Evidence for a solar wind origin of auroral ions from low-energy ion measurements

Abstract

A rocket-borne ion spectrometer capable of separating and detecting H+, He+, and He++ with energies between 2 and 24 kev has been flown into an early-evening proton aurora from Fort Churchill, Canada. The measurements made with a Black Brant 4 at altitudes to 775 km enabled the positive-ion composition of the primary auroral precipitation to be determined above the masking effects of atmospheric interactions. The proton differential energy spectrum was found to peak at 8 kev with an intensity of 1.5×105 (cm2 sec ster kev)−1, which is orders of magnitude less than some previously obtained values between 2 and 10 kev. No statistically significant flux of He+ was observed. The He++ intensity at 15 kev was measured to be 7±3.5×103 (cm2 sec ster kev)−1 and an upper limit of 5×103 was set at 7.5 kev. The He++/H+ ratios determined from these data were combined with the two previously reported measurements at higher energies to obtain an auroral He++ differential energy spectrum. The H+ and He++ energy spectra were used to calculate an integrated He++/H+ flux ratio of approximately 4%. Some comments on characteristics of auroral particle acceleration mechanisms are presented. The observation of auroral He++ and H+ ions of approximately the same ratio as in the solar wind, and the apparent lack of He+ ions, is interpreted as strong evidence for a direct solar wind origin of the particles.