Further low-energy auroral-ion composition measurements

Sounding rocket observations of auroral-ion composition in the premidnight sector during an auroral ‘breakup’ are reported. Proton H+ and doubly charged helium ions He++ were detected above the atmosphere in the 2- to 20-kev energy range, whereas no He+ ions were observed. Energy spectra of H+ and He++ obtained during the rocket flight were found to be peaked at the same energy per unit charge (6 kev), and the average ion-flux ratio He++/ H+ was determined to be 3%. Maximum intensities of 2 × 105 cm−2 sec−1 ster−1 kev−1 for H+ and 2 × 104 cm−2 sec−1 ster−1 kev−1 for He++ were recorded at 6 kev per unit charge. These results strongly suggest that the auroral ions originated in the solar wind, were injected into the magnetosphere with approximately the same probability per ion incident on the magnetospheric boundary, and fell through an electrostatic potential of about 6 kv before being detected.