Entry of solar cosmic rays into the polar cap atmosphere

Worldwide ƒmin records are used to analyze onset patterns of PCA owing to enhanced ionization caused by penetrating solar particles. The initial phase of PCA consists of at least three characteristic stages that occur successively in different zones of the polar region: the first stage is observed as a slightly enhanced ionization near the geomagnetic pole; the second stage as a remarkable development of PCA in the polar cap above 65° (corrected geomagnetic latitude); and the third stage as an extension of the enhanced ionization down to latitudes of 60° or lower. A diffusion model of interplanetary space suggests that the three stages are due to differential arrival of solar electrons, protons, and α particles at the polar atmosphere. The importance of α particles to the third stage is established by using known incident spectra of both solar protons and α particles. An empirical relation between particle rigidity and geomagnetic latitude shows a uniform reduction of the Störmer rigidity cutoff by some 70% in the interval 55–65° in corrected geomagnetic latitude and a disappearance of cutoff in the polar cap above 65°. The former suggests a quiet-time ring current, while the latter may be explained either by a direct merging of the polar cap magnetic field with the interplanetary field or by a particle-energy coupling due to a diffusion process such as in a postulated long geomagnetic tail. Onset patterns of PCA are used as a test for these theoretical models of the geomagnetosphere. Transient times of a few hours usually required for the first and second stages seem to favor the particle-diffusion model.