Coordinated Cluster and ground-based instrument observations of transient changes in the magnetopause boundary layer during an interval of predominantly northward IMF: relation to reconnection pulses and FTE signaturesReportar como inadecuado




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1 Solar Terrestrial Physics Division 2 Department of Physics and Astronomy 3 MSSL - Mullard Space Science Laboratory 4 IRF 5 Department of Physics Oslo 6 EISCAT Scientific Association 7 Blackett Laboratory 8 CESR - Centre d-étude spatiale des rayonnements 9 HAO - High Altitude Observatory 10 University of Calgary 11 AFRL 12 Department of Physics and Astonomy 13 FMI - Finnish Meteorological Institute 14 University of Tromso 15 Communications Research Centre 16 Remote Sensing Group 17 Space Science Centre 18 Department of Physics Minneapolis 19 DMI - Danish Meteorological Institute 20 Istituto di Fisica dello Spazio Interplanetario

Abstract : We study a series of transient entries into the low-latitude boundary layer LLBL of all four Cluster spacecraft during an outbound pass through the mid-afternoon magnetopause XGSM, YGSM, ZGSM ˜ 2, 7, 9 RE. The events take place during an interval of northward IMF, as seen in the data from the ACE satellite and lagged by a propagation delay of 75 min that is welldefined by two separate studies: 1 the magnetospheric variations prior to the northward turning Lockwood et al., 2001, this issue and 2 the field clock angle seen by Cluster after it had emerged into the magnetosheath Opgenoorth et al., 2001, this issue. With an additional lag of 16.5 min, the transient LLBL events correlate well with swings of the IMF clock angle in GSM to near 90°. Most of this additional lag is explained by ground-based observations, which reveal signatures of transient reconnection in the pre-noon sector that then take 10–15 min to propagate eastward to 15 MLT, where they are observed by Cluster. The eastward phase speed of these signatures agrees very well with the motion deduced by the cross-correlation of the signatures seen on the four Cluster spacecraft. The evidence that these events are reconnection pulses includes: transient erosion of the noon 630 nm cusp-cleft aurora to lower latitudes; transient and travelling enhancements of the flow into the polar cap, imaged by the AMIE technique; and poleward-moving events moving into the polar cap, seen by the EISCAT Svalbard Radar ESR. A pass of the DMSP-F15 satellite reveals that the open field lines near noon have been opened for some time: the more recently opened field lines were found closer to dusk where the flow transient and the poleward-moving event intersected the satellite pass. The events at Cluster have ion and electron characteristics predicted and observed by Lockwood and Hapgood 1998 for a Flux Transfer Event FTE, with allowance for magnetospheric ion reflection at Alfvénic disturbances in the magnetopause reconnection layer. Like FTEs, the events are about 1 RE in their direction of motion and show a rise in the magnetic field strength, but unlike FTEs, in general, they show no pressure excess in their core and hence, no characteristic bipolar signature in the boundary-normal component. However, most of the events were observed when the magnetic field was southward, i.e. on the edge of the interior magnetic cusp, or when the field was parallel to the magnetic equatorial plane. Only when the satellite begins to emerge from the exterior boundary when the field was northward, do the events start to show a pressure excess in their core and the consequent bipolar signature. We identify the events as the first observations of FTEs at middle altitudes.





Autor: M. Lockwood - A. Fazakerley - H. Opgenoorth - J. Moen - A. P. Van Eyken - M. Dunlop - J.-M. Bosqued - G. Lu - C. Cully - P. Eglit

Fuente: https://hal.archives-ouvertes.fr/



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