Modeling a force‐free flux transfer event probed by multiple Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft

Abstract

A flux transfer event (FTE) was encountered by all five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft just outside the dusk magnetopause on 20 May 2007. In the present paper, we fit the magnetic field observations from multiple satellites to a flux rope model in order to obtain the critical geometric parameters of this FTE. The model used includes both electromagnetic force and pressure gradient force and reduces to a force‐free model when the magnetic tension is balanced mainly by the magnetic pressure gradient. The cross‐sectional scales of the FTE were estimated to be 0.6 × 1.3 RE. The core field direction is found to differ only slightly from the results of the minimum variance analysis and the Grad‐Shafranov technique. The maximum electric current density within the FTE was ∼100 nA/m2 and the total current was 0.3 MA, which is comparable with the current per RE flowing on the local magnetopause. The magnetic flux inside the FTE was ∼4 × 105 Wb. Magnetic field data and plasma data show that this FTE was fully embedded in the boundary region between the magnetosheath and the magnetosphere. The magnetic configuration and the flow distribution around the FTE suggest that X lines were present both ahead of and behind the FTE, but we cannot determine whether these X lines formed synchronously. Our observations favor the conclusion that multiple X line models are the candidate mechanisms for the generation of this FTE, and some of the FTE's features cannot be understood in the frame of any single X line FTE model.