Motion of flux transfer events: a test of the Cooling model

Abstract

Abstract. The simple model of reconnected field line motion developed by Cooling et al. (2001) has been used in several recent case studies to explain the motion of flux transfer events across the magnetopause. We examine 213 FTEs observed by all four Cluster spacecraft under a variety of IMF conditions between November 2002 and June 2003, when the spacecraft tetrahedron separation was ∼5000 km. Observed velocities were calculated from multi-spacecraft timing analysis, and compared with the velocities predicted by the Cooling model in order to check the validity of the model. After excluding three categories of FTEs (events with poorly defined velocities, a significant velocity component out of the magnetopause surface, or a scale size of less than 5000 km), we were left with a sample of 118 events. 78% of these events were consistent in both direction of motion and speed with one of the two model de Hoffmann-Teller (dHT) velocities calculated from the Cooling model (to within 30 • and a factor of two in the speed). We also examined the plasma signatures of several magnetosheath FTEs; the electron signatures confirm the hemisphere of connection indicated by the model in most cases. This indicates that although the model is a simple one, it is a useful tool for identifying the source regions of FTEs.

The azimuthal extent of three flux transfer events

“…Observations at scales between 100 and 5000 km have enabled the study of FTE motion at the magnetopause (Owen et al, 2001;Wild et al, 2005a;Dunlop et al, 2005;Fear et al, 2005Fear et al, , 2007, FTE structure (Hasegawa et al, 2006;Owen et al, 2008), further study of the statistics of FTE occurrence (Wang et al, , 2006Fear et al, 2005Fear et al, , 2007 and conjugated magnetopause/ionospheric studies using the SuperDARN radar network (Wild et al, 2001(Wild et al, , 2003(Wild et al, , 2005b(Wild et al, , 2007Marchaudon et al, 2004;Amm et al, 2005). In this paper we present the first observations of FTEs from the Cluster 10 000 km separation magnetopause crossing season, enabling in situ examination of the scale lengths of an FTE in all three dimensions.…”

“…This was done by finding the time delay which maximised the cross-correlation between each pair of B N time series, and then adjusting the lag by eye if necessary to achieve the best alignment of three key points: the midpoint of the bipolar B N signature and the positive and negative peaks. These time differences are given in Table 1, along with the uncertainties on each timing measurement, within which all three key points are aligned, as described by Fear et al (2007). Using the procedure described by Harvey (1998, p. 311), the velocity of the FTE at 19:33 UT was found to be (399, 97, 16) LMN km s −1 , and the velocity of the .…”

“…Since it was observed in the Northern Hemisphere the force exerted on the FTE by the magnetosheath flow must have outweighed the magnetic tension force on the kinked magnetic field lines. Such instances are not uncommon (Daly et al, 1984;Fear et al, 2007). For such a situation to occur, the magnetosheath flow at the reconnection site must be super-Alfvénic, which has the further consequence that reconnection there must be non-steady (Cowley and Owen, 1989).…”

“…Owen et al, 2001, who in a case study estimated D V to be of order 0.8 R E ). Fear et al (2007) calculated the scale size between the positive and negative peaks of the bipolar signature of 142 FTEs which were observed by all four Cluster spacecraft (a similar, but not identical measure to D V ), and found that most FTEs were between 0.6 and 2.2 R E , although smaller events were under-represented as a consequence of the requirement of the FTE to be observed by all four Cluster spacecraft. An FTE's scale size normal to the magnetopause (D N ) can be estimated either by a suitable conjunction of different spacecraft (e.g.…”

MESSENGER observations of large flux transfer events at Mercury

“…Here we report a comprehensive survey of the MESSENGER magnetic field data for the occurrence of FTEs. From definitions developed for Earth's magnetosphere [e.g., Wang et al, 2005;Fear et al, 2007], six FTEs were identified during the two flybys with all, save one, strongly resembling flux ropes. Unfortunately, MESSENGER does not make the high-time-resolution plasma moment measurements necessary to analyze these FTEs using the Grad-Shafranov (GS) reconstruction technique [Zhang et al, 2008;Eriksson et al, 2009].…”