Magnetospheric Multiscale Satellite Observations of Parallel Electron Acceleration in Magnetic Field Reconnection by Fermi Reflection from Time Domain Structures

The same time domain structures (TDS) have been observed on two Magnetospheric Multiscale Satellites near Earth’s dayside magnetopause. These TDS, traveling away from the $X$ line along the magnetic field at $4000\mathrm{km}/\mathrm{s}$, accelerated field-aligned $\sim 5\mathrm{eV}$ electrons to $\sim 200\mathrm{eV}$ by a single Fermi reflection of the electrons by these overtaking barriers. Additionally, the TDS contained both positive and negative potentials, so they were a mixture of electron holes and double layers. They evolve in $\sim 10\mathrm{km}$ of space or 7 ms of time and their spatial scale size is 10–20 km, which is much larger than the electron gyroradius ($<1\mathrm{km}$) or the electron inertial length (4 km at the observation point, less nearer the $X$ line).

Figure 1

The electric field component parallel to the background magnetic field measured on the four MMS spacecraft during a 150 ms interval.

Figure 2

The electric field (top panel) and electric potential (bottom panel) associated with a single TDS that passed spacecraft MMS4 and, 7.9 ms later, passed MMS1. The electric potentials of the bottom panel were obtained by integrating the electric fields of the top panel and by assuming that the distance between successive data points was the TDS speed of $4000\mathrm{km}/\mathrm{s}$ and the data rate was $8192\text{points}/\mathrm{s}$.

Figure 3

(a)–(c) give the three components of the magnetic field on MMS 1 at the time of the TDS (the vertical dashed line), and (d) illustrates the relative positions of spacecraft MMS1 and MMS4 in the reconnection geometry.

Figure 4

The upper panel gives the flux of 131 eV electrons at three pitch angles as a function of time before and after the TDS passage shortly after 0.6 s. The bottom panel gives the ratio of the fluxes shortly before and after the TDS as a function of energy.

Figure 5

Distribution functions of electrons before and after TDS passage for 174° pitch angle electrons (top panel) and 90° electrons (bottom panel).

Figure 6

The top panel give the final electron energy versus its initial energy resulting from Fermi reflection of the electron by either a $4000$ or $5000\mathrm{km}/\mathrm{s}$ TDS. The bottom panel gives the minimum initial electron energy that can be scattered by the TDS as a function of the maximum negative potential in the TDS.