Cosmic‐ray propagation processes: 2. The energetic storm‐particle event

Seven examples of the observation of intense fluxes of ≈10-Mev charged particles associated with disturbances in the interplanetary medium during 1966 are discussed, and detailed data from four of them are presented. It is shown that there is an approximately one-to-one correspondence between such events and the onset of Forbush decreases initiated by solar flares. The particle fluxes exhibit characteristic time profiles, with a typical time scale of 6 hours. The particle fluxes are strongly anisotropic, and the direction of the anisotropy shows drastic temporal variations during the onset of the Forbush decrease. The fluxes of particles of energy ≈10 Mev are frequently considerably greater, while the energy spectra are usually noticeably softer than those observed during the preceding flare effect. A bidirectional anisotropy is commonly observed following the decreasing intensity phase of a Forbush decrease, the two directions of maximum flux being aligned parallel and antiparallel to the interplanetary magnetic field vector. It is shown that the energetic storm particles do not suffer any significant trapping in the magnetic regime associated with the onset of the Forbush decrease. It is shown that the various facts are consistent with the Parker ‘blast wave’ model for the interplanetary magnetic field associated with the Forbush decrease, while the Gold ‘magnetic bottle’ model is unable to explain the observed temporal variations in the anisotropic character of the particle fluxes. Studies of the net particle flux moving radially away from the sun suggest that the energetic storm particles are suffering acceleration within the magnetic field (i.e. within the shock front) associated with the onset of the Forbush decrease. It is suggested that the recurrent low-energy cosmic-ray events observed by other workers might consist of energetic storm particles associated with recurrent Forbush decrease events.