| Summary: | Using Cluster data, from the WHISPER and CODIF instruments, for the interval spanning 2001 { 2012, empirical models describing the electron density, average ion mass, and plasma mass density distribution along closed geomagnetic field lines are determined. The models represent the region spanning 5:9 ≤ L < 9:5, with dependences on L shell and MLT (Magnetic Local Time) included. An average mass density model, describing spatial variations for typical quiet time conditions, was produced by independently modelling field-aligned variations in the electron density and average ion mass, and combining these to infer the corresponding model for mass density. The resulting average models illustrates some key features of the electron density and average ion mass spatial distributions. Dependences of the electron density, average ion mass, and total plasma mass density on the ring current intensity were also examined. Using a similar approach as for the average models, the field-aligned distributions were quantified, including variations with Dst index, providing information on how the spatial distributions vary due to storm-related processes. A key result obtained is the observed general decrease in mass density, accompanied by an increase in average ion mass, during disturbed conditions. An application of the mass density models in improving estimates of field line eigenfrequencies is explored, using the time-of-flight technique. The analysis highlights the contribution of mass density variations in determining the frequency of standing Alfvén waves on closed geomagnetic field lines, as well as the magnetic field variations. The results provide information on the spatial distribution of field line eigenfrequencies, as well as dependences on geomagnetic activity. Furthermore, the validity of the improved time-of-flight calculations are illustrated through a comparison to a statistical analysis of ground magnetometer FLR (Field Line Resonance) observations.
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