Correlation between the He/H ratios in upstream particle events and in the solar wind

Abstract

We have compared the alpha particle to proton ratio measured at a fixed energy per charge (∼30 keV/e) in 29 well‐developed diffuse ion events upstream of the earth's bow shock with the same ratio observed simultaneously in the solar wind. The data were obtained with the Max‐Planck‐Institut/University of Maryland energetic particle sensor and the Los Alamos National Laboratory/Max‐Planck‐Institut plasma instrument aboard ISEE 1. We have also correlated the energetic He+2 to H+ intensity ratio in these upstream events with the same intensity ratio measured in the ring current population close to the magnetopause during the same spacecraft orbit. The high correlation of the ratio in upstream events with the ratio in the solar wind and the much weaker correlation with that in the ring current population strongly suggests, independent of other arguments, that the solar wind is the primary source for diffuse upstream ions. We also find that the ratio of the He+2 to H+ intensities at ∼30 keV/e is on the average enhanced by a factor of 1.6 over the solar wind ratio, although the three events with the lowest Alfven Mach numbers show a depletion relative to the solar wind ratio. Our results are consistent with Fermi acceleration of a seed particle population, which is extracted from the solar wind, generally with little or no compositional bias, and then moderately enhanced in helium during the acceleration process.