Cosmic ray recovery and solar minimum phase of solar cycle 22: An interim report

Abstract

Cosmic ray observations in space now cover four successive solar minima and extend to heliocentric distances beyond 65 AU. In the outer heliosphere, comparisons between the 1996 and 1987 data show a reversal in the sign of the latitudinal gradients, smaller radial intensity gradients, and the peak in the energy spectra of anomalous H and He+ is at much lower energies but accompanied by a large reduction in their intensity at higher energies. There is also a marked difference in the cosmic ray time history over successive solar minima. Qualitatively, all of these observations are consistent with the expected effects of gradient and curvature drifts on these particles. At 1 AU the H and He spectra returned to their 1977 levels in mid‐1996, and the relative comparison of the four solar minima spectra clearly show the imprint of a 22‐year modulation cycle. In the outer heliosphere in 1997 the galactic and anomalous cosmic ray intensities at 51 AU are well below their peak intensities in 1987 at 42 AU (except for low‐energy anomalous cosmic rays), and in fact, are comparable to the Pioneer 10 intensities at 15 AU in 1977/1978. The data over successive solar minima suggest that the turbulence over the solar poles plays an important role in qA > 0 epochs. In qA < 0 periods the neutral current sheet provides ready access to cosmic ray ions in the outer heliosphere, but this path becomes increasingly difficult inside ∼20 AU. At the present time, solar minimum conditions still prevail, and there may be further increases in these energetic particle populations in the outer heliosphere.