Formation and motion of a transpolar arc in response to dayside and nightside reconnection

We trace the formation and subsequent motion of a transpolar arc in response to dayside and nightside reconnection. Both high- and low-latitude dayside reconnection are observed, as well as periods of substorm and nonsubstorm nightside reconnection, during the 7-hour interval of interest on 19 January 2002. We speculate that the arc is formed by a burst of nonsubstorm nightside reconnection and that its subsequent motion is controlled predominantly by the rate of dayside high-latitude reconnection, siphoning open flux from the dusk sector polar cap to the dawn sector. The observations allow us to quantify the rates of reconnection: on the nightside, 35 and 100 kV during nonsubstorm- and substorm-related bursts, respectively; on the dayside, 30 and 100 kV for high- and low-latitude reconnection. The latter values give effective merging line lengths of 1 and 5.5 R E for northward and southward interplanetary magnetic field, respectively. We suggest that transpolar arc motion will be controlled not only by the B y component of the IMF but also by the relative magnitude of the B z component, when ∣B y ∣ > B z motion will be dawnward for B y < 0 nT and duskward for B y > 0 nT; however, when B z > ∣B y ∣, we expect that the arc will move toward the noon-midnight meridian of the polar cap.