Semiannual and solar activity variations of daytime plasma observed by DEMETER in the ionosphere-plasmasphere transition region

摘要

Using the plasma data of Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite and the NRLMSISE-00 atmospheric model, we examined the semiannual and solar activity variations of the daytime plasma and neutral composition densities in the ionosphere-plasmasphere transition region (~670–710 km). The results demonstrate that the semiannually latitudinal variation of the daytime oxygen ions (O~+) is basically controlled by that of neutral atomic oxygen (O), whereas the latitude distributions of the helium and hydrogen ions (He~+ and H~+) do not fully depend on the neutral atomic helium (He) and hydrogen (H). The summer enhancement of the heavy oxygen ions is consistent with the neutral O enhancement in the summer hemisphere, and the oxygen ion density has significantly the summer-dense and winter-tenuous hemispheric asymmetry with respect to the dip equator. Although the winter enhancements of the lighter He~+ and H~+ ions are also associated with the neutral He and H enhancements in the winter hemisphere, the high-density light ions (He~+ and H~+) and electrons (e~-) mainly appear at the low and middle magnetic latitudes (|λ|<50°). The equatorial accumulations of the light plasma species indicate that the light charged particles (He~+, H~+, and e~-) are easily transported by some equatorward forces (e.g., the magnetic mirror force and centrifugal force). The frequent Coulomb collisions between the charged particles probably lead to the particle trappings at different latitudes. Moreover, the neutral composition densities also influence their ion concentrations during different solar activities. From the low-F_(10.7) year (2007–2008) to the high-F_(10.7) year (2004–2005), the daytime oxygen ions and electrons increase with the increasing neutral atomic oxygen, whereas the daytime hydrogen ions tend to decrease with the decreasing neutral atomic hydrogen. The helium ion density has no obvious solar activity variation, suggesting that the generation (via the neutral He photoionization) and loss (via the charge exchange with neutral nitrogen N_2 and/or the recombination with electrons) of the daytime He~+ ions are comparable during different solar activities.