Onset and amplitude of the 11‐year solar modulation of cosmic ray intensities at the Earth and at Voyagers 1 and 2 during the period from 1997 to 2003

Abstract

We have examined the intensity changes of galactic and anomalous cosmic rays at five rigidities and at three heliocentric radii, 1 AU, ∼60 AU, and ∼80 AU, between the maximum 11‐year solar cycle intensity observed in 1997–1998 and the minimum intensity observed in 2001. The main results of this paper are as follows. (1) The onset of the intensity decrease in the outer heliosphere is delayed by 1–2 years from that observed at Earth. Most of the intensity decrease at both the Earth and Voyagers 1 and 2 occurs at the time of the solar magnetic polarity change from positive to negative. (2) The amplitude of this 11‐year intensity decrease is strongly dependent on heliospheric radius. It is only ∼1/3 as large at Voyager 2 located at ∼60 AU and ∼1/6 as large at Voyager 1 located at ∼80 AU for all galactic particles. This radial dependence extrapolates to a zero modulation in this positive solar magnetic polarity cycle at a distance ∼90–100 AU. This strong radial dependence of the intensity changes is in contrast to that observed in the previous negative polarity cycle where the radial effects were much smaller. (3) The dependence on the rigidity, P, of the modulation observed at all the three locations follows a simple P−1 dependence between ∼0.6 and 2.5 GV. The relative modulation of anomalous O nuclei is ∼5.35 times that for galactic nuclei at the same rigidity of 2.4 GV. This is just the ratio of speeds of anomalous O and galactic H and He nuclei at this rigidity.