Coalescence of magnetic flux ropes (MFRs) is suggested as a crucial mechanism for electron acceleration in various astrophysical plasma systems. However, how electrons are being accelerated via MFR coalescence is not fully understood. In this paper, we quantitatively analyze electron acceleration during the coalescence of three MFRs at Earth’s magnetopause using in-situ Magnetospheric Multiscale (MMS) observations. We find that suprathermal electrons are enhanced in the coalescing MFRs than those in the ambient magnetosheath and non-coalescing MFRs. Both first-order Fermi and E|| acceleration were responsible for this electron acceleration, while the overall effect of betatron mechanism decelerated the electrons. The most intense Fermi acceleration was observed in the trailing part of the middle MFR, while E|| acceleration occurred primarily at the reconnection sites between the coalescing MFRs. For non-coalescing MFRs, the dominant acceleration mechanism is the E|| acceleration. Our results further consolidate the important role of MFR coalescence in electron acceleration in space plasma.

Magnetic reconnection is a fundamental process known to play a crucial role in electron acceleration and heating, however, the mechanism of electron energization during reconnection is still not fully understood. This study introduces a novel electron acceleration mechanism in which electrons can be accelerated by secondary reconnection in the separatrix region. The secondary reconnection occurs in a thin current sheet resulted from the shear of the out-of-plane Hall magnetic fields of the primary magnetopause reconnection. It results in intense electron energy fluxes towards the primary X-line. This result will likely be an important piece in the puzzle of particle acceleration by reconnection.

How are particles being energized by turbulent electromagnetic fields is an outstanding question in plasma physics and astrophysics. This paper investigates the electron acceleration mechanism in strong turbulence (δB/B0 ~ 1) in the Earth’s magnetosheath based on the novel observations of the Magnetospheric Multiscale (MMS) mission. We find that electrons are magnetized in turbulent fields for the majority of the time. By directly calculating the electron acceleration rate from Fermi, betatron mechanism, and parallel electric field, it is found that electrons are primarily accelerated by the parallel electric field within coherent structures. Moreover, the acceleration rate by parallel electric fields increases as the spatial scale reduces, with the most intense acceleration occurring over about one ion inertial length. This study is an important step towards fully understanding the turbulent energy dissipation in weakly collisional plasmas.