Coalescence of Macroscopic Flux Ropes at the Subsolar Magnetopause: MMS Observations

Flux ropes with different scale sizes are frequently observed in Earth's magnetosphere. It is now well accepted that they result from magnetic reconnection and play a crucial role in dissipating energy during the reconnection process. Sometimes, multiple flux ropes can be produced, which might coalesce sporadically into larger ropes, releasing an even larger amount of energy. In this presentation, we report one of the first direct observations of the coalescence of macroscopic flux ropes at the subsolar magnetopause by the Multiscale Magnetospheric (MMS) Mission. The merging of two large-scale flux ropes with sizes of about 0.5 RE were identified by the occurrence of a quadrupolar signature in the normal magnetic field component measured by the MMS spacecraft as they were in the magnetosheath after they crossed the magnetopause. The inner bipolar component was weaker than the outer fields indicating that the magnetic field was dissipating between the two flux ropes. The thickness of the merging sheet between the two flux ropes was of the order of the local ion inertial length and the current was mainly carried by electrons. While the ion frozen-in condition was broken, electrons were still magnetized in the merging sheet. The enhancement of electron-driven waves (e.g., electromagnetic whistler and electron cyclotron harmonic waves) observed in the vicinity of the merging sheet, suggests that the coalescence involves multi-scale processes. We discuss these observations in the context of kinetic simulations.