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The D-shaped ion velocity distribution generated by the time-of-flight (ToF) effect can be considered as an important characteristic of reconnection exhausts in the magnetopause and magnetotail reconnection processes. In this study, we reported the D-shaped velocity distribution of O+ ions produced by the ToF effect in the magnetotail reconnection exhaust based on the observations from the Cluster spacecraft. The observed cutoff velocity of O+ ions is smaller than that of H+ ions at the same time, which is different from the previous theoretical prediction. We suggested that the difference between the two cutoff velocities is probably due to O+ ions having a larger reconnection diffusion region than H+ ions. We also demonstrated a remote-sensing method to estimate the spatial scale ratio between the O+ and H+ diffusion regions and the distance from the observation site to the center of the magnetotail reconnection region.

Plain Language Summary The reconnection processes at the Earth's magnetopause and magnetotail can convert the magnetic energy into the kinetic energy of the plasma, thus producing high-speed reconnection exhausts. The exhausting ions usually show a D-shaped velocity distribution caused by the time-of-flight effect. Previous works usually focused only on exhausting protons from the reconnection site, while O+ ions could also be abundant during geomagnetic active times. In this study, we analyzed a reconnection event to investigate the behaviors of O+ and H+ ions in the magnetotail reconnection exhaust region using the observations from the Cluster spacecraft. It is suggested that different behaviors of H+ and O+ ions are probably due to O+ ions having a larger reconnection diffusion region than H+ ions. In addition, we proposed a remote-sensing method to estimate the spatial scale ratio between the O+ and H+ diffusion regions and the distance from the observation site to the center of the magnetotail reconnection region.