MMS observations of energetic electrons associated with a pair of flux ropes in Earth\u27s plasma sheet

The properties and acceleration mechanisms of energetic electrons (\u3e 50 keV) associated with a pair of tailward traveling flux ropes and their accompanied reconnection X-lines are investigated with MMS in Earth’s magnetotail. Energetic electrons are enhanced on the boundaries and inside the flux ropes. The power-law spectra of energetic electrons near the X-lines and in flux ropes are harder than those on flux rope boundaries. The energy of the adiabatic electron is a few keV around the X-lines, tens of keV on the flux rope boundaries, and a few MeV inside the flux ropes. The X-lines cause strong energy dissipation, which may account for the energetic electron beams around them. The first-order Fermi acceleration due to the contraction of the field line is strong near the X-lines, which causes the enhanced electron parallel temperature. Betatron acceleration due to the compression of magnetic field is strong near the flux rope boundaries, where the energetic electrons are enhanced in the perpendicular direction. The Fermi-reflection can occur in the regions between the flux rope pair. However, the two flux ropes moving away from each other instead of mering might influence the energy gain of electrons. The acceleration and deceleration of the Betatron and Fermi processes exist simultaneously indicating that the magnetic field and plasma are turbulent around the flux ropes, which can be driven by the frequently appearing X-lines. Electrostatic structures are observed inside the flux ropes, which correspond to potential drops comparable to electron thermal energy. Because of strong helical field lines, energetic electrons inside the flux ropes can be transported from other places