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

Abstract

We illustrate a semi−automated procedure to detect the field line resonance (FLR) frequencies and the derived equatorial plasma mass den− sities 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 latitudi− nally separated. Equatorial plasma mass densities are then inferred by solving the toroidal MHD wave equation using the TS05 Tsyga− nenko 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 sta− tions, 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 convec− tion. 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 forma− tion 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.

An Empirical Model for the Dayside Magnetospheric Plasma Mass Density Derived From EMMA Magnetometer Network Observations

“…The results of this new analysis are qualitatively identical to the ones presented by Del Corpo et al (), and we refer the reader to their work for a comprehensive discussion. Here we summarize only the main aspects that are crucial for deriving the model.…”

“…Del Corpo et al () presented a survey of EMMA observations taken over 165 days (in the year interval 2012–2017) and encompassing a wide range of geomagnetic conditions (Vellante & Heilig, ). EMMA is a network specifically set up to monitor the plasmasphere (Lichtenberger et al, ) by means of cross‐spectral analysis of ULF signals detected by pairs of stations slightly separated in latitude and approximately aligned along the same magnetic meridian (Baransky et al, ; Waters et al, ).…”

“…Depending on the period analyzed, Del Corpo et al () applied the procedure to a variable number of station pairs (approximately 20 pairs for each period for a total of 41 different station pairs used for the entire analysis) and arranged the results in three subsets roughly representing three different levels of geomagnetic activity. They used as indicator of the geomagnetic activity level the maximum value of the index in the 24 hr preceding the observation.…”

“…The technique assumes that the electric conductivity at ionospheric height is infinite, but this approximation is not valid during nighttime when the conductivity is relatively low. For this reason, Del Corpo et al () inverted only FLR frequencies selected between 1 hr before sunrise and 1 hr after sunset at ground level. For the present study we used a more objective criterion, that is, we limited the analysis to the interval between the sunrise and sunset times evaluated at 120 km altitude, which we assume being the height at which the wave reflection occurs.…”

“…Recently, Del Corpo et al () illustrated a semi‐automated procedure to infer equatorial plasma mass densities in the inner magnetosphere by using the FLR frequencies detected from Ultra Low Frequency (ULF) measurements recorded at the European quasi‐Meridional Magnetometer Array (EMMA, Lichtenberger et al, ). They also performed a statistical analysis of the inferred density for different geomagnetic conditions, geocentric distances, and local time sectors.…”