Electron Rolling-pin Distribution Inside Magnetic Hole

Magnetic holes (MHs), characterized by depressions in the magnetic field magnitude, are transient magnetic structures ubiquitous in space plasmas. The electron pitch-angle distribution inside the MHs is key to diagnosing the MH properties and has been suggested to mainly exhibit a pancake-type distribution showing pitch angles near 90 degrees. Here, we present the first observation of electron rolling-pin distribution-showing electron pitch angles mainly at 0 degrees, 90 degrees, and 180 degrees-within an electron-scale MH, by using Magnetospheric Multiscale mission high-resolution measurements. With a second-order Taylor expansion method, the magnetic field topology of the MH is reconstructed, and the characteristics of the rolling-pin distribution inside the MH are investigated. We find that the rolling-pin distribution primarily appears near the MH center and at energies ranging from 110 to 1200 eV. We interpret the rolling-pin formation as a consequence of the combination of local-scale electron trapping and global-scale Fermi acceleration. These results can improve current understanding of electron dynamics in the MHs.