Distortion of the outer boundary of the closed region in the Tsyganenko magnetic field model

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Using the Tsyganenko magnetic field model (TSYGANENKO, Planet. Space Sci., 37, 5, 1989) we make an attempt to determine the outer boundary of the closed region when the interplanetary magnetic field (IMF) is southward. As a simple magnetic field model including the effect of IMF B_z<0, the B_z component of a constant value of minus a few nanoTeslas is added to the magnetic field in the Tsyganenko model with low K_p values. In this paper, if the magnetic field strength, B, is not less than 2 nT in the whole range of a field line (namely the minimum B along a field line is greater than 2 nT), this field line is judged to be "firmly" closed. The firmly closed field lines are thought to be definitely closed as long as the fluctuation amplitude of B_z (around its average level) in the interplanetary (solar wind) magnetic field is less than 2 nT. The outer boundary of the firmly closed region is then constituted by field lines with the minimum B of 2 nT. This boundary is found to be close to (just inside of) the open-closed boundary, which can be determined with accuracy of 0.01° in latitude of the foot point of a field line. It is found that a circle with the center at a latitude of about 85° on the midnight meridian can be fitted to the outer boundary of the firmly closed region, as it is projected to the ionosphere. Interestingly this circle coincides with a typical auroral circle; the auroral circles are those delineating the poleward boundary of the quiet auroral belt, which were earlier identified from the statistical analysis of satellites' auroral images by MENG et al. (J. Geophys. Res., 82, 164, 1977). Importantly we find that the outer boundary of the firmly closed region is "distorted" on the nightside in the sense that the ionospheric projection of the average magnetic drift velocity of a plasma with isotropic pressure is not parallel to the boundary; more specifically, that of an isotropic ion fluid has an equatorward component on the duskside boundary and a poleward one on the dawnside boundary, respectively. This kind of the boundary distortion may be one of the possible causes of the generation of the nightside region 1 field-aligned current, which has been first suggested by HRUSKA (J. Geophys. Res., 91, 371, 1986) and recently, further studied by YAMAMOTO and INOUE (Proc. NIPR Symp. Upper Atmos. Phys., 11, 106, 1998).
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Distortion of the outer boundary of the closed region in the Tsyganenko magnetic field model

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