Cold ionospheric ions in the external dayside magnetosphere

During periods of quiet magnetic activity, a cold plasma layer with densities reaching 1-3 cm(-3) is encountered on the magnetospheric side of the dayside magnetopause. Direct density measurements from the plasma frequency indicate that this layer can have a width exceeding 1 R-E in the direction normal to the magnetopause. Plasma composition measurements indicate that the major detected ions are H+, He+ and O+. These cold ionospheric ions show a repetitive pattern of energy changes. While the magnetopause is approaching the satellites, their energy increases from below the detector low-energy threshold up to about 100 eV for protons. After the passage of the satellites into the magnetosheath and just following their re-entry into the magnetosphere, the ion energy decreases from about 100 eV for protons down to the lowest detectable energy. This behavior is interpreted as the effect of the electric field associated with the magnetopause motion. The ion motion is set up when the magnetopause is compressed and relaxed when the boundary is going out. Altogether the measurements clearly show that there are hidden plasma populations inside the dayside magnetosphere, at least during quiet geomagnetic conditions. This paper emphasizes the importance to use the determination of the plasma frequency to probe the magnetospheric density. The use of biased low-energy particle detectors located far enough from the satellite body should allow to probe the distribution function of these low-energy ions in future missions.