A comparison of the 0.1–17 keV/e ion composition in the near equatorial magnetosphere between quiet and disturbed conditions

Abstract

Data from the plasma composition experiment on ISEE 1 were used to investigate the relative number densities and energy characteristics of H+, He++, He+, and O+ in the near‐equatorial magnetosphere at L ≤ 12 during magnetically quiet and disturbed conditions. The ‘quiet’ data were selected on the basis of both Dst and Kp over ∼5 day periods preceeding the data accumulations, and the ‘active’ data were from periods of Dst ≲ −100γ. The ions included in this study had energies in the range of 0.1 ≤ E/Q ≲ 17 keV/e. The data naturally subdivide into two largely different sets in terms of a dipole L parameter. (1) At L ≲ 5 the ion density is dominated by three species, H+, O+, and He+, appearing in variable ratios. The He++ is a small component and is generally obscured by background counts caused by penetrating MeV electrons. The O+ is the most abundant species in well over half of the plasma samples, whereas the H+ is more abundant in the remaining samples. No sample with statistically significant counts shows He+ to be the most abundant ion, although the He+ exceeds the H+ in density in several cases. The O+/H+ and He+/H+ ratios increase toward the inner edge of the ring current population and are, on the average, highest during quiet conditions. Both of these features are qualitatively consistent with a decay process dominated by charge exchange, but a comparison with published charge exchange rates seems to imply that other processes are also involved. All three ion species have a similar mean energy (∼2–7 keV) within the energy window of the experiment, and all three have a low‐energy component (0.1 ≤ E/Q ≲ 1 keV/e) that usually dominates the number density at the inner edge, at about L ∼ 3–4 during quiet conditions and L ∼ 2–3 during disturbed conditions. (2) At L > 5 all four species may be present simultaneously in highly variable ratios, but the H+ is usually the most abundant, particularly during quiet conditions. As a rule, the O+/H+ and He+/H+ ratios decrease with increasing L, whereas the He++/H+ ratio slowly increases. Typical values in the outer magnetosphere (L ∼ 10–12) are 1–3% for the He++/H+ and He+/H+ ratios during both quiet and disturbed conditions, whereas the O+/H+ ratio may range from below 1% to over 200%, depending on the magnetic activity level. The highest O+/H+ ratios are found here during the early main phase of magnetic storms, which is contrary to the conditions at L < 5. The H+, O+, and He+ all have a similar mean energy in this region (∼4–6 keV), whereas the He++ is 2‐3 times more energetic.