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ACE News #66 - Nov 27, 2002

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A Double Flux-Rope Magnetic Cloud

A recent study by Hu and Sonnerup [in press] showed that a magnetic cloud at 1 AU has the topology of a non-axisymmetric, cylindrical flux rope. As illustrated in the top panel, the method they employ to determine cloud geometry utilizes magnetic field and plasma data collected during the interval when the spacecraft passes through the cloud. A cross-section of the cloud, in the plane perpendicular to the axis (z) along which the structure has approximate invariance, is obtained from this single line of data.

A magnetic cloud event observed August 12, 2000 (day 225, lower left panel) by the MAG and SWEPAM instruments on ACE has recently been examined. The recovered cross-section reveals a configuration of double magnetic flux ropes with a single X-point in between (lower right panel). The contour lines show the transverse magnetic field lines, while the colors show the axial magnetic field distribution as indicated by the color bar. The yellow arrows along y=0 denote the measured transverse magnetic field vectors utilized as initial input into the numerical solver. The 2½-D current density distribution is also obtained. The two flux ropes apparently have different sizes, different maximum axial magnetic field strengths, and different axial current distributions.

For this event, the total axial magnetic flux and the total axial current bounded by the highlighted white contour line are 7.6 × 1012 T-m2, and 1.1 × 109 A, respectively. The axial current density near the center of the left and right flux ropes, where Bz reaches local maximum, is -4.2 × 10-12 A/m2, and -3.1 × 10-12 A/m2, respectively. This analysis self-consistently reveals unique 2½-D solutions with a single pass through one of these clouds without additional constraints.

Contributed by Qiang Hu and the ACE MAG Team at the University of Delaware.

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Last modified 27 November 2002, by Andrew Davis