M. Piersanti

Comparison of equatorial plasma mass densities deduced from field line resonances observed at ground for dipole and IGRF models

The technique to remotely sense the plasma mass density in magnetosphere using field line resonance frequencies detected by ground-based magnetometers has become more and more popular in the last few years. In this paper we examine the error that would be committed at low and middle latitudes (Lu2009<u20094) in estimating the equatorial plasma mass density if dipole field lines are assumed instead of the more realistic representation given by International Geomagnetic Reference Field (IGRF) lines. 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 about −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. We also propose simple modifications of the standard technique, such as the use of an effective dipole moment or the eccentric dipole model, which allow to keep using the dipole field geometry but with a significant error reduction.

On the transmission of waves at discrete frequencies from the solar wind to the magnetosphere and ground: A case study

We analyze a case event in which several fluctuations at discrete frequencies (fu2009≈u20091.3–1.5,u20093.3–3.6,u20094.4–4.6,u2009andu20095.9–6.2u2009mHz; i.e., close to the “CMS” frequencies) were observed in the magnetosphere, after the impact of a sharp shock wave that, in the interplanetary medium, was followed by intense fluctuations in the solar wind parameters. The comparison between interplanetary, geosynchronous, and ground-based observations revealed that following the Sudden Impulse, magnetospheric modes at the same discrete frequencies were detected at geostationary orbit by spacecraft located in the morning and the dawn sector, and, ubiquitously, at ground-based stations: all of them revealed a one-to-one correspondence with those ultimately identified in the high-velocity stream following the shock wave. It reveals that the occurrence of such global modes is directly related to the transmission of external fluctuations and the observed geomagnetic fluctuations might be interpreted as the ground magnetic response to magnetospheric compressional modes forced by oscillations of the solar wind pressure at the same frequencies. By contrast, we did not find any evidence for magnetospheric oscillations possibly related to other mechanisms such as the velocity shear, the impact of the shock wave itself, etc.

On the discrimination between magnetospheric and ionospheric contributions on the ground manifestation of sudden impulses

The definite identification of the characteristics of the geomagnetic response to solar wind pressure changes represents an interesting element of magnetospheric dynamics that is also important in the Space Weather context. In the present analysis the aspects of the global response in ground-based observations have been examined for three case events, discriminating between magnetospheric and ionospheric contributions in ground manifestations of sudden impulses (SI). The separation between the magnetospheric and ionospheric contributions is obtained by a comparison between the observations at geostationary orbit and the predictions of the Tsyganenko and Sitnov (2005) model for the different magnetospheric current systems (from the magnetopause, ring current, tail current, etc.). The magnetopause current is the key element for the SI variation observed at geosynchronous orbit in a wide local time sector and practically represents the DL field of magnetospheric origin. The expected DL field is then subtracted, at each ground station, from the experimental measurements, in order to obtain a confident estimate of the residual DP field at different latitudes and local times. After evaluating the contribution of the field-aligned currents, we estimate the ionospheric current flow pattern of the preliminary and main impulses (PIIC and MIIC). The patterns of PIIC and MIIC fields are consistent with those proposed by Araki (1994). Some “anomalous” ground manifestations can be interpreted in terms of the combined effect of the irregular configuration of the boundary of the vortices of the ionospheric currents, of the rapid temporal evolution of the entire pattern, and of the station rotation beneath the pattern.

Applying a curl-B technique to Swarm vector data to estimate nighttime F region current intensities

The innovative geometry of European Space Agency Swarm constellation opens the way for new investigations based on magnetic data. Since the knowledge of a vector field on two spherical surfaces allows calculating its curl, we propose a new technique to estimate the curl of the ionospheric magnetic field measured by Swarm satellites A and B, orbiting the Earth at two different altitudes from March to September 2014. Using this technique, we mapped the amplitude of the radial, meridional, and zonal components and of total intensity of the ionospheric current density at the satellites altitudes, i.e., the F region of the ionosphere, during two local nighttime intervals: before and after midnight. Most of the obtained results are consistent with some of the known features of nighttime F region currents; others need further investigation. The proposed technique could contribute in selecting magnetic data with minimum contamination from nighttime F region electric currents for magnetic modeling purposes.