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Motion of Flux Transfer Events: a Test of the Cooling Model R Motion of flux transfer events: a test of the Cooling model R. C. Fear, S. E. Milan, A. N. Fazakerley, C. J. Owen, T. Asikainen, M. G. G. T. Taylor, E. A. Lucek, H. Rème, I. Dandouras, P. W. Daly To cite this version: R. C. Fear, S. E. Milan, A. N. Fazakerley, C. J. Owen, T. Asikainen, et al.. Motion of flux transfer events: a test of the Cooling model. Annales Geophysicae, European Geosciences Union, 2007, 25 (7), pp.1669-1690. hal-00330141 HAL Id: hal-00330141 https://hal.archives-ouvertes.fr/hal-00330141 Submitted on 30 Jul 2007 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Ann. Geophys., 25, 1669–1690, 2007 www.ann-geophys.net/25/1669/2007/ Annales © European Geosciences Union 2007 Geophysicae Motion of flux transfer events: a test of the Cooling model R. C. Fear1, S. E. Milan1, A. N. Fazakerley2, C. J. Owen2, T. Asikainen3, M. G. G. T. Taylor4, E. A. Lucek5, H. Reme` 6, I. Dandouras6, and P. W. Daly7 1Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK 2Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK 3Department of Physical Sciences, University of Oulu, P.O. Box 3000, 90014, Finland 4Research and Scientific Support Department, European Space Agency, 2201AZ Noordwijk, The Netherlands 5Blackett Laboratory, Imperial College, Prince Consort Road, London, SW7 2BZ, UK 6CESR/CNRS, 9 Avenue du Colonel Roche, B.P. 4346, 31028 Toulouse Cedex 4, France 7Max-Planck-Institut fur¨ Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany Received: 30 March 2007 – Revised: 29 June 2007 – Accepted: 13 July 2007 – Published: 30 July 2007 Abstract. The simple model of reconnected field line mo- 1 Introduction tion developed by Cooling et al. (2001) has been used in sev- eral recent case studies to explain the motion of flux transfer Magnetic reconnection at the Earth’s magnetopause events across the magnetopause. We examine 213 FTEs ob- (Dungey, 1961, 1963) is the dominant process of energy served by all four Cluster spacecraft under a variety of IMF and momentum transfer from the solar wind into the ter- conditions between November 2002 and June 2003, when the restrial magnetosphere (Cowley, 1984). The basic process spacecraft tetrahedron separation was ∼5000 km. Observed of reconnection as envisaged by Dungey (1961, 1963) velocities were calculated from multi-spacecraft timing anal- was steady state. Indeed, reconnection may occur as a ysis, and compared with the velocities predicted by the Cool- continuous process for hours at a time (Frey et al., 2003; ing model in order to check the validity of the model. After Phan et al., 2004). However, magnetopause reconnection excluding three categories of FTEs (events with poorly de- commonly occurs as a transient or time-varying process, fined velocities, a significant velocity component out of the as first observed by Haerendel et al. (1978) in the form of magnetopause surface, or a scale size of less than 5000 km), signatures they called “flux erosion events”. we were left with a sample of 118 events. 78% of these Independently, Russell and Elphic (1978, 1979) identified events were consistent in both direction of motion and speed signatures of transient reconnection at the low-latitude mag- with one of the two model de Hoffmann-Teller (dHT) veloc- netopause which they called “flux transfer events” (FTEs). ities calculated from the Cooling model (to within 30◦ and These signatures were interpreted as open flux ropes, formed a factor of two in the speed). We also examined the plasma by reconnection, which moved away from the reconnection signatures of several magnetosheath FTEs; the electron sig- site under the net effect of the force exerted by the solar natures confirm the hemisphere of connection indicated by wind flow and the j×B magnetic tension force. They were the model in most cases. This indicates that although the identified by inspecting the magnetic field data in the magne- model is a simple one, it is a useful tool for identifying the tosheath or magnetosphere in a boundary normal coordinate source regions of FTEs. system (introduced by Russell and Elphic, 1978), in which Nˆ is normal to the magnetopause and directed away from Keywords. Magnetospheric physics (Magnetopause, cusp, Earth. An FTE exhibits a bipolar signature in the BN com- arid boundary layers; Solar wind-magnetosphere interac- ponent. In the simplest case, the bipolar signature is formed tions) – Space plasma physics (Magnetic reconnection) by the draping of unreconnected magnetic field lines around the FTE. The flux erosion events reported by Haerendel et al. (1978) were shown to be flux transfer events by Rijnbeek and Cowley (1984). The polarity of the BN signature depends upon the mo- Correspondence to: R. C. Fear tion of the FTE relative to the unperturbed magnetic field. ([email protected]) In the magnetosheath, a “standard” or “direct” signature (a Published by Copernicus Publications on behalf of the European Geosciences Union. 1670 R. C. Fear et al.: FTE motion positive followed by a negative deflection in BN ) occurs if ern Hemisphere, whilst reverse polarity FTEs are generally the FTE velocity has a component antiparallel to the local observed in the Southern Hemisphere (Rijnbeek et al., 1984), magnetosheath magnetic field, whereas a “reverse” signa- although the division between these events is often inclined ture (negative/positive) occurs if the velocity has a compo- to the magnetic equator (Berchem and Russell, 1984). Rus- nent parallel to the magnetic field (Rijnbeek et al., 1982). If sell et al. (1985) showed the polarity, and hence motion, of the FTE is observed from inside the magnetosphere, a stan- FTE signatures which occurred when the IMF was southward dard signature occurs if the FTE velocity has a component to be consistent with low-latitude reconnection even when parallel to the geomagnetic field and a reverse component if there is a dominant IMF BY component. the component is antiparallel. There is often a significant in- More recent surveys have extended to the post-terminator crease or decrease in |B|, and an imbalance in the total pres- region. Kawano and Russell (1997a,b) studied a database 2 sure (pgas+B /2µ0) countered by the magnetic tension in of 1246 FTEs, of which 79 occurred in the post-terminator the draped magnetic field lines (Paschmann et al., 1982). A region (XGSM<0, |ZGSM|<15 RE) when the IMF was north- bipolar BN signature is also observed when the reconnected ward. It was proposed that most of these events could be flux tube was crossed, explained by a helicity in the recon- explained by a tilted, subsolar component reconnection line nected flux rope (Sonnerup, 1987). if open flux tubes in the subsolar region were immediately Several alternative reconnection-based models have been closed by a process of “re-reconnection” (Nishida, 1989), proposed which explain the observations: Lee and Fu (1985) thus preventing northward IMF FTEs from being observed proposed a model where helical flux tubes were generated in the subsolar region. When the IMF was more strongly by multiple reconnection lines (X-lines). Southwood et al. northward, Kawano and Russell (1997b) concluded that the (1988) and Scholer (1988) independently proposed a model polarities and IMF BY dependency could also be explained if based on a single X-line, where the magnetopause boundary the FTEs were generated near the polar cusps at an antiparal- layer thickens and then thins as a result of a variation in the lel reconnection site, but then somehow moved equatorward reconnection rate, producing a bulge which propagates un- and tailward. der the same magnetosheath flow and j×B effects. This two The anisotropy of plasma signatures associated with mag- dimensional model does not produce a tube of reconnected netosheath FTEs can be used to determine the hemisphere flux, but can extend a considerable distance along the mag- of connection of an FTE. Plasma populations originating netopause. from the magnetosphere and with a field-aligned anisotropy It has also been suggested that FTE-style signatures can were first observed inside magnetosheath FTEs by Daly et al. be formed by magnetopause waves, although this has been (1981), consistent with the spacecraft being on open, recon- hotly debated (e.g. Sibeck et al., 1989; Lanzerotti, 1989; nected magnetic field lines. A parallel beam observed on Sibeck, 1990; Elphic, 1990; Sckopke, 1991; Lockwood, open magnetic field lines in the magnetosheath implies a con- 1991; Sibeck, 1992; Smith and Owen, 1992; Kawano et al., nection to the Southern Hemisphere, whereas an antiparallel 1992; Elphic et al., 1994; Song et al., 1994, 1996; Sibeck and beam implies connection to the Northern Hemisphere. (A Newell, 1995, 1996; Sanny et al., 1996). In the context of plasma signature is also observed in magnetospheric FTEs, this debate, Kawano et al. (1992) introduced a “characteris- e.g. Paschmann et al. (1982), although the scenario is compli- tic time” (tchar, defined as the time between the positive and cated by the mirroring of plasma at the ionosphere.) Further- negative peaks in the BN signature) to distinguish between more, Daly et al. (1984) found that there was sometimes an longer events with bipolar BN signatures (tchar>90 s) which inconsistency between the direction of motion inferred from were found to occur over a wide range of McIlwain L-shells the FTE polarity (standard or reverse) and the anisotropy and were not correlated to periods of reconnection as evi- of high-energy ion signatures (above 25 keV).
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