Direct Measurement of Electron Heating in Electron-Only Reconnection in a Laboratory Mini-Magnetosphere

Abstract

We report on the experimental observation of electron heating in electron-only magnetic reconnection in laser-driven laboratory mini-magnetospheres on the Large Plasma Device (LAPD) at the University of California, Los Angeles. In this experiment, a fast-flowing plasma impacts a pulsed magnetic dipole embedded within LAPD's magnetized ambient plasma, creating an ion-scale magnetosphere and driving electron-only magnetic reconnection between the background and dipole field lines. The electron velocity distribution is measured across the reconnection region using non-collective Thomson scattering, enabling determination of electron temperature and density. Significant electron heating is observed in the electron diffusion region, increasing from an initial temperature of 1.8 eV to 9.5 eV, corresponding to a 40\% conversion of Poynting flux into electron enthalpy flux. Particle-in-cell simulations that provide insights into the heating mechanisms are also presented.

Figures (3)

• Figure 1: a) Schematic of the experiment on the LAPD. b) Scattering vector $\mathbf{k_s}- \mathbf{k_i}$. c) Example of a Thomson scattering spectrum obtained by averaging over 400 shots (black) and its gaussian fit used to retrieve density and temperature (red). d) Initial magnetic field in the z-direction $B_z$ and magnetic field lines in the reconnection (Y-Z) plane, measured with a B-dot probe. The centered vertical and horizontal lines represent the TS measurement regions.
• Figure 2: (a)-(b) Electron temperature and density measured by Thomson scattering along the y and z lineouts across the reconnection zone as represented on Fig. \ref{['fig:setup']}c at $t = 650$ ns. Each position data point is obtained from a spectrum averaged over 400 shots. (c) Electron velocity along $\mathbf{k}$, $V_k$ at $y=-10.5$ cm and $z= -1.5$ cm (outflow) for different times, measured from the Doppler shift of the TS spectra. (d) Electron temperature at the reconnection point $y=-10.5$ cm and $z = 0$ with the dipole turned ON and OFF, for different times of the experiment.
• Figure 3: Simulation result at simulation time $t= 3.9~\omega_{ci}^{-1}$ and $x=0$. The black streamlines represent the magnetic field lines and (a) Normalized electron density (b) Normalized out of plane current density $J_x$. (c) Electron temperature normalized to the initial temperature. The trajectories of two electrons gaining a significant amount of energy in the reconnection layer are plotted, with their energy in green-blue color scale. (d)-(f) Components of the electron temperature $T_{ex},T_{ey},T_{ez}$ respectively along the $x,y,z$ directions, normalized to the initial temperature. (g)-(h) Vertical lineouts of the out of plane current $J_x$ (black), reconnection magnetic field $B_z$ (green) and electron temperature $T_e$ (red): (top) in the experiment, at $t=650$ ns and (bottom) in the simulation.