Evolution of an Electron Current Layer Prior to Reconnection Onset

Electron current layers (ECLs) are the sites where magnetic reconnection (MR) initiates in a current sheet (CS). Using three-dimensional particle-in-cell (3DPIC) simulations, we study the plasma processes that occur in an ECL as it evolves rapidly over a time scale much shorter than the ion cyclotron period. The processes include its thinning, generation of electrostatic instabilities, trapping and heating of electrons in growing waves, its re-broadening, generation of anomalous resistivity and eventually the generation of large-amplitude magnetic fluctuations. These fluctuations could be interpreted in terms of electron tearing and/or Weibel instabilities, which are commonly invoked as mechanisms for the MR onset. The widths of the broadened ECL are compared with those measured in the MRX experiment, showing excellent agreement. The waves heat both the electrons and ions. The heating produces a hard power law energy spectrum of electrons locally in the ECL. When such heated electrons flow out of the thin ECL and mix with colder electrons, the energy spectrum shows an energetic tail with power law index depending on the history of the background plasma. The processes described here could be relevant to measurements from the forthcoming MMS mission.