In Situ Observations of Particle Energization during the Coalescence of Magnetic Flux Ropes at Earth’s Magnetopause

Coalescence of magnetic flux ropes (MFRs) is an efficient pathway for particle acceleration and energy conversion during magnetic reconnection. However, the associated particle energization mechanism and energy partition remain poorly understood. Using Magnetospheric Multiscale (MMS) observations of two coalescing MFRs at Earth’s magnetopause, we quantitatively evaluate electron and ion energization across distinct energy ranges. We find that electron energization was dominated by a parallel electric field within the thermal population, while ion energization was primarily governed by Fermi and betatron mechanisms within both thermal and suprathermal populations. The suprathermal energization fraction was significantly larger for ions than for electrons, indicating distinct acceleration pathways and energy partitions between the two species during MFR coalescence. These results improve our understanding of species- and energy-dependent energization during MFR coalescence.