Electron-only magnetic reconnection in lunar-relevant laser-driven mini-magnetospheres*

Mini-magnetospheres are ion-scale structures that are ideal for studying the kinetic-scale physics of collisionless space plasmas. Such ion-scale magnetospheres can be found on local regions of the Moon, associated with the interaction of the solar wind with the lunar crustal magnetic field.

  We present laboratory observation of magnetic reconnection in laser-driven lunar-like ion-scale magnetospheres on the Large Plasma Device (LAPD) at UCLA. In our experiment, we use a high-repetition rate (1 Hz), nanosecond laser to drive a fast moving plasma that expands into the field generated by a pulsed magnetic dipole embedded into a background plasma and magnetic field [1]. The dipole and background fields are oriented to be anti-parallel, allowing a magnetic reconnection geometry. Taking advantage of the high-repetition rate, we acquire time resolved, volumetric data of the magnetic and electric fields, measured with magnetic flux and emissive probes around the reconnection point. We notably find that Hall physics plays an important role in mini-magnetospheres, and that the observed reconnection is dominated by electron dynamics [2]. We use spatially resolved Thomson scattering to further investigate the plasma density evolution, as well as heating associated with the mini-magnetosphere and reconnection process. Using particle-in-cell simulations reproducing the experiment, and comparing the terms of the generalized Ohm’s law,  we uncover the microphysics driving this reconnection and show that the reconnection electric field is mostly sustained by the electron pressure anisotropy on the electron scale.

By comparing several dimensionless parameters, we show that our experiment operates in a physical regime relevant to lunar mini-magnetospheres and therefore can contribute to understanding the physics driving reconnection on the Moon in a context where in-situ data is scarce and limited.

[1] D. B. Schaeffer et al. Physics of Plasmas 29, 042901 (2022)

[2] L. Rovige et al. The Astrophysical Journal, In press (2024)