Cluster observations of low plasma density magnetospheric regions -a statistical study

The polar cap and magnetospheric lobe regions are interfaces between plasma sources in the solar wind and ionosphere. However, due to the high potential obtained by spacecraft in these regions in-situ particle measurements here are notoriously difficult. The Cluster-mission has enabled a study of both the polar cap as well as the near-Earth magnetospheric lobes throughout the waning part of solar cycle 23. We have obtained new and more accurate plasma densities in these regions based on spacecraft potential measurements from the EFW-instrument. The plasma density values are found from such observations by converting from the potential measurements using a functional relationship between these two parameters. From the ensuing data base the density distribution can be obtained for both regions. Based on this material, statistical investigations throughout the polar cap and magnetospheric lobe regions are carried out with respect to solar wind driving as well as internal magnetospheric processes. Similarities and variation in solar cycle dependency are discussed.