Physics of auroral phenomena : proceedings of the 33rd Annual seminar, Apatity, 02 - 05 March, 2010 / [ed.: A.G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 206 с. : ил.

MODELS OF THE GEOMAGNETIC FIELD AND MAGNETOMETER MEASUREMENTS OF CLUSTER FGM IN THE MAGNETOSPHERIC TAIL M .Forster1, Y .I.Feldstein2, L.I. G romova2, L.A. D remukhina2, A.E. Levitin2, S.E. Haaland 3’4 'Helmholtz Centre Potsdam, GFZ German Research Centre fo r Geosciences, 14473 Potsdam, Germany 2 IZMIRAN, 142090 Troitsk, Moscow region, Russia 3Max Planck Institute fo r Solar System Research, 37191 Katlenburg-Lindau, Germany 4Department o f Physics and Technology, University o f Bergen, Norway Abstract. The Electron Drift Instrument (EDI) on board Cluster measured the plasma drift vector within the magnetosphere. The EDI drift data were then mapped along the magnetic field lines into the upper ionosphere (-400 km) by use of the Tsyganenko-2002 (T01) geomagnetic field model. They provided the basis for statistical ionospheric convection pattern at high latitudes for various orientations of the interplanetary magnetic field (IMF) and constituted in this way a magnetospheric convection model. For estimating the correctness of the model assumptions, the T01, the Tsyganenko-1996 (T96) and the Paraboloid Magnetic Field (PM, Alexeev et al, 1996) model values of the geomagnetic field were compared with the flux gate magnetometer (FGM) measurements on board Cluster. The Cluster FGM data are not part of the T01 and T96 data bases and can therefore be used for an independent verification of these models. A comparison of Cluster FGM data with T01 model values was performed by Woodfield et al. [2007] from perigee up to a geocentric distance of 8 Re. Here, we show a corresponding comparison between observed (Cluster) and modelled (T01, T96, and PM) magnetic field values for the magnetospheric tail region between 8 Re and 15 Re. The results of the drift vector mapping by use of the T01 and T96 models were compared with analogous mappings using the PM geomagnetic field model. Introduction Four identical spacecraft, Cluster C1-C4, were launched in summer 2000 into a high-inclination polar elliptical orbit with perigee at around 4 Re and apogee near 19 Re geocentric distance, and an orbital period of about 57 hours. The Electron Drift Instrument (EDI) on board Cluster measured the 2D plasma drift in the plane perpendicular to the local geomagnetic field, while the Fluxgate Magnetometer (FGM) probe recorded the components of the full geomagnetic field vector. The EDI data were sorted with respect to the orientation o f the interplanetary magnetic field (IMF) near the magnetopause into 8 distinct sectors. The spatially distributed EDI measurements were then mapped along the geomagnetic field lines to a common reference level at 400 km altitude into the high-latitude ionosphere using the Tsyganenko T01 magnetic field model [Tsyganenko, 2002]. The EDI data treatment and the procedure to relate the remote ACE observations of the solar wind and IMF to actual values at the magnetopause are described in the companion papers of Haaland et al. [2007] and Forster et al. [2007] as well as in Forster et al. [2008]. Forster et al. [2009] describe a methodology to derive four basic convection pattens (BCPs) for various orientations of the IMF, which constitutes an ionospheric convection model for any IMF value. Possible sources o f incorrect sampling for the EDI Cluster convection model might be caused by the magnetic field model, which is used for the projection of the EDI drift vectors into the ionosphere. To estimate possible induced errors, we compare subsequently the magnetic field magnitude measured by the Cluster satellite with magnetic field model values of the T96 [ Tsyganenko, 1996], the T01 [Tsyganenko, 2002] and PM model [Alexeev et al., 1996; Feldstein et al., 2005] along the same trajectories. The Cluster FGM data are not part of the T01 and T96 data base and can therefore be used for an independent verification o f the model. For the calculation of the internal geomagnetic field contribution, the IGRF-2005 model was used. Modelling of the magnetic field in the magnetospheric tail The FGM instrument on board of Cluster consists o f two triaxial fluxgate magnetic field sensors on one of the two radial booms of 5 m length of each spacecraft [Balogh et al, 2001]. There are four operative ranges of the FGM instrument, which covers magnetic field values within ±4000 nT with a precision of ± 0.1 nT. Figure 1 shows the comparison of Cluster-3 FGM observations with the three (T01, T96, and PM) geomagnetic field models along the orbital trace from the far tail at the night side with -20 Re geocentric distance to -7 Re during geomagnetically quiet conditions on 10-11 August 2007 (left panels) and a similar orbit on 19-20 August 2006 during disturbed conditions (right panels). The time resolution of the data points presented here is 10 min. The characteristics of the interplanetary medium near the magnetopause during these two orbits are presented in Figure 2. The difference between the magnetic field magnitude measured by Cluster FGM and the PM model values, B(C1)-B(PM), is generally positive during different activity levels (dotted lines in bottom panels of Figure 1). “P hysics o f Auroral P henom ena * Proc. XXXIII A n n u a l Sem inar, A patity, pp. 5 9 - 62, 2011 © Kola Science Centre, Russian Academy of Science, 2011 Polar Geophysical Institute 59