COMPARISONS OF EIGHT YEARS MAGNETIC FIELD DATA FROM CLUSTER WITH TSYGANENKO MODELS

Eight years of magnetic field data, taken while the four Cluster spacecraft pass through, or adjacent to, the equatorial ring current, have been surveyed to investigate the effects on the Earth’s magnetic field components of the externally driven current systems connecting the ionospheric, cusp and ring current regions. We compare the modeled magnetic field, expected at Cluster, and the measured behavior using different global field models (Tsyganenko 1989, 1996, and 2001), and data from the four Cluster spacecraft, to better identify the result of the changing magnetospheric location (also due to the progressive southward dropping of the Cluster orbit and changing magnetospheric response). Residual field values, obtained by differencing the data and model field components, are used. The study shows that these residuals can reach ~20 nT near perigee and take two forms: well-defined smooth trends over the region of the ring current and frequently sampled, sharp, bipolar signatures, associated either with region 2 field aligned currents (FACs) connecting to the ionospheric current system (nightside), or with low-altitude cusp FACs (dayside). These deviations from the expected model field show that there are ring current and FAC components which are not fully modeled. The sharp bipolar signatures are not always sampled, implying a localized extent. The 4-spacecraft sample different signatures at the large separations achieved during the later years. Using the four-spacecraft technique, we infer that the region 2 FACs, with a size of ~0.17-0.54 RE, and cusp FACs, with a size of ~0.06-0.12 RE, are very stable and are almost stationary.