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Träfflista för sökning "WFRF:(Dunlop M) srt2:(2001-2004)"

Sökning: WFRF:(Dunlop M) > (2001-2004)

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1.
  • Lockwood, M, et al. (författare)
  • 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 signatures
  • 2001
  • Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 19:10-12, s. 1613-1640
  • Forskningsöversikt (refereegranskat)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 ([X(GSM), Y(GSM), Z(GSM)] approximate to [2, 7, 9] R(E)). 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 well-defined 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 cor-relate well with swings of the IMF clock angle (in GSM) to near 90degrees. 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 Alfvenic disturbances in the magnetopause reconnection layer. Like FTEs, the events are about 1 R(E) 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.
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  • Andre, M., Behlke, R., Wahlund, J.E., Vaivads, A., Eriksson, A., Tjulin, A., Carozzi, T. D., Cully, C., Gustafsson, G., Sundkvist, D., Khotyaintsev, Y., Cornilleau-Wehrlin, N., Rezeau, L., Maksimovic, M., Lucek, E., Balogh, A., Dunlop, M., Lindqvist, P.A. (författare)
  • Multi-spacecraft observations of broadband waves near the lower hybrid frequency at the Earthward edge of the magnetopause.
  • 2001
  • Ingår i: Annales Geophysicae. ; 19:6, s. 1471-1481
  • Tidskriftsartikel (refereegranskat)abstract
    • Broadband waves around the lower hybrid frequency (around 10 Hz) near the magnetopause are studied, using the four Cluster satellites. These waves are common at the Earthward edge of the boundary layer, consistent with earlier observations, and can have a
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  • Andre, M., et al. (författare)
  • Multi-spacecraft observations of broadband waves near the lower hybrid frequency at the Earthward edge of the magnetopause
  • 2001
  • Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 19:12-okt, s. 1471-1481
  • Tidskriftsartikel (refereegranskat)abstract
    • Broadband waves around the lower hybrid frequency (around 10 Hz) near the magnetopause are studied, using the four Cluster satellites. These waves are common at the Earthward edge of the boundary layer, consistent with earlier observations, and can have amplitudes at least up to 5 mV/m. These waves are similar on all four Cluster satellites, i.e. they are likely to be distributed over large areas of the boundary. The strongest electric fields occur during a few seconds, i.e. over distances of a few hundred km in the frame of the moving magnetopause, a scale length comparable to the ion gyroradius. The strongest magnetic oscillations in the same frequency range are typically found in the boundary layer, and across the magnetopause. During an event studied in detail, the magnetopause velocity is consistent with a large-scale depression wave, i.e. an inward bulge of magnetosheath plasma, moving tailward along the nominal magnetopause boundary. Preliminary investigations indicate that a rather flat front side of the large-scale wave is associated with a rather static small-scale electric field, while a more turbulent backside of the large-scale wave is associated with small-scale time varying electric field wave packets.
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  • Marklund, G.T., Ivchenko, N., Karlsson, T., Fazakerley, A., Dunlop, M., Lindqvist, P.A., Buchert, S., Owen, C., Taylor, M., Vaivalds, A., Carter, P., Andre, M. and Balogh, A. (författare)
  • Temporal evolution of the electric field accelerating electrons away from the auroral ionosphere.
  • 2001
  • Ingår i: Nature. ; 414:6865, s. 724-727
  • Tidskriftsartikel (refereegranskat)abstract
    • The bright night-time aurorae that are visible to the unaided eye are caused by electrons accelerated towards Earth by an upward-pointing electric field(1-3). On adjacent geomagnetic field lines the reverse process occurs: a downward-pointing electric fie
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  • Marklund, Göran T., et al. (författare)
  • Temporal evolution of the electric field accelerating electrons away from the auroral ionosphere
  • 2001
  • Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 414:6865, s. 724-727
  • Tidskriftsartikel (refereegranskat)abstract
    • The bright night-time aurorae that are visible to the unaided eye are caused by electrons accelerated towards Earth by an upward-pointing electric field(1-3). On adjacent geomagnetic field lines the reverse process occurs: a downward-pointing electric field accelerates electrons away from Earth(4-11). Such magnetic-field-aligned electric fields in the collisionless plasma above the auroral ionosphere have been predicted(12), but how they could be maintained is still a matter for debate(13). The spatial and temporal behaviour of the electric fields-a knowledge of which is crucial to an understanding of their nature-cannot be resolved uniquely by single satellite measurements. Here we report on the first observations by a formation of identically instrumented satellites crossing a beam of upward-accelerated electrons. The structure of the electric potential accelerating the beam grew in magnitude and width for about 200 s, accompanied by a widening of the downward-current sheet, with the total current remaining constant. The 200-s timescale suggests that the evacuation of the electrons from the ionosphere contributes to the formation of the downward-pointing magnetic-field-aligned electric fields. This evolution implies a growing load in the downward leg of the current circuit, which may affect the visible discrete aurorae.
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  • Resultat 1-10 av 14

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