The cosmic ray intensity above the atmosphere near the geomagnetic pole

Abstract

With the help of new high altitude intensity measurements of Meredith, Van Allen and Gottlieb with single Geiger counters near the north geomagnetic pole, an assessment of existing knowledge of the low rigidity end of the primary cosmic ray spectrum is presented. The new Iowa rocket experiments fully confirm and substantially extend previous evidence for the marked flattening of the integral primary cosmic ray spectrum below a magnetic rigidity of about 1.5–109 volts. In particular, they indicate a complete or nearly complete absence of the major components (H, He) of the primary radiation in the following spectral regions: (a) For hydrogen, the magnetic rigidity range 1.2–109 volts to 0.18-109 volts (kinetic energy range 590 MeV to 18 MeV). (b) For helium, the magnetic rigidity range 1.2 · 109 volts to 0.37·109 volts (kinetic energy range 700 MeV to 72 MeV). The low end of these ranges is far below that attainable by any means except rocket-borne apparatus. This observed absence of low rigidity primaries is not inconsistent with a solar dipole moment as large as 0.6–1034 gauss-cm3. But there may be an entirely different physical cause. Due to the low relative abundance of primary nuclei ofZ > 2, the present data are not of sufficient accuracy to conclusively exclude their presence in the low rigidity region of the spectrum. A crucial test of the solar cut-off hypothesis is available in this connection. Preliminary roeket flights of pulse ionization chambers have been made with the intention of investigating the intensities of low rigidity heavy nuclei.