The Impact of IMF By Variations on the Evolution of Lobe Reconnection Pattern and the Subsequent Development of the Conjugate Transpolar Arcs

Under a northward interplanetary magnetic field (IMF) with B_x ∼ 0, conjugate transpolar auroral arcs (TPAs) often appear on opposite polar cap sides in both hemispheres. These TPAs often show movement in response to IMF B_y changes. For example, a negative-to-positive IMF B_y shift moves northern TPAs from dawn to dusk, with southern TPAs moving oppositely. A popular model links these TPAs to lobe reconnection changes, but it's based on single-hemisphere observations and lacks confirming evidence from both hemispheres. In this study, utilizing simultaneous DMSP/SSUSI observations from both hemispheres, we examined a well-chosen event to track the formation and evolution of conjugate TPAs and compared the results with global MHD simulations. We found that: initially, when IMF B_y was ∼ −7 nT, conjugate TPAs were observed in the dawn/dusk side in the Northern/Southern Hemisphere, and MHD simulations showed that single-lobe reconnection occurs at the dawnside/duskside lobe in the Northern/Southern Hemisphere, respectively. As the IMF B_y gradually changed to 0, the TPAs in both hemispheres shifted to the polar cap center and dual-lobe reconnection began to appear in the simulation results. We also notice that there is a transitional phase where both single- and dual-lobe reconnections co-exist before dual-lobe reconnection dominates. Based on these results, we argue that here we provide strong evidence for Milan's model that the evolution of conjugate TPAs is associated with changes in the location and pattern of lobe reconnection driven by variations in IMF B_y.