Magnetospheric plasma density inferred from field line resonances: Effects of using different magnetic field models

The technique for remote sensing the plasma mass density in magnetosphere by geomagnetic field line resonances detected at ground-based stations is getting more and more popular after the establishment in the last few years of extended magnetometer arrays, such as the EMMA network recently formed in the framework of the EU FP-7 PLASMON project [1]. It is important therefore to quantify the level of accuracy associated to such technique. In this study we examine the effect of using different magnetic field models. First the equatorial plasma mass density estimates obtained using the dipole approximation are compared with those obtained using the IGRF model for low-mid latitudes. It is found that the use of the centered dipole model may result in an error in the inferred density appreciably larger than what is usually assumed. In particular it has a significant longitudinal dependence being, for example, greater than +30% in the atlantic sector and ~ -30% at the opposite longitude sector for field lines extending to a geocentric distance of 2 Earth radii. This may result in an erroneous interpretation of the longitudinal variation in plasmaspheric density when comparing results from ground-based arrays located at different longitudes. Simple modifications of the technique are proposed which allow to keep using the dipole approximation but with a significant error reduction. Then the results of using the T01 Tsyganenko model [2] are compared with those based on the IGRF model. With respect to previous studies we take into account the different equatorial crossing points of the IGRF and T01 field lines traced from a given ground position by considering reasonable radial gradients of the equatorial density. For average solar wind/magnetospheric conditions, mass densities computed using the IGRF model result to be moderately overestimated (less than 20%) for L values <; 4. The uncertainty obviously increases for higher L values and the bias may become negative for st- ep radial variations of the equatorial density. For storm-time conditions the error dramatically increases beyond L ~ 4, but may remain within ~ 20% for L < 4 assuming radial variations of the equatorial density which are typical for such magnetospheric conditions. We also present an analysis of a real event using measurements provided by the European magnetometer network EMMA.