Evolution of macro- and micro-scale turbulence during magnetic reconnection

Abstract:

Magnetic reconnection is a key process in collisionless plasmas that converts magnetic energy to plasma kinetic and thermal energies through a rapid change of magnetic field topology. Past simulations and observations demonstrated non-steady evolutions of this process through a formation of multiple reconnection X-lines and subsequent formation of multiple magnetic islands or magnetic flux ropes. In this study, to investigate the turbulent evolution of magnetic reconnection, we perform a large-scale fully kinetic simulation of a thin current sheet considering a power-law spectrum of initial fluctuations in the magnetic field at MHD scales. The results show that multiple ion- and larger-scale islands, formed by the initial fluctuations, collide and merge into larger ones, which prevents a free growth of reconnection leading to the decrease of the peak reconnection rate. As predicted by theories in the electron-MHD regime, the decreased reconnection rate results in a smaller aspect ratio of the electron diffusion region (EDR), where the topology change of magnetic field lines occurs accompanied by the magnetic diffusion. The simulation further show that electron-scale magnetic islands are repeatedly formed near this extended EDR. This is because the length of the extended EDR becomes closer to or exceeds the wavelength of the fastest growing mode of the electron tearing instability. Interestingly, these micro-scale islands formed near the EDR evolve accompanying a significant magnetic diffusion, as recently observed by the Magnetospheric Multiscale (MMS) mission. These macro-scale-turbulence-induced micro-scale fluctuations near the EDR contribute to the formation of a unique power-law spectrum from the electron to sub-ion scales, whose slope is different from the ones at larger scales. These new findings indicate the importance of non-steady features of the EDR to comprehensively understand the energy conversion and cascade processes in collisionless reconnection.