The spontaneous formation of magnetic islands is observed in driven, antiparallel magnetic reconnection on the Terrestrial Reconnection Experiment. We here provide direct experimental evidence that the plasmoid instability is active at the electron scale inside the ion diffusion region in a low collisional regime. The experiments show the island formation occurs at a smaller system size than predicted by extended magnetohydrodynamics or fully collisionless simulations. This more effective seeding of magnetic islands emphasizes their importance to reconnection in naturally occurring 3D plasmas.

Fully kinetic simulations are applied to the study of 2D anti-parallel reconnection, elucidating the dynamics by which the electron fluid maintains force balance within both the ion diffusion region (IDR) and the electron diffusion region (EDR). Inside the IDR, magnetic field-aligned electron pressure anisotropy (pe∥≫pe⊥) develops upstream of the EDR. Compared to previous investigations, the use of modern computer facilities allows for simulations at the natural proton to electron mass ratio mi/me=1836. In this high-mi/me-limit, the electron dynamics change qualitatively, as the electron inflow to the EDR is enhanced and mainly driven by the anisotropic pressure. Using a coordinate system with the x-direction aligned with the reconnecting magnetic field and the y-direction aligned with the central current layer, it is well known that for the much studied 2D laminar anti-parallel and symmetric scenario the reconnection electric field at the X-line must be balanced by the ∂pexy/∂x and ∂peyz/∂z off-diagonal electron pressure stress components. We find that the electron anisotropy upstream of the EDR imposes large values of ∂pexy/∂x within the EDR, and along the direction of the reconnection X-line, this stress cancels with the stress of a previously determined theoretical form for ∂peyz/∂z. The electron frozen-in law is instead broken by pressure tensor gradients related to the direct heating of the electrons by the reconnection electric field. The reconnection rate is free to adjust to the value imposed externally by the plasma dynamics at larger scales.

Magnetic reconnection is explored on the Terrestrial Reconnection Experiment (TREX) for asymmetric inflow conditions and in a configuration where the absolute rate of reconnection is set by an external drive. Magnetic pileup enhances the upstream magnetic field of the high-density inflow, leading to an increased upstream Alfvén speed and helping to lower the normalized reconnection rate to values expected from theoretical consideration. In addition, a shock interface between the far upstream supersonic plasma inflow and the region of magnetic flux pileup is observed, important to the overall force balance of the system, thereby demonstrating the role of shock formation for configurations including a supersonically driven inflow. Despite the specialized geometry where a strong reconnection drive is applied from only one side of the reconnection layer, previous numerical and theoretical results remain robust and are shown to accurately predict the normalized rate of reconnection for the range of system sizes considered. This experimental rate of reconnection is dependent on system size, reaching values as high as 0.8 at the smallest normalized system size applied.

Magnetic reconnection (Dungey, 1953) is the process of changing the topology of magnetic field lines in the presence of a plasma, often permitting an explosive release of magnetic energy. Well-known examples include solar flares (Priest & Forbes, 2000) and auroral substorms in the Earth's magnetosphere (Vasyliunas, 1975). Although reconnection often governs the global dynamics of plasma systems, the reconnection process occurs in localized electron diffusion regions (EDRs), where the motion of the electron fluid decouples from the magnetic field, breaking the frozen-in law of magnetohydrodynamics. The origin of this process in the collisionless regime, where conventional resistive friction is absent, remains controversial. For example, laminar kinetic models predict that the EDRs are characterized by intense current layers with widths as narrow as the kinetic scales associated with the electron orbit motion (Pritchett, 2001;Vasyliunas, 1975). In other models, the scattering of electrons by electric field fluctuations associated with high-frequency instabilities is proposed to widen the current layers and enhance the anomalous transfer of momentum from the electrons to the ions (Hoshino, 1991;Huba et al., 1977;Papadopoulos, 1977).Significant insight into reconnection physics is provided by fully kinetic numerical models. In three-dimensional (3D) configurations, it has been argued that turbulence can cause local suppression of the effective

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