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.