ИСТИНА

On the base of simultaneous measurements from the THEMIS satellites, the radial profile of the magnetic field and the position of the inner edge of the magnetospheric tail were determined during selected events in 2009. The magnetic field of the currents other than the tail current was subtracted from measurements. It was found that during quiet times the inner edge of the tail current sheet is located in the night side magnetosphere, at distances of about 10 Re. In the vicinity of the inner edge of the tail current the magnetic field Bx and Bz components were about -20 nT, while in the distant tail, at Xgsm -30 Re, these values were about -10 nT. During geomagnetic disturbances the inner edge of the tail current sheet shifted towards the Earth to a distance of about 7 Re. At the same time strong currents in the magnetotail were detected. During the disturbance of February 14, 2009 (min Dst -35 nT), the Bx component of the magnetic field near the inner edge of the tail current sheet was about 70 nT, and the Bz component was about -50 nT. Solar wind conditions on 14.02.2009 were consistent with those during moderate magnetic storms with minimum Dst of about -100 nT. However, strong currents, measured in the geomagnetic tail did not cause the expected Dst-effect. Calculations in terms of the magnetospheric magnetic field model A2000 give a tail current contribution to the magnetic field at the Earth’s surface of about -20 nT. Actually, the magnetospheric current systems (magnetopause and cross-tail currents) were located at larger geocentric distances than typical during the 2009 extremely quiet epoch. The event of February 14 was accompanied by solar wind and magnetospheric conditions characteristic of a moderate storm. However, only a very small disturbance on the Earth’s surface was detected consistent with an “inflated” magnetosphere.