Energetic (0.1‐ to 16‐keV/e) magnetospheric ion composition at different levels of solar F10.7

Abstract

Data obtained in the near‐equatorial magnetosphere, between L = 3 and R = 23 RE, by the plasma composition experiment on ISEE‐1 are examined for possible effects of varying solar activity, as measured by the daily F10.7 index. The data consist of velocity moments for H+, He++, He+, and O+ ions, primarily number densities and mean energies, integrated over the 0.1‐ to 16‐keV/e energy range. These are grouped into four ranges of F10.7, less than 100, 100 to 150, 150 to 200, and greater than 200, using two methods. In one the data are averaged over geomagnetic activity, in the other the data are restricted to times of AE<200 γ. In both cases, the strongest effect is found in the number densities of the He+ and the O+, which increase by factors of about 3–5 and 5–10, respectively, over the full range of the F10.7, the rate of increase varying somewhat with location. The peak density of the O+ is about 20 times that of the He+ and is the highest at L ∼ 3–5 (>1 cm−3). Both species show a decreasing energy with increasing F10.7 at R<10 RE, from about 4–5 keV at low F10.7 to about 2–3 keV at high F10.7. The O+ shows a similar but weaker trend at greater distances as well. These effects are presumably all caused by the increase in the solar EUV irradiation of the Earth' atmosphere over the rising phase of the solar cycle (cycle 21), in ways that have been discussed elsewhere in the literature in conjunction with similar effects in other data sets. A third species that shows an increasing density over the time period of these data is the He++ (by about a factor of 3). It is argued, based on published solar wind data, that this effect is due to a variation of the helium abundance in the solar wind. The fourth species, the H+, is generally the dominant one at L>7 and appears to have both solar and terrestrial origins. It does not show a strong increase in density anywhere, however. In the plasma sheet it actually decreases slightly with increasing F10.7 (about 25% over the full range of the F10.7), an effect that is ascribed in part to a known variation of the solar wind proton density and in part to the apparent tendency of the plasma sheet to maintain a constant total pressure (including all ion species). At low L the H+ density appears to be dominated by a terrestrial source, but shows little or no increase with the F10.7, suggesting that this source is essentially saturated with respect to the EUV (F10.7>80).