A Statistical Study of Magnetospheric Plasma Mass Loading Using the Cluster Spacecraft

Abstract:

The study of variations in magnetospheric plasma mass density is motivated by its crucial role in the many different dynamical processes that occur in the magnetosphere, such as determining the propagation of wave modes implicated in radiation belt energisation and decay. Using Cluster data, from the WHISPER and CIS instruments for the interval spanning 2000-2012, the field-aligned variations in the electron density and average ion mass are examined. The combination of the resulting models are used to infer an empirical model for the plasma mass density along closed geomagnetic field lines in the outer plasmasphere and plasmatrough (5.9 ≤ L < 9.5), including dependences with L shell and MLT (Magnetic Local Time). The findings indicate some key features concerning the variations in number density and ion composition in this region. This includes a previously unconsidered localised peak in electron density at the magnetic equator, at least within the plasmatrough.

The study is extended further by examining variations in the data with geomagnetic activity, and including these dependences in the electron density and average ion mass models. The resulting model for the inferred plasma mass density spatial distribution provides information on the mass loading processes in this region during different levels of geomagnetic activity. In addition, the mass density model is utilised to investigate corresponding variations in the properties of magnetospheric ULF (Ultra Low Frequency) pulsations, demonstrating a possible application of the model.