Effect of O ⁺ on Perpendicular Anisotropy of Mixed Ions in the Near Magnetotail

Abstract Previous research has indicated significant correlations between the perpendicular anisotropy of mixed ions and the mirror and ion cyclotron instabilities in the near magnetotail. To further explore the potential influence of on perpendicular anisotropy and instabilities of mixed ions, we conducted an event analysis of data from the Fast Plasma Investigation (FPI), Hot Plasma Composition Analyzer (HPCA) and Fluxgate Magnetometer (FGM) aboard the Magnetospheric Multiscale (MMS) satellite during its orbit at on 19 August 2022. Although the two instabilities could be identified by the macroscopic plasma parameters of both the indistinguishable mixed ions and the single‐component and , differences between them were obvious. We find that the perpendicular anisotropy of the mixed ions can be affected by a minor population, while the corresponding cyclotron instability almost disappeared when the content exceeded 2.8%. Wavelet analysis of the magnetic field indicated that enhancements of power spectral density did not appear even though conditions were satisfied for the criteria of the two instabilities. Kinetic details of the perpendicular anisotropy were revealed in the velocity distribution function. Considering that this event occurred during an intense substorm, we propose that the rapidly lost and with energies up to 10 keV might precipitate into the ionosphere and may contribute to auroral activity. The newly refilled ions below 10 keV are composed of earthward from the magnetotail and outward from the ionosphere, thus continuously replenishing particles in the magnetosphere for the ongoing strong substorm.