Direct measurement of auroral alpha particles

Abstract

A rocket-borne magnetic spectrometer capable of separating singly charged alpha particles and protons with energies greater than 50 kev has been flown into a moderate auroral substorm at Fort Churchill, Manitoba. The measurements were carried out in a Black Brant VB rocket at altitudes up to 360 km. Below this altitude a beam of 50-kev alpha particles incident on the atmosphere will be near a state of charge equilibrium in which 30% of the alpha particles will be singly charged and about 70% neutral. Throughout the rocket flight, the spectrometer detected an appreciable flux of alpha paricles. Typical measured intensities of precipitating electrons (>20 kev), protons (>50 kev), and alpha particles (>50 kev) were 106 cm−2 ster−1 sec−1, 1.4 × 105 cm−2 ster−1 sec−1, and 2.6 × 104 cm−2 ster−1 sec−1, respectively. The alpha particle to proton ratio varied from approximately 0.06 to 0.3 during the flight with an estimated uncertainty of no more than a factor of 2. The α/p ratio measured here, when compared to measured solar wind values, suggests that the ‘thermalization’ process in the magnetosheath produces alphas and protons of equal velocity rather than equal energy distributions.