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We analyze a magnetic signature associated with the leading edge of a burstybulk flow observed by Cluster at −19 RE downtail on 22 August 2001. Adistinct rotation of the magnetic field was seen by all four spacecraft. Thisevent was previously examined by Slavin et al. (2003b) using bothlinear force-free modeling as well as a curlometer technique. Extending thiswork, we apply here single- and multi-spacecraft Grad-Shafranov (GS)reconstruction techniques to the Cluster observations and find good evidencethat the structure encountered is indeed a magnetic flux rope and containshelical magnetic field lines. We find that the flux rope has a diameter ofapproximately 1 RE, an axial field of 26.4 nT, a velocity of≈650 km/s, a total axial current of 0.16 MA and magnetic fluxes oforder 105 Wb. The field line twist is estimated as half a turn perRE. The invariant axis is inclined at 40° to the ecliptic plane and 10° to the GSM equatorial plane.The flux rope has a force-free core and non-force-free boundaries. When wecompare and contrast our results with those obtained from minimum variance,single-spacecraft force-free fitting and curlometer techniques, we find ingeneral fair agreement, but also clear differences such as a higherinclination of the axis to the ecliptic. We further conclude thatsingle-spacecraft methods have limitations which should be kept in mind whenapplied to THEMIS observations, and that non-force-free GS and curlometertechniques are to be preferred in their analysis. Some properties we derivedfor this earthward– moving structure are similar to those inferred byLui et al. (2007), using a different approach, for a tailward-moving fluxrope observed during the expansion phase of the same substorm

The structure of an earthward propagating magnetic flux rope early in its evolution: comparison of methods