Remote Monitoring of the Plasma Mass Density in the Inner Magnetosphere using Field Line Resonances detected at the European Magnetometer Network EMMA

The knowledge of the spatial distribution and temporal variation of the concentration and composition of the plasma populating the Earth's magnetosphere represents an essential information for understanding the dynamic processes which take place in the magnetosphere. Such an information can be efficiently provided through the detection of geomagnetic field line resonance (FLR) frequencies by cross-spectral analysis of ULF signals recorded at ground-based magnetometer stations closely spaced in latitude and approximately aligned along the same magnetic meridian. An automated real time implementation of this technique is currently used on measurements provided by EMMA, a network of 25 magnetometer stations extending from Central Italy to North Finland (1.5 < L < 6.5) which was established in 2012. The results obtained during some selected intervals which include geomagnetic storm events are presented. The dynamics of the plasmasphere during these events is investigated by examining the time evolution of the FLR-derived radial profiles of the equatorial plasma mass density. Daytime refilling rates at different L-values are evaluated as well as day-to-day refilling during the recovery phase following events of plasmasphere erosion. We also show a comparison of FLR-derived plasma mass densities with in situ electron density measurements by Van Allen Probes during favourable conjuction events, providing useful information on the radial variation of the plasma composition.