Structure and Coalescence of Magnetopause Flux Ropes and Their Dependence on IMF Clock Angle: Three‐Dimensional Global Hybrid Simulations

Abstract

Flux ropes are ubiquitous at Earth's magnetopause and play important roles in energy transport between the solar wind and Earth's magnetosphere. In this study, structure and coalescence of the magnetopause flux ropes formed by multiple X line reconnection in cases with different southward interplanetary magnetic field (IMF) clock angles are investigated by using three‐dimensional global hybrid simulations. As the IMF clock angle decreases from 180°, the axial direction of the flux ropes becomes tilted relative to the equatorial plane, the length of the flux ropes gradually increases, and core field within flux ropes is formed by the increase in the guide field. The flux ropes are formed mostly near the subsolar point and then move poleward toward cusps. The flux ropes can eventually enter the cusps, during which their helical structure collapses, their core field weakens gradually, and their axial length decreases. When the IMF clock angle is large (i.e., the IMF is predominantly southward), the flux ropes can coalesce and form new ones with larger diameter. The coalescence between flux ropes can occur both near the subsolar point when they are newly formed and away from the subsolar point (e.g., in the southern hemisphere) when they move toward cusps. However, when the IMF clock angle is small (≤135°), we do not find coalescence between flux ropes.

Re‐Reconnection Processes of Magnetopause Flux Ropes: Three‐Dimensional Global Hybrid Simulations

“…Note that Case 1 in this paper is the same as Case 3 in Guo et al (2021), in which the FR coalescence has been well identified. Moreover, the simulation domain of Case 1 contains a small dayside cusp region, and it is not enough to simulate the re-reconnection between FR and the cusp field lines.…”

“…Using 3‐D global hybrid simulations, Guo et al. (2021) studied the 3‐D structures and evolution of FRs formed by magnetopause reconnection with different southward IMF clock angles, and they found that whether and where FR coalescence occurs are controlled by the IMF clock angle. However, the detailed coalescence process, that is, the topological change of magnetic field lines during the re‐reconnection, is still unclear.…”

“…When the FRs enter the cusp, their field lines can reconnect with the cusp magnetic field lines (i.e., tail lobe magnetic field lines) through another kind of re‐reconnection (Omidi & Sibeck call it secondary reconnection), during which the plasma in the FRs can enter the ionosphere, as shown by the 2‐D global hybrid simulations in Omidi and Sibeck (2007). However, the axial (west‐east) length of FRs can reach more than a dozen (where is the Earth’s radius, Guo et al., 2021). The structure and nature of the re‐reconnection between FR and cusp field lines in 3‐D is also unclear.…”

Three‐Dimensional Global Hybrid Simulations of High Latitude Magnetopause Reconnection and Flux Ropes During the Northward IMF

“…Moreover, J. Guo et al. (2021a, 2021b) used a 3‐D global hybrid simulation to show that when the IMF is southward, the number of flux ropes is 4–8, the length of flux ropes can be up to 12 and the diameter of the flux ropes can be up to 2 . However, in this study, we find that when the IMF is northward, the number of flux ropes is 1–2, the length of flux ropes is only about 3 and the diameter of the flux ropes is less than 1 .…”

“…Magnetopause reconnection can form flux transfer events (FTEs; Russell & Elphic, 1978), which is an effective mechanism for energy transport in the solar wind‐magnetosphere coupling. FTEs are usually interpreted as magnetopause flux ropes with helical magnetic field lines formed by multiple X‐line reconnection (Z. F. Guo et al., 2020; J. Guo et al., 2021a; Hasegawa et al., 2010; Lee & Fu, 1985; Øieroset et al., 2016; Tan et al., 2011; Wang et al., 2019).…”