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).

Circulation of Heavy Ions and Their Dynamical Effects in the Magnetosphere: Recent Observations and Models

“…From low to high geomagnetic activity the O + density increases by a factor of ∼10 at solar minimum and of ∼31 at solar maximum while from low to high solar EUV flux it increases by a factor of ∼3.5. Lennartsson (1989) empirical formula is about 3 times higher than the estimate by Maggiolo and Kistler (2014), but displays a similar increase rate with F10.7. Lennartsson and Shelley (1986) obtained an empirical function describing the O + /H + density ratio variation as a function of Kp corresponding to an average ratio about 2 times higher than the one estimated from Maggiolo and Kistler (2014) observations but with a similar increase rate.…”

“…+ Energy The typical O + energy in the plasma sheet has been estimated to be around 3-4 keV both in the dayside magnetosphere (Bouhram et al 2005) and in the tail plasma sheet (Lennartsson and Shelley 1986;Lennartsson 1989). Ohtani et al (2011) provide much higher estimates of the O + energy (up to 10-20 keV) but from the extrapolation of high energy measurements, which as conceded by the authors may be associated with some errors.…”

“…Solar and Geomagnetic Activity Dependence Statistical evidence for the influence of the solar EUV irradiance (F10.7 index) on the O + ion density in the plasma sheet was provided by , Lennartsson and Shelley (1986), Lennartsson (1989), Denton et al (2005), , , Ohtani et al (2011) and Maggiolo and Kistler (2014). The instruments, observation periods, regions, energy ranges and species investigated in these studies and those in Sect.…”

“…Lennartsson and Shelley (1986) found no variation of the O + average energy with geomagnetic activity using AE index contrary to Ohtani et al (2011) who reported a positive correlation between the O + energy and the Sym-H index. Both Lennartsson (1989) and Ohtani et al (2011) reported observations indicating a decrease of the O + average energy with increasing solar EUV flux. Lennartsson (1989) estimated that the O + decreases from ∼4-5 keV for low F10.7 to ∼2-3 keV for high F10.7 and showed that this energy decrease is weaker at high geocentric distances than at low geocentric distances.…”

“…Both Lennartsson (1989) and Ohtani et al (2011) reported observations indicating a decrease of the O + average energy with increasing solar EUV flux. Lennartsson (1989) estimated that the O + decreases from ∼4-5 keV for low F10.7 to ∼2-3 keV for high F10.7 and showed that this energy decrease is weaker at high geocentric distances than at low geocentric distances. Ohtani et al (2011) reported that in contrast to H + , the O + average energy is decreasing toward Earth.…”