Triple crossings of a string of magnetic islands at duskside magnetopause encountered by AMPTE/IRM satellite on 8 August 1985

Abstract

[1] There have been a number of reports on the existence of pearl-like magnetic island structures at the magnetopause current layer based on the analyses of single spacecraft data and two-dimensional reconstruction method of solving the Grad-Shafranov equation as a spatial initial value problem. This paper presents an unusual event of multiple magnetopause crossings encountered by AMPTE/IRM satellite at the duskside equatorial plane on 8 August, 1985. In a total of 11 magnetopause crossings spanning for nearly 2 hours, crossing 3, 4, and 9 display similar features of a string of magnetic islands imbedded within the overall tangential discontinuity-like current layers. In these crossings, the deHoffmann-Teller velocities form approximately 90°from the magnetopause normal that the in-and-out magnetopause motion becomes subsiding for the satellite to pick up the pearl-like plasmoids with island width of about 6-12 ion inertial length. In particular, crossing 3 and 9 are 1 hour apart but have almost the same magnetopause normal and deHoffmann-Teller velocity as well as similar invariant axis. A region of cold plasma adjacent to the magnetopause within the magnetosphere, the low-latitude boundary layer, is seen in all three crossings.

THEMIS observations of a secondary magnetic island within the Hall electromagnetic field region at the magnetopause

“…We employ magnetohydrostatic GS reconstruction [e.g., Hau and Sonnerup , 1999; Sonnerup et al , 2004; Hu and Dasgupta , 2005; Hasegawa et al , 2006; Teh and Hau , 2007] to derive the properties and shape of the magnetic island under the assumptions that its structure is steady‐state and two dimensional. The cross‐sectional map is constructed using spacecraft data as spatial initial values for the integration of the GS equation.…”

A statistical study of magnetopause structures: Tangential versus rotational discontinuities

“…A total of 328 magnetopause crossings have been identified from analyzing the AMPTE/IRM plasma and magnetic field data (with time resolution of 4.37 sec) of the period between Aug 1984 and Aug 1986. Using the same data, we have selected a number of magnetopause crossings for recovering the two‐dimensional magnetic field maps [ Hau and Sonnerup , 1999; Teh and Hau , 2004, 2007]. In this study the procedures and criteria used to identify the magnetopause crossings are: (1) the plasma and magnetic field data in the range of 5–13 R E are first plotted, and (2) after excluding the bow shock, the magnetopause is recognized as the layer across which there are pronounced changes in the magnetic field magnitude or direction and/or in the number density and/or temperature in the entire plot.…”

“…For some selected events 2D Grad‐Shafranov (GS) reconstruction methods [e.g., Hau and Sonnerup , 1999] are applied to infer the corresponding plasma and magnetic field configurations from single spacecraft data. As shown, magnetopause structures often involve single or multiple X‐type nulls and/or magnetic islands in the transverse field, a feature also found in earlier event study [e.g., Hasegawa et al , 2004; Teh and Hau , 2007]. In many cases such structures have insignificant flow in the HT frame which may be interpreted as the result of tearing‐mode instability after it has reached its saturated state [ Hau and Chiou , 2001].…”

Reconstruction of a magnetic flux rope from THEMIS observations

“…Such a useful tool called Grad‐Shafranov reconstruction has been developed by Bengt Sonnerup and his colleagues to examine magnetic flux ropes near the magnetopause, in the solar wind, and in the magnetotail (see Sonnerup et al [2006] for an overview). The technique is initially employed to analyze data from a single satellite [ Sonnerup and Guo , 1996; Hau and Sonnerup , 1999; Hu and Sonnerup , 2001, 2003; Teh and Hau , 2004, 2007] and is now extended to treat measurements from multiple satellites [ Sonnerup et al , 2004; Hasegawa et al , 2005, 2006, 2007]. The assumptions in employing this technique were listed by Hau and Sonnerup [1999], with additional caveats given by Hasegawa et al [2007] based on tests with simulation data.…”