IONIZATION STATES OF LOW-ENERGY COSMIC-RAYS - RESULTS FROM SPACELAB 3 COSMIC-RAY EXPERIMENT

We have determined the ionization states of a total of 75 low-energy (15-145 MeV nucleon-1) cosmic ray events in an experiment conducted onboard Space Shuttle Spacelab 3 mission during 1985 April-May. The geomagnetic field was used as a rigidity filter in this experiment to place an upper limit on the ionization state (Z*) of individual cosmic ray particles using the relation Z* less-than-or-equal-to Mpc/R(c), where p is the measured momentum per nucleon of an ion, M is the mass of the ion, and R(c) is the computed threshold rigidity for the arrival location and direction of the ion in the Spacelab orbit. Results for 27 of these events have been reported earlier (Biswas et al.; Singh et al.). The data presented in this paper more than double the event statistics and confirm the presence of three distinct groups of energetic particles in the near-Earth space: (i) low-energy (15-25 MeV nucleon-1) anomalous cosmic rays that are either singly ionized or consistent with their being in singly ionized state, (ii) fully ionized galactic cosmic ray ions, and (iii) partially ionized iron and sub-iron group ions. The partially ionized heavy ions accounts for about 20% of all the iron and sub-iron group ions detected at the Spacelab orbit within the magnetosphere in the energy interval 25-125 MeV nucleon-1. We present arguments to show that they are indeed a part of the low-energy galactic cosmic rays present in the interplanetary space. The source of these ions could be the low-energy (1-10 MeV nucleon-1) partially ionized galactic cosmic rays that are accelerated to the observed energies in the interstellar space and/or at the heliospheric boundary.