Why doesn’t the observed field-aligned current saturate with increasing interplanetary electric field?

Theoretical studies indicate that electric potential saturation is regulated by the limited rate of reconnection at the magnetopause associated with field-aligned currents (FACs), which predicts the possible saturation of FACs for large interplanetary electric field (IEF). However, recent statistical studies have shown that observed FACs increase linearly with IEF. To reconcile this disparity, numerical experiments are conducted to explore the response of FACs under large IEF. Results show that the FACs may exhibit either linear or saturated responses, which is associated with the Alfvén Mach number. The magnetospheric closure paths of region 1 currents vary significantly between the linear and saturated conditions. In the linear case, essential parts of the region 1 current extend toward dayside and connect to the magnetopause currents, whereas these parts of currents almost disappear in the saturation case. Saturation usually occurs under sub-Alfvénic solar wind, so the observed FACs generally grow linearly with IEF.