Geotail observations of magnetic flux ropes in the plasma sheet

Abstract

[1] Examination of Geotail measurements in the near-tail (X > À30 R E ) has revealed the presence of small flux ropes in the plasma sheet. A total of 73 flux rope events were identified in the Geotail magnetic field measurements between November 1998 and April 1999. This corresponds to an estimated occurrence frequency of $1 flux rope per 5 hours of central plasma sheet observing time. All of the flux ropes were embedded within high-speed plasma sheet flows with 35 directed Earthward, hV x i = 431 km/s, and 38 moving tailward, hV x i = À451 km/s. We refer to these two populations as ''BBF-type'' and ''plasmoid-type'' flux ropes. The flux ropes were usually several tens of seconds in duration, and the two types were readily distinguished by the sense of their quasisinusoidal ÁB z perturbations, i.e., Ç for the ''BBF'' events and ± for the ''plasmoid'' events. Most typically, a flux rope was observed to closely follow the onset of a high-speed flow within $1-2 min. Application of the Lepping-Burlaga constant-a flux rope model (i.e., J = aB) to these events showed that approximately 60% of each class could be acceptably described as cylindrical, force-free flux ropes. The modeling results yielded mean flux rope diameters and core field intensities of 1.4 R E and 20 nT and 4.4 R E and 14 nT for the BBF and plasmoid-type events, respectively. The inclinations of the flux ropes were small relative to the GSM X-Y plane, but a wide range of azimuthal orientations were determined within that plane. The frequent presence of these flux ropes in the plasma sheet is interpreted as strong evidence for multiple reconnection X-lines (MRX) in the near-tail. Hence, our results suggest that reconnection in the near-tail may closely resemble that at the dayside magnetopause where MRX reconnection has been hypothesized to be responsible for the generation of flux transfer events.

High-speed flowing plasmas in the Earth’s plasma sheet

“…In the meantime, earthward flowing plasmoids or magnetic structures have also been observed [41,42]. Plasmoids are observed singly or in groups [41,43].…”

“…In the meantime, earthward flowing plasmoids or magnetic structures have also been observed [41,42]. Plasmoids are observed singly or in groups [41,43]. Frequently, the observed flux ropes are believed to be evidence of a near-Earth multi-X line reconnection process that can produce more than one plasmoid in the plasma sheet.…”

“…High-speed flows are thought to be magnetic structures since the plasma parameters are similar for flows and structures [24]. Slavin et al [41] suggested there is a close relationship between these two and divided the flux rope events into "BBF-type" and "Plasmoid-type" events ( Figure 3). They found that flux ropes with a scale of 2-5 R E could be observed every 5 hours.…”

“…According to their model, the BBF-type structures would have an average scale of 1.4 R E with a field of 20 nT in the center of the structure, while for plasmoid-type structures, the magnitudes are 4.4 R E and 14 nT. Slavin et al [41] also analyzed a BBF structure with a scale of 1 R E and concluded that this structure was not force-free because it had apparently interacted with the background plasma. Tailward flowing structures form plasmoid structures that are often observed in the magnetotail during substorms [46].…”

Periodic traveling compression regions during quiet geomagnetic conditions and their association with ground Pi2

“…The association with pulsed reconnection was based on the observation that the lobe Pi2 comprised a series of traveling compression regions (TCRs) or nightside flux transfer events (NFTE), which are thought to be the remote signature of reconnection (e.g. Slavin et al, 1984Slavin et al, , 2003aSergeev et al, 1993;Taguchi et al, 1998). It was thus further suggested by Keiling et al that one type of substorm-related ground Pi2 is associated with TCRs/NFTEs.…”