The ionization state of oxygen ions in anomalous cosmic rays: results from the ANURADHA experiment in Spacelab-3

Abstract

We report the first results on the determination of the ionization states of oxygen ions in the anomalous cosmic rays (ACR) from the measurements of their flux in the cosmic-ray experiment in Spacelab-3 (SL-3) mission of NASA flown at 350 km altitude during 29 April-6 May, 1985. The detectors used were specially prepared CR-39 plastics of very high sensitivity for recording tracks of ions with Z>2. The measured orbit averaged flux of ACR oxygen is (2.9±1.3)×10⁻⁴ particles m⁻²sr⁻¹s⁻¹ (MeV N⁻¹) at an energy of 23 MeV N⁻¹. We made an independent estimate of the expected ACR oxygen flux at SL-3 orbit from interplanetary data and compared this with the measured flux to infer the ionization states of ACR oxygen ions. The flux and energy spectra of ACR oxygen at 1 AU outside the magnetosphere is obtained from the data of Voyager-2, during the same epoch as the SL-3 flight, and using the measured radial intensity gradient of 15%/AU for ACR oxygen between 1-17 AU. We calculate the geomagnetic transmission factors for ACR oxygen ions of charge states O⁺¹, O⁺², etc., from the known cut-off rigidities in the world grid and using the SL-3 trajectories for 116 orbits in the 6-day mission to obtain the expected flux at SL-3 for different charge states. When these flux values are compared with our measured flux, the averge ionization state of ACR oxygen ions in the energy interval of 20-26 MeV N⁻¹ is obtained as O⁺¹.