The annual and longitudinal variations in plasmaspheric ion density

Abstract

[1] This paper shows that at solar maximum, equatorial ion densities at L = 2.5 are substantially higher at American longitudes in the December months than in the June months. This arises because the configuration of the geomagnetic field causes a longitude-dependent asymmetry in ionospheric solar illumination at conjugate points that is greatest at American longitudes. For example, at À60 E geographic longitude the L = 2.5 field line has its foot point near 65 geographic latitude in the Southern Hemisphere but near 42 latitude in the Northern Hemisphere. We investigated the consequent effects on equatorial electron and ion densities by comparing ground-based observations of ULF field line eigenoscillations with in situ measurements of electron densities (from the CRRES and IMAGE spacecraft) and He + densities (IMAGE) for L = 2.5 at solar maximum. Near À60 E longitude the electron and ion mass densities are about 1.5 and 2.2 times larger, respectively, in the December months than in the June months. Over the Asia-Pacific region there is little difference between summer and winter densities. Plasmaspheric empirical density models should be modified accordingly. By comparing the electron, helium, and mass densities, we estimate the annual variation in H + , He + , and O + concentrations near À3 E longitude and À74 E longitude. In each case the He + concentration is about 5% by number, but O + concentrations are substantially higher at À3 E longitude compared with À74 E. We speculate that this may be related to enhanced ionospheric temperatures associated with the South Atlantic anomaly.

Comparison of equatorial plasma mass densities deduced from field line resonances observed at ground for dipole and IGRF models

“…Incidentally, it is also worth mentioning that the mass loading factors deduced by Menk et al [2012] at À74°E and À3°E geographic longitudes by comparing the inferred plasma mass densities with in situ measurements of electron densities at L = 2.5 should likewise be reduced by~20% for both longitudes. This would lead to predict O + ions concentrations less than those estimated by Menk et al [2012].…”

“…These corrections would not affect, however, the December/June density ratios (annual variation), with largest values (~2) at~À60°E geographic longitude (Φ~10°) which is probably caused by seasonal variation in plasmaspheric plasma temperatures and meridional winds [Richards et al, 2000;Menk et al, 2012]. Incidentally, it is also worth mentioning that the mass loading factors deduced by Menk et al [2012] at À74°E and À3°E geographic longitudes by comparing the inferred plasma mass densities with in situ measurements of electron densities at L = 2.5 should likewise be reduced by~20% for both longitudes. This would lead to predict O + ions concentrations less than those estimated by Menk et al [2012].…”

“…This may result in an erroneous interpretation of the longitudinal variation in plasmaspheric mass density when comparing results from ground-based arrays located at different longitudes. Previous studies of this kind have been conducted by Clilverd et al [2007] and by Menk et al [2012]. In particular, Menk et al [2012] examined a range of geographic longitudes extending from about À120°E to 160°E (~À60°≤ Φ ≤~230°) for L = 2.5 and separately for December and June months.…”

“…Previous studies of this kind have been conducted by Clilverd et al [2007] and by Menk et al [2012]. In particular, Menk et al [2012] examined a range of geographic longitudes extending from about À120°E to 160°E (~À60°≤ Φ ≤~230°) for L = 2.5 and separately for December and June months. For December, the highest mass density values were found in the geographic longitude range À70°E-0°( Φ~0°-75°), a behavior similar to that observed by in situ measurements of electron and He + densities.…”

Radial dependence of ionization losses of protons of the Earth's radiation belts

“…In the solar maximum N e (L) is 1.3-1.5 times higher than in the minimum (e.g., Rasmussen and Schunk, 1990). Average values of N e (L) are 1.5-2.2 times more in December than in June (e.g., Rasmussen and Schunk, 1990;Menk et al, 2012).…”