We investigate an event with oscillations of electron counts at 500 eV, with a period of ∼3 s measured by the electron drift instrument (EDI) on Cluster. These oscillations are detected at the overshoot of a quasi‐perpendicular bow shock and in its downstream region. During this event, the Alfvén Mach number is supercritical, and the proton β is >1. The correlation of the electron counts between three pitch angle ranges (∼10°, ∼90°, and ∼170°) is in‐phase. However, the correlation between the electron counts and the magnitude of the magnetic fields is antiphase. As EDI has a higher time resolution than that of conventional particle instruments, the correlation at a period of ∼3 s is examined for the first time. The oscillations are inferred as mirror mode waves with oblique propagation angles to the magnetic field. The oscillation of electron counts in the parallel direction might indicate the oscillation of the parallel electric potential. We have estimated this electric potential by Liouville's mapping, taking into account the shape of the electron distribution function as well as using the linear hot plasma dispersion relation. We have found that this kind of parallel electric potential is related to heating of particles with a comparable magnitude as the cross‐shock potential. The elliptical polarization of these waves indicates a possible interaction with gyrating particles so that the waves would be related to the particle dynamics. Finally, in‐phase correlation between parallel and perpendicular counts is explained in terms of Liouville's mapping.
