Velocity of magnetic neutral lines in the magnetotail

The Geotail spacecraft often observed intermittent enhancements of ion density and velocity in the distant (>70RE) tail lobe [Shirai, H., Takada, T.K., Kamide, Y, Mukai, T. Enhancement of lobe ion density and velocity associated with plasmoids. J. Geophys. Res. 106, 29,935–29,942, 2000]. Each enhancement occurred alter the passage of a plasmoid and was accompanied by a south-to-north variation in Bz. These signatures suggest that a magnetic neutral line formed in the near-Earth region moves tailward after the plasmoid is ejected. In this paper, we analyze the timing of the ion enhancement in detail. From the timing, we estimate the average velocity of the neutral line motion. The estimated velocity is about 130–580 km/s, which is around 20–70% of a typical value of the plasmoid velocity (i.e., about 760 km/s). We also compare the neutral line velocity in the Earth ’s magnetotail with the rise velocity of a solar flare loop [Shibata, K., Masuda, S., Shimojo, M, et al., Hot plasma ejection associated with compact-loop solar flares. Ap. J. Lett. 451, L83–L85, 1995]. The neutral line velocity in the magnetotail is larger than the loop velocity (4–20 km/s). However the ratio of the neutral line velocity to the plasmoid velocity in the tail is similar to the ratio of the loop velocity to the plasmoid velocity in solar flares suggesting the presence of the same physical mechanism behind them.