Abstract
AbstractUsing 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.
Highlights
Variations in magnetospheric plasma mass density provide information on the morphology of the magnetosphere and the different dynamical processes occurring
Using ion density data obtained by the CODIF 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 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
Summary
Variations in magnetospheric plasma mass density provide information on the morphology of the magnetosphere and the different dynamical processes occurring. The magnetospheric mass density plays a crucial role in determining the propagation of wave modes implicated in radiation belt energization and decay [Meredith et al, 2003; O’Brien et al, 2003]. The magnetospheric mass density is a significant factor in influencing dayside reconnection rates [Borovsky and Denton, 2006], and has implications for the coupling of the solar wind to the magnetosphere. An important application for models of the magnetospheric mass density is in determining the frequencies of magnetospheric ultralow-frequency waves and, the response time of the magnetosphere to perturbations. As well as varying with changes in the number density of the electrons and ions, the ion composition contributes significantly to the plasma mass density. It is of scientific interest to understand the plasma ion composition and its spatial variations
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