Determining the axial direction of high-shear flux transfer events: Implications for models of FTE structure

摘要

[1] Flux transfer events are bursts of dayside reconnection, which give rise to local perturbations of the magnetic field that can be observed by spacecraft near the magnetopause. Although it is commonly accepted that flux transfer events are caused by reconnection, various models exist to explain their structure. A key difference between mechanisms is that when the magnetic shear across the magnetopause is close to 180°, some models will give rise to a structure whose axis is oriented north-south, whereas others will result in a dawn-dusk-oriented structure. Several techniques for determining the axial direction of such structures have been suggested: minimum variance analysis on the magnetic field, minimization of the axial electric field, and Grad-Shafranov reconstruction. We apply these techniques to a series of flux transfer events observed by Cluster at a high-shear magnetopause crossing on 27 March 2007. Minimum variance analysis on the signatures caused by the draping of unreconnected magnetic flux around the flux transfer events consistently results in an axial direction that is dawn-dusk oriented. However, the electric field technique, applied to flux transfer events that are penetrated by the spacecraft, results in a mixture of north-south and dawn-dusk axes. Testing these axial directions with Grad-Shafranov reconstruction suggests that the axes of events which appear to be oriented dawn-dusk might be more reliably determined than the axes of events which appear to be north-south. Using the Grad-Shafranov method alone to determine the axial direction of the penetrated events gives dawn-dusk axial directions, consistent with the results of minimum variance analysis on the draping events. Overall, these observations are consistent with the formation of flux transfer events by processes involving relatively long reconnection lines (e.g., single or multiple X-line reconnection) rather than elbow-shaped flux tubes, as originally envisaged. Furthermore, examination of one particular flux transfer event that was observed by Cluster 1 but not by Cluster 2 (which was closer to the magnetopause) allows us to preclude the elbow-shaped model for this event.