2011 THEMIS SCIENCE NUGGETS

Flux transport, dipolarization and current sheet evolution during a double-onset substorm

by Rumi Nakamura

Introduction

A magnetospheric substorm is the sudden release of magnetotail energy that was transported from the dayside into the auroral ionosphere and night-side magnetosphere and is a spectacular process involving solar wind-magnetosphere-ionosphere coupling. During substorms, high-speed plasma flows and magnetic field dipolarization, which is enhancement in BZ, are observed in the near-Earth tail. Consequently the distribution of the tail current changes locally and/or globally.

Substorms occasionally have multiple onsets observed both in the ionosphere and the magnetotail. Although there are a number of studies of double-onset substorms, the evolution of the current sheet and flux transport processes in the magnetotail during such events are not yet well resolved. Based on simultaneous measurements in the magnetotail and the ionosphere, we inferred the changes in current sheet configuration, the possible location of the reconnection site, and flux transport characteristics to understand the evolution of multiple-onset substorm.

Results

Figure 1. Auroral and magnetospheric observations during the double-onset substorm on Feb. 16, 2008. Keogram from (a) SNKQ, and (b) KUUJ, (c-d) BX from THEMIS spacecraft in the midtail P1-2(THB-C) and in the inner magnetosphere P3-5(THD-E,A), respectively, and (e-f) BZ in the same format as BX.(g) Cumulative flux transport from P2(THC) and P5(THE) calculated by integrating the y component of the electric field. The flux transport value for P2(THC) is multiplied by two to show the temporal change more clearly. (h) Δ BX (local current density) and L (current sheet thickness) determined using P3-5(THD-E,A) data. The vertical lines indicate the onset times at 0220 and 243 UT.

Click each image to enlarge.

The double-onset substorm (at 0220 UT and 0243 UT) discussed in this paper occurred during a gradual northward interplanetary magnetic field (IMF) turning on 16 February 2008. At these times, THEMIS and GOES spacecraft were distributed between 6.6 and 18 RE downtail. Figure 1 shows the overview of the aurora and THEMIS observations. The weak westward electrojet onset at 0220 UT was associated with auroral intensification at SNKQ, whereas the stronger (> 500 nT) high-latitude electrojet onset at 0243 UT was associated with auroral expansion at SNKQ and KUUJ starting more poleward than the previous onset (Figures 1a-b). In spite of these different levels of activities, strong near-Earth dipolarizations were observed for both events (Figure 1f). Detailed analysis showed some differences in orientation and associated flow characteristics for these dipolarizations, attributed to the different ambient plasma condition and field configuration between these two events.

Figure 2. Sketch of the magnetotail configuration and possible location of the reconnection in the thin current sheet based on the THEMIS spacecraft observed around the 0220 UT onset (top panel), the 0230 UT and the 0238 UT activation events (second and third panels), and the 0243 UT onset (bottom panel).

Click each image to enlarge.

The changes in the tail current sheet configuration during these onsets were deduced from the magnetic field disturbance (Figure 1c-f), the flux transport (Figure 1g) and also from the current sheet thickness in the near-Earth region (Figure 1h). Figure 2 illustrates the possible magnetic field configuration and the location of some THEMIS spacecraft at four times deduced from these observations. At around onset of the first small substorm expansion (0220 UT), a thin current sheet developed at the inner probes region when the enhanced flux transport associated with negative BZ excursion started in the midtail region (Figure 2 top panel). Following the 0220 UT onset, the magnetic flux accumulated in the near-Earth tail region. At 0230 UT (second panel), the next flux transport enhancements started. This flux transport was associated with negative BZ and tailward flows in the midtail, indicating that the X-line formed Earthward of P2(THC). The continuous flux transport from the midtail caused a flux pileup and compressed configuration in the near-Earth region and led to formation of a thin current sheet with smaller BZ in the midtail region, making conditions favorable for another onset of reconnection tailward of the first location at around 0238 UT (third panel). At 0243 UT, when additional enhancement in the flux transport was observed, the midtail reconnection eventually involved lobe flux, which led to global reconfiguration of the tail, and the second onset was observed (bottom panel). In this way, the thin current sheet developed tailward due to multiple enhancements in the flux transport and consequently, the second onset took place tailward of the first one.

Conclusion

Based on simultaneous multi-point observations by the THEMIS and GOES spacecraft and ground-based observations, we inferred the current sheet evolution during a double onset substorm. The first onset, which took place when the thin current sheet was extended relatively Earthward (reaching the 11 RE region), initiated an effective Earthward flux transport of closed flux. As a result, the closed flux was carried away from the midtail region and piled up in the near-Earth region. The flux pileup caused compression and thickening of the near-Earth current sheet, while the thin current sheet region was shifted tailward of 18 RE. We suggest that this internal redistribution of magnetic flux results in production of a thin current sheet with a smaller BZ component in the midtail region. This reconfiguration leads to favorable conditions for lobe reconnection. Lobe reconnection associated with the second onset caused a global reconfiguration of the magnetotail. Since the flux pileup leads to tailward shift of the thin current sheet region, the second onset occurred tailward of the first onset region. These observations of the double-onset substorm suggest that the plasma sheet can be preconditioned not only by the IMF driver but by the internal magnetotail processes.

Source

Nakamura, R., et al. (2011), Flux transport, dipolarization, and current sheet evolution during a double-onset substorm, J. Geophys. Res., 116, A00I36, doi:10.1029/2010JA015865.

Biographical Note

Rumi Nakamura is a group leader at the Space Research Institute, Austrian Academy of Sciences. Her current research interests include plasma transport and acceleration and dynamics of current sheet in the magnetotail.


Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu