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.