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Electron scale magnetic cavities are electron vortex structures formed in turbulent plasma, while the evolution and electron dynamics of these structures have not been fully understood. Recently, high-energy, angular, and temporal electron measurements from Magnetospheric Multiscale have enabled the application of an energetic particle sounding technique to these structures. This study analyzes an electron scale magnetic cavity observed by Magnetospheric Multiscale on 7 May 2015 in the plasma sheet. A comprehensive sounding technique is applied to obtain the geometry and propagation velocities of the boundaries. The result shows that the scale size of the structure is similar to 90 km, and the leading and trailing boundaries are moving in the same direction but with different speeds (similar to 11.5 +/- 2.2 and similar to 18.1 +/- 3.4 km/s, respectively). The speed difference suggests a shrinking of the structure that may play a significant role in magnetic energy dissipation and electron energization of electron scale magnetic cavities.

Plain Language Summary age Summary Electron scale magnetic cavities are extremely small vortices frequently found in space plasma environments, where a cavity of the depressed magnetic field contains energetic electrons forming a ring-like current at electron scale. The formation of these structures is thought to be linked to energy cascade in plasma turbulence, while the mechanism and process of the energy transformation are not clear. In this paper we discuss an electron scale magnetic cavity in the terrestrial plasma sheet with a modified particle sounding technique proposed and applied to the boundaries, suggesting that the structure is shrinking This analysis may provide new insight into understanding the evolution and electron energization of these structures.