solar modulation in 1990–1991. These radial profiles are then compared with those measured in 1987 at a period of minimum modulation in order to examine the radial dependence of the 11‐year solar modulation effects. It is found that the radial intensity profiles observed in 1990–1991 can be described to first order by simple reduction factors of the radial profiles observed in 1987. These reduction factors range from ∼2.5 at the highest rigidities to ∼10 at the lowest rigidities measured here. These reduction factors describe the magnitude of the 11‐year modulation and imply that the intensities at the estimated location of the termination shock (TS), here taken to be at 88 AU, are reduced by a corresponding factor to those at the Earth at each energy. Therefore these TS intensities range from only 0.4 to 0.05 of the estimated IS intensities at the highest to lowest rigidities, respectively. This implies that most of the overall (11‐year and residual) modulation at this time and at these rigidities is occurring beyond the TS. Some of this 11‐year modulation (∼50% or less) occurs within this location. This part of the modulation is usually described in the present modulation models by invoking changing heliospheric current sheet tilt and changes in the drift and diffusion coefficients. However, overall, the 11‐year solar modulation, at least at the rigidities examined here, is most properly described as an outer heliospheric modulation phenomenon occurring mainly at or beyond the TS with a secondary (weaker) modulation occurring inside the TS. This is similar to the situation described by Webber and Lockwood [this issue], where the dominant modulation at times of minimum (residual) solar modulation also appears to occur in the outer heliosphere near to or beyond the TS. This large outer heliosphere modulation remains largely unexplained in present modulation theories. A new paradigm in solar modulation theory is required to more fully account for the large fraction of the 11‐year and/or residual solar modulation that is occurring in the outer heliosphere near to or beyond the TS.