Primary Cosmic-Ray Proton and Alpha Flux near the Geomagnetic Equator

Abstract

The vertical flux of primary protons and of alpha particles has been measured at Guam, Marianas Islands ($\ensuremath{\lambda}=3\ifmmode^\circ\else\textdegree\fi{}$N) using a \ifmmode \check{C}\else \v{C}\fi{}erenkov scintillation detector carried to a residual pressure of 6.1 g/${\mathrm{cm}}^{2}$ by a Skyhook balloon. The primary alpha flux, ${J}_{0\ensuremath{\alpha}}$ was found to be, ${J}_{0\ensuremath{\alpha}}=18.0\ifmmode\pm\else\textpm\fi{}2$ particles/${\mathrm{m}}^{2}$-sec-sterad at 0 g/${\mathrm{cm}}^{2}$ atmos depth. The flux of primary protons, ${J}_{0p}$ was found to be, as an upper limit (without returning albedo correction), ${J}_{0p}=115\ifmmode\pm\else\textpm\fi{}12$ particles/${\mathrm{m}}^{2}$-sec-sterad at 0 g/${\mathrm{cm}}^{2}$ atmos depth, and as a lower limit (with returning albedo corrections), ${J}_{0p}=95\ifmmode\pm\else\textpm\fi{}12$ particles/${\mathrm{m}}^{2}$-sec-sterad at 0 g/${\mathrm{cm}}^{2}$ atmos depth. This flight completes an extensive latitude survey of the alpha and proton components by this laboratory using identical detectors at all latitudes. The alpha energy spectrum can be represented from 0.150 Bev/nucleon to 7.3 Bev/nucleon by ${{J}_{0\ensuremath{\alpha}}(\ensuremath{\geqq}E)=\ensuremath{-}(1.5)(415)\ensuremath{\int}{E}^{\ensuremath{\infty}}\frac{[1\ensuremath{-}\mathrm{exp}(\ensuremath{-}80{E}^{\ensuremath{'}3})]}{{(1+{E}^{\ensuremath{'}})}^{2.5}},d{E}^{\ensuremath{'}}}{\mathrm{particles}/{\mathrm{m}}^{2}\ensuremath{-}\mathrm{s}\mathrm{e}\mathrm{c}\ensuremath{-}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{d},}$ where ${J}_{0\ensuremath{\alpha}}(\ensuremath{\geqq}E)$ is the vertical flux of primary alphas with kinetic energy $\ensuremath{\geqq}E$ (measured in Bev/nucleon). This energy spectrum is in excellent agreement with the measured low-energy alpha differential energy spectrum. Above 0.35 Bev/nucleon the alpha energy spectrum is accurately represented by ${J}_{0\ensuremath{\alpha}}(\ensuremath{\geqq}E)=\frac{415}{{(1+E)}^{1.5}}$ particles/${\mathrm{m}}^{2}$-sec-sterad. The proton energy spectrum in the latitude range 0-41\ifmmode^\circ\else\textdegree\fi{}N (15.2-4.0 Bev/nucleon) is well represented by ${J}_{0p}(\ensuremath{\geqq}E)=\frac{6600}{{(1+E)}^{1.5}}$ particles/${\mathrm{m}}^{2}$-sec-sterad. Thus the flux of protons and alphas has the same energy dependence over a wide range of energies. Values of the flux of splash albedo at $\ensuremath{\lambda}=3\ifmmode^\circ\else\textdegree\fi{}$N are given and estimates are made of the magnitude of the returning albedo correction.