Abstract
Abstract. The structure and formation mechanism of a total of five Flux Transfer Events (FTEs), encountered on the equatorward side of the northern cusp by the Cluster spacecraft, with separation of ~5000 km, are studied by applying the Grad-Shafranov (GS) reconstruction technique to the events. The technique generates a magnetic field/plasma map of the FTE cross section, using combined magnetic field and plasma data from all four spacecraft, under the assumption that the structure is two-dimensional (2-D) and time-independent. The reconstructed FTEs consist of one or more magnetic flux ropes embedded in the magnetopause, suggesting that multiple X-line reconnection was involved in generating the observed FTEs. The dimension of the flux ropes in the direction normal to the magnetopause ranges from about 2000 km to more than 1 RE. The orientation of the flux rope axis can be determined through optimization of the GS map, the result being consistent with those from various single-spacecraft methods. Thanks to this, the unambiguous presence of a strong core field is confirmed, providing evidence for component merging. The amount of magnetic flux contained within each flux rope is calculated from the map and, by dividing it by the time interval between the preceding FTE and the one reconstructed, a lower limit of the reconnection electric field during the creation of the flux rope can be estimated; the estimated value ranges from ~0.11 to ~0.26 mV m-1, with an average of 0.19 mV m-1. This can be translated to the reconnection rate of 0.038 to 0.074, with an average of 0.056. Based on the success of the 2-D model in recovering the observed FTEs, the length of the X-lines is estimated to be at least a few RE.
Highlights
A flux transfer event (FTE) (Russell and Elphic, 1978; Haerendel et al, 1978), observed by a spacecraft situated near the magnetopause, is characterized by a bipolar pulse in the magnetic field component, Bn, normal to the average magnetopause surface
Optimal Grad-Shafranov reconstruction, a technique to generate a 2-D map of plasma and magnetic field structures using multi-spacecraft data, has been applied to five FTEs encountered by Cluster near the northern cusp
Under the assumption that the orientation of the flux rope axis is roughly the same as that of X-line(s), which led to the FTEs, the result indicates the existence of an X-line, both poleward and equatorward of the flux rope
Summary
A flux transfer event (FTE) (Russell and Elphic, 1978; Haerendel et al, 1978), observed by a spacecraft situated near the magnetopause, is characterized by a bipolar pulse in the magnetic field component, Bn, normal to the average magnetopause surface. (3) Southwood et al (1988) and Scholer (1988) suggested that the FTE signature may result from an impulsive burst of reconnection along an extended X-line, without invoking a localization in local time of the reconnection process, as in the Russell-Elphic model In this scenario, a temporal variation in the reconnection rate leads to the formation of a bulge in the magnetopause, which is observed by a spacecraft as the bulge propagates along the boundary into an unperturbed reconnection layer. A new X-line forms near the location of the old X-line formation, the result being the creation of a flux rope between the old and new Xlines Since this process repeats itself, this model accounts for the quasi-periodic occurrence of FTEs seen in observations
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