Measurement of an internal azimuthal plasma current in a collisionless low $\beta$ plasma expanding in a magnetic nozzle is presented. The electric field is removed from the plasma ensuring a negligible electron $\mathbf{E}\times \mathbf{B}$ drift resulting in a purely diamagnetic electron azimuthal current. The electron energy probability function is non-Maxwellian, having an energetic tail component in addition to the thermal bulk electrons. The measured azimuthal current is significantly larger than the electron diamagnetic current estimated by considering only the bulk electrons. This can be well explained by considering the energetic tail electrons, which have a density of only about five percent of the total density. These results experimentally demonstrate that the energetic tail electrons are major contributors to the diamagnetism of the plasma even if their density is a small fraction of the total electron density.

• Received 14 December 2022
• Accepted 27 March 2023

DOI:https://doi.org/10.1103/PhysRevResearch.5.L022029

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Published by the American Physical Society