Geotail observations of magnetic flux ropes in the plasma sheet

Abstract

Examination of Geotail measurements in the near‐tail ( X > −30 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, 〈 V x 〉 = 431 km/s, and 38 moving tailward, 〈 V x 〉 = −451 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 Δ B z perturbations, i.e., ∓ 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‐α flux rope model (i.e., J = α B ) 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.

Geotail observations of magnetic flux ropes in the plasma sheet

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