High‐energy cosmic ray intensity waves

Abstract

A new mode of galactic cosmic ray modulation has been observed. It manifests itself as waves of isotropic intensity variations of high‐energy (∼150 GeV) galactic cosmic rays. The wave periods are in synchronism with the alternating toward and away polarity states of the sectored interplanetary magnetic field. Since data with the requisite precision for observing the phenomenon first became available in 1982, isotropic intensity waves have occurred during the latter half of three successive years when the long‐term modulation was maximum (1982) and recovering rapidly (1983 and 1984), but were not readily discernible in 1985, after the steady state intensity had recovered almost to the solar minimum level. The variational spectrum Δ j(p)/j(p) = kp γ (where p is magnetic rigidity) is hard, with γ ≈ 0. Although the largest Forbush decrease ever recorded occurred while the 1982 episode was in progress, isotropic intensity waves are not causally related to that effect. However, the isotropic intensity waves observed thus far are in phase in the southern hemisphere with much smaller concurrent variations of the north‐south anisotropy. The new phenomenon bespeaks a difference, not yet identified, in the plasma regimes on opposite sides of the neutral sheet.