The deviation of BT from the model, as shown in the bottom panel of Figure 1, cannot be mistaken as a spatial pattern, because the deviation took place when the spacecraft was around the apogee: was fairly flat when the deviation was observed. In addition, preceding and following orbits without substorm activity do not show a similar deviation near apogee (not shown). This deviation therefore must be associated with the concurrent substorm activity. Similar to the observations in the magnetotail lobe, BT first increased, and then recovered toward the quiet value, in the polar magnetosphere. Thus the same explanation could be applied to this polar phenomenon: magnetic flux tubes reconnected at the dayside magnetopause, under southward IMF, are carried downtail, expand the radius of the post-terminator magnetosphere and magnetotail, increasing the angle between the tail magnetopause and the solar wind flow, and increasing the pressure of the solar wind on the boundary.
The exact timing of the BT increase and decrease, in comparison with the ground signatures, is of interest, and we examine it using Figure 2. The middle panel of Figure 2 shows again the magnetic field observed by POLAR, but this time the difference between the observation and the model, , is shown, and a new coordinate system is adopted, called the field-aligned (FA) coordinate system. Here, the unit vector is parallel to , the unit vector is defined as , where is the position vector of the spacecraft, and the unit vector satisfies . Around 0400 UT, the model magnetic field was southward and sunward and the spacecraft was located near the GSM Z axis, (see the bottom panel of Figure 1), thus is directed roughly dawnward, and is directed roughly northward. We note (bottom curve) is almost the same as , because is almost parallel to the axis. For the sake of physical clarity, we will use the expression , or , instead of throughout this paper.
(roughly dawnward) did not change much when started changing around 0312 UT (line A superposed on the figure), but (roughly northward, perpendicular to the ambient field) started increasing at that time. That is, the magnetic field vector became less tilted (ambient field was southward and sunward). This deflection may be an effect of the pileup of the reconnected flux at the polar magnetopause northward of the satellite. That is, the piled-up flux would push the pre-existing polar magnetosphere tailward, and due to the finite conductivity of the ionosphere, the footprint of the field lines moved slower than the field lines in the polar magnetosphere, thus leading to less tilt.