3.2.6 X-component magnetogram at Boulder
In order to examine the local substorm activity, we also performed the
similar analysis for H-component magnetogram at Boulder (BOU), shown in
Figure 10. Although there is around 1-hr time difference from Boulder
and Athabasca, the magnetic field variation at Boulder might provide an
estimate of the middle latitude variation of X-component magnetogram at
the longitude zone of Athabasca. Similar to the ASY-H index, the
X-component magnetogram at Boulder began to increase gradually 1-hr
prior to the arc detachment for all arcs; however, an abrupt increase
~30-mins prior to the detachment is observed only for
STEVE. The X-component magnetogram exhibits an enhancement of
~20 nT for STEVE. By contrast, the enhancement of
X-component magnetogram was gradual, broad, and weak
(~10 nT) for red arc and very weak for red+green arc
(~3 nT). Thus, the mid-latitude positive bay occurred
prominently for STEVE as compared to red and red+green arc. The
detachment of all arcs occurred immediately after the appearance of
positive bay structure.