Abstract
Abstract. In this paper we examined the continuous motions of a near-Earth neutral line during the recovery phase of the 5 October 2000 substorm. Estimation was based on the PSBL ion beam model proposed by Onsager (1991) and the Geotail observations. Estimated distances from the Earth ranged from 20 to 60 RE and retreated tailward at velocities of 250 and 300 km/s. This event initiated with the arrival of solar wind discontinuity. Simultaneous observations of electromagnetic field and electrons indicate the existence of earthward propagating waves associated with field-aligned currents. Based on these observations, we suggest that the source of the PSBL ion beams was the retreating near-Earth neutral line formed by the compression of the magnetosphere. Two scenarios of near-Earth neutral line motion in the tail dynamics are also proposed. One is the formation of plural neutral lines to create a long plasmoid. The other is the oscillation of one neutral line between the near-Earth region and the mid-tail stagnant plasmoid.
Highlights
Magnetic reconnection in the tail plasma sheet is one of the basic components of substorm dynamics and it changes the structure of the tail plasma sheet through the formation of neutral lines (Hones, 1977)
We closely examine the plasma sheet expansion from 12:00 UT to 12:40 UT with plasma and electromagnetic field measurements made by Geotail (Fig. 3)
At 12:11 UT the estimated distance suddenly decreased from 40 RE to 25 RE at the speed of 2200 km/s. This short and rapid motion is beyond the limitation of the analysis because it is faster than the lower-energy cutoffs of the earthward beam and its duration is comparable to the particle flight time (1–2 min)
Summary
Magnetic reconnection in the tail plasma sheet is one of the basic components of substorm dynamics and it changes the structure of the tail plasma sheet through the formation of neutral lines (Hones, 1977). Reliance on single point measurements requires a statistical approach, such as a timing analysis to discuss the speed of the neutral line retreat. Another approach to estimate the motion of the neutral line is to use counter-streaming field-aligned beams in the PSBL (Forbes et al, 1981; Onsager et al, 1991; Elphic et al, 1995). In this paper we discuss the motion of a near-Earth neutral line by applying the Onsager model to the PSBL ion beam observations made by Geotail. We visually surveyed Geotail observations from 1995 to 2003, and found the longestduration counter-streaming ion beam event during the 5 October 2000 substorm.
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