A statistical study of magnetospheric ion composition along the geomagnetic field using the Cluster spacecraft for L values between 5.9 and 9.5

Using ion density data obtained by the CODIF (ion Composition and Distribution Function analyser) instrument on board the Cluster spacecraft, for the interval spanning 2001–2005, an empirical model describing the average ion mass distribution along closed geomagnetic field lines is determined. The empirical model describes the region spanning 5.9≤L < 9.5, with dependences on L shell and magnetic local time included, and represents ions in the energy range of 0.025 to 40 keV/charge. The data reduction process involves the identification and rejection of CODIF data contaminated by penetrating energetic radiation belt particles, found to frequently occur for L < 5.9. Furthermore, a comparison of data with observations of the cold plasma population in the region provides evidence that the CODIF data set is representative of the full plasma population. The variations in average ion mass along the field lines were modeled using a power law form, which maximizes toward the magnetic equatorial plane, with observed power law index values ranging between approximately −2.0 and 0.0. The resulting model illustrates some key features of the average ion mass spatial distribution, such as an average ion mass enhancement at low L in the evening sector, indicating the transport of high‐latitude heavy ion outflows to the closed inner magnetosphere.