Observing the cold plasma in the earth's magnetosphere with the EMMA network

We illustrate a semi-automated procedure to detect the field line resonance (FLR) frequencies and the derived equatorial plasma mass densities in the inner magnetosphere from ULF measurements recorded at the European quasi-Meridional Magnetometer Array (EMMA). FLR frequencies are detected using the standard technique based on cross-phase and amplitude ratio spectra from pairs of stations latitudinally separated. Equatorial plasma mass densities are then inferred by solving the toroidal MHD wave equation using the TS05 Tsyganenko magnetic field model and assuming a 1/r dependence of the mass density along the field line. We also present a statistical analysis of the results obtained from 165 non-consecutive days of observations at 8 station pairs covering the range of magnetic L-shells 2.4-5.5 and encompassing a wide range of geomagnetic conditions. The rate of FLR detection maximizes around local noon at each pair of stations, reaching the highest values (~95%) around L = 3. A clear diurnal modulation of the FLR frequency is observed at all L values. At the lowest latitudes, the variation is characterized by a rapid decrease in the early morning hours, a stagnation in the middle of the day, and an increase in the evening hours. At higher latitudes, a continuous and more pronounced decrease of the FLR frequency is observed during all daytime hours reflecting a permanent state of recovery of flux tubes depleted by events of enhanced magnetospheric convection. Consistently, the radial profiles of the inferred equatorial mass density show a density increase from morning to afternoon which gets more pronounced with increasing distance and with the level of the preceding geomagnetic activity. The results also confirm the formation of the plasmapause at geocentric distances that decrease as the disturbance level increases. Mean mass density distributions in the equatorial plane are also shown in 2-D maps for different geomagnetic conditions, as well as for a representative stormy day.