Laboratory study of asymmetric reconnection including diamagnetic suppression

Magnetic reconnection plays a fundamental role in nearly all magnetized plasmas as it enables magnetic energy to be converted into high-speed flows and thermal energy. It allows the magnetic field lines to change topology in collisionless plasmas, thereby controlling the spatial and temporal evolution of explosive phenomena such as solar flares and coronal mass ejections. Reconnection at the dayside magnetopause and in the magnetotail is of special interest for the Sun-Earth connection and plasma conditions in the near-Earth environment because it is the dominant process that couples the solar wind to the Earth's magnetosphere. Especially at the dayside magnetopause, reconnection is known to be asymmetric where the shocked solarwind plasma has a number density much larger than the number density observed on the magnetospheric side of the boundary layer. In the presence of an out-of-plane guide magnetic field, this asymmetry and resulting pressure asymmetry across the boundary layer is believed to have a stabilizing effect on reconnection. In a highly cited paper by Swisdak et al., 2003 the phenomenon was first investigated using fully kinetic simulations. It was found that reconnection is suppressed when the diamagnetic drift in the plane of the reconnection exhausts exceeds the Alfven speed. The effect has been studies in numerous theoretical and numerical papers. Furthermore, in situ observations primarily form the Earth's magnetosphere also appear consistent with the original findings in [Swisdak et al., 2003]. To the best of our knowledge, diamagnetic suppression has never been studied or reproduced in a dedicated laboratory. In this proposal, using the Terrestrial Reconnection EXperiment (TREX), we will experimentally address the following science goals: 1) To characterize the role of diamagnetic suppression of reconnection in asymmetric configuration including a variable guide magnetic field. 2) To document experimentally the 3D electrostatic and magnetic structures of the reconnection region for parameter ranges including large plasma beta, density asymmetry and a guide magnetic field. Complementary to spacecraft observations, laboratory experiments have the advantage that the reconnection physics can be isolated and studied under controlled and repeatable plasma conditions. Funded in part by a current H-TIDeS award, recent upgrades to TREX have made it the first reconnection experiment to realize plasma conditions in a collisionless kinetic regime. Contrary to data obtained in the more collisional experiments, our experiment confirm key aspects of kinetic simulation results, such as the narrow electron layers that develop within the electron diffusion region. Thus, the TREX configuration provides an unparalleled opportunity to study reconnection in the laboratory under collisionless conditions directly relevant to space plasmas including those observed in the Earth's magnetosphere. The present proposal is focused on characterizing the normalized reconnection rate vfor a large range of plasma parameters spanning a 3D parameter space including regions where diamagnetic suppression is expected to extinguish the reconnection process. This will provide data complementary to those of spacecraft missions and numerical simulations, and we expect closure to our science questions. Furthermore, we foresee that the dataset to be compiled on the normalized reconnection rate may further the development of space weather models. The proposed work addresses the actions under Goals 1 and 4 for Solar and Heliospheric Physics in the last Decadal Survey: Goal 1: Determine the origins of the Sun's activity and predict the variations of the space environment. Goal 4: Discover and characterize fundamental processes that occur both within the heliosphere and throughout the universe. Given our experience of operating the experiment, we consider the proposal to be low-risk-high-yield.

Support NASA's Heliophysics strategic science objectives to understand the Sun and its interactions with Earth and the solar system, including space weather. This will be achieved by developing/demonstrating instrumentation technology necessary to address the following science goals: Explore the physical processes in the space environment from the Sun to the Earth and throughout the solar system; Advance our understanding of the connections that link the Sun, the Earth, planetary space environments, and the outer reaches of our solar system; Develop the knowledge and capability to detect and predict extreme conditions in space to protect life and society and to safeguard human and robotic explorers beyond Earth.

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