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.