Magnetopause X‐Line Extent and Dayside Convection During IMF Southward Turning: Global MHD and MHD‐AEPIC Comparisons

Abstract Magnetic reconnection plays a fundamental role in transporting energy, momentum, and plasma from the solar wind to the magnetosphere‐ionosphere system. During an interplanetary magnetic field (IMF) southward turning, reconnection forms at localized magnetopause X‐lines that subsequently expand azimuthally. However, the physical mechanisms governing the X‐line extent remain unclear. In this study, we investigate the evolution of the magnetopause X‐line and the associated dayside ionospheric convection during an IMF southward turning using a global magnetohydrodynamic (MHD) model with adaptively embedded particle‐in‐cell physics (MHD‐AEPIC), and compare the results with an ideal MHD simulation and Super Dual Auroral Radar Network (SuperDARN) observations. The ideal MHD simulation produces strong and narrow post‐noon flows associated with a smooth, continuous X‐line. The MHD‐AEPIC simulation presents multiple dynamic X‐lines, leading to broader and weaker ionospheric flows that peak in the pre‐noon region, which is consistent with the SuperDARN observation. Field‐line tracing demonstrates that ionospheric flows are more consistent with the magnetopause off‐equatorial flows, while the flows in the X‐line region exhibit azimuthally wider and more intermittent flow signatures. A force analysis shows that the poleward acceleration is dominated by the force, with additional modulation by upstream magnetosheath flows. The X‐line spreading is most consistent with the Doppler‐shifted Alfvén speed along the guide field in the inertial flow frame. Transient magnetic flux ropes introduce short‐lived distortion of the magnetopause off‐equatorial flows. These results demonstrate that kinetic‐scale physics is essential for capturing the meso‐scale structure, extent, and asymmetry of dayside reconnection‐driven ionospheric convection.