Modulation of galactic cosmic rays at solar maximum: Observations

Abstract

The spectra of galactic cosmic rays observed anywhere inside the heliosphere are modulated by the sun because the charged particles must fight their ways inward against the radially expanding solar wind. In the past half a century, galactic cosmic ray fluxes at Earth were continuously monitored by neutron monitors on the ground and instruments on spacecraft in the Earth orbit. It was found that cosmic ray intensities are the lowest at the maximum of the 11-year solar activity cycle when the solar and heliospheric magnetic fields are complex and disturbed by many events on the sun. Our knowledge of comic ray modulation in the global heliosphere was greatly enhanced in the past two decades, thanks to an array of heliospheric missions, Voyagers and Pioneers that covered the outer heliosphere up to ∼75 AU radial distance from the sun and Ulysses that covered the inner heliosphere over almost the entire range of heliographic latitude. It is particularly fortunate that Ulysses was at the highest heliographic latitudes in 2000–2001 making observations of the polar regions of the heliosphere at the solar maximum for the first time. With the observations from the heliospheric network missions, both the spatial variation and the temporal variation of cosmic ray modulation in the three dimensional heliosphere could be investigated. In this paper, observational results of cosmic rays during recent solar maximum periods are reviewed. Comparisons to the cosmic ray observations during the solar minimum are made to show how the modulation changes with the solar activity cycle and solar magnetic fields. Implications of these results to our understanding of the heliospheric magnetic field and solar wind structures and the mechanisms of long-term cosmic ray modulation are discussed.