Abstract
On December 3, 1996, in the time interval with low geomagnetic activity, the Interball-1 satellite crossed the neutral sheet of geomagnetic tail from north to south at the radial distance ≈25R E. Characteristics of proton and electron fluxes 20 keV measured by DOK2 spectrometer are presented. High anisotropy of proton fluxes was observed. Fluxes of protons and their anisotropy are modulated by magnetic field. In a region where the total magnetic field is low and proton fluxes are high the anisotropy is lower and energy spectra of protons are harder than in another region. In the latter one (after 20:17 UT) when the total magnetic field is higher mainly due to an increase of |B x | (B x 0, the proton fluxes in earthward direction predominate. In the time with B x <0 we observe the proton fluxes in the tail direction. This is in a good agreement with the measurements of bipolar fluxes on Geotail reported by Petrukovich et al. [J. Geophys. Res. 26 (1999) 2909]. We suppose that the cause of these bipolar proton fluxes is the reconnection of geomagnetic field lines during the growth phase of geomagnetic substorm.
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