Laboratory Observations of Electron Heating and Non-Maxwellian Distributions at the Kinetic Scale during Electron-Only Magnetic Reconnection

Non-Maxwellian electron velocity distribution functions composed of a warm bulk population and a cold beam are directly measured during electron-only reconnection with a strong out-of-plane (guide) magnetic field in a laboratory plasma. Electron heating is localized to the separatrix, and the electron temperature increases continuously along the separatrix. The measured gain in enthalpy flux is 70% of the incoming Poynting flux. The electron beams are oppositely directed on either side of the $X$ point, and their velocities are comparable to, and scale with, the electron Alfvén speed. Particle-in-cell simulations are consistent with the measurements. The experimental results are consistent with, and go beyond, recent observations in the magnetosheath.

Figure 1

(a) The PHASMA experiment. Green arrows in the dashed box show the incident $\overrightarrow{k_i}$ and scattered $\overrightarrow{k_s}$ wave vectors of TS. (b) Bias current versus time for the two flux ropes. (c) Axial current density near the $X$ point with the error bars given by the color band. (d) Emission light intensity recorded by the fast camera downstream of the bias plate. (e),(f) Reconnecting magnetic field topology (black lines) and axial current density (colors) at $t=15\mu \mathrm{s}$ and $47\mu \mathrm{s}$. Green dots indicate locations of TS measurements. Magenta arrows denote inflows (shorter open) and outflows (longer solid), suggesting push-type (e) and pull-type (f) reconnection. The dotted rectangle is used to calculate energy fluxes.

Figure 2

(a) EVDFs at $x=7\mathrm{mm}$ (black circles) and $x=1\mathrm{mm}$ (red circles). The color bands show the measurement deviation. Solid lines are Maxwellian fits, while the vertical dashed lines are the thermal speeds of each fit. (b) $T_e$ at the green dots in Fig. 1 at $t=47\mu \mathrm{s}$. The black arrow is $1.8\delta$ from the $X$ point where the enthalpy increase is calculated. The gray arrow is the location used for testing the scaling of electron enthalpy increase with $B_{\mathrm{recx}}$. (c) The variance of $T_e$ along the separatrix obtained from a 2D PIC simulation.

Figure 3

(a)–(c) Reconnecting magnetic field topology (black lines) and axial current density (colors) at $t=42\pm 1\mu \mathrm{s}$, $46\pm 1\mu \mathrm{s}$, and $51\pm 1\mu \mathrm{s}$. Solid circles, blank circles, and solid stars denote the outflow, inflow, and separatrix regions, respectively. (d) $T_e$ measurements at $x=11\mathrm{mm}$ (red) and $x=-12\mathrm{mm}$ (black) as a function of time. Yellow shaded rectangles denote times corresponding to (a)–(c).

Figure 4

Measured electron enthalpy density increase $[\gamma /(\gamma -1)]n_e{k}_B\mathrm{\Delta }{T}_e$ versus (a) reconnecting magnetic enthalpy $B_{\mathrm{recx}}^2/{\mu }_0$ and (b) ratio of guide field to reconnecting field $B_g/B_{\mathrm{recx}}$.

Figure 5

EVDFs for the $B_{\mathrm{recx}}=15\mathrm{G}$ discharge showing oppositely directed beams on either side of the $X$ point. (a) $x=-3\mathrm{mm}$ (black circles) and (b) $x=7\mathrm{mm}$ (red circles). Dashed lines are Maxwellian fits for the bulk and beam and the solid line is their sum. The dotted vertical lines denote speeds of $V_{\mathrm{Ae}}/2$ and $V_{\mathrm{Ae}}$. (c) EVDF measured at $x=-3\mathrm{mm}$ for the $B_{\mathrm{recx}}=10\mathrm{G}$ discharge.