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Ann. Geophys., 21, 2233-2258, 2003
www.ann-geophys.net/21/2233/2003/
© European Geosciences Union 2003
Polar, Cluster and SuperDARN evidence for high-latitude merging during southward IMF: temporal/spatial evolution
N. C. Maynard1, D. M. Ober1, W. J. Burke2, J. D. Scudder3, M. Lester4, M. Dunlop5, J. A. Wild4, A. Grocott4, C. J. Farrugia6, E. J. Lund6, C. T. Russell7, D. R. Weimer1, K. D. Siebert1, A. Balogh8, M. Andre9, and H. Rème10
1Mission Research Corporation, Nashua, New Hampshire, USA
2Air Force Research Laboratory, Hanscom Air Force Base, Massachusetts, USA
3Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA
4University of Leicester, Leicester, UK
5Rutherford-Appleton Laboratory, Didcot, UK
6EOS, University of New Hampshire, Durham, New Hampshire, USA
7IGPP, UCLA, Los Angeles, California, USA
8Imperial College, London, UK
9Uppsala University, Upsalla, Sweden, USA
10CESR, Toulouse, France
Abstract. Magnetic merging on the dayside
magnetopause often occurs at high latitudes. Polar measured fluxes of
accelerated ions and wave Poynting vectors while skimming the subsolar
magnetopause. The measurements indicate that their source was located to
the north of the spacecraft, well removed from expected component merging
sites. This represents the first use of wave Poynting flux as a merging
discriminator at the magnetopause. We argue that wave Poynting vectors,
like accelerated particle fluxes and the Walén tests, are necessary, but
not sufficient, conditions for identifying merging events. The Polar data
are complemented with nearly simultaneous measurements from Cluster in the
northern cusp, with correlated observations from the Super-DARN radar, to
show that the locations and rates of merging vary. Magnetohydrodynamic (MHD)
simulations are used to place the measurements into a global context. The
MHD simulations confirm the existence of a high-latitude merging site and
suggest that Polar and SuperDARN observed effects are attributable to both
exhaust regions of a temporally varying X-line. A survey of 13
merging events places the location at high latitudes whenever the
interplanetary magnetic field (IMF) clock angle is less than ~150°. While
inferred high-latitude merging sites favor the antiparallel merging
hypothesis, our data alone cannot exclude the possible existence of a
guide field. Merging can even move away from equatorial latitudes when the
IMF has a strong southward component. MHD simulations suggest that this
happens when the dipole ilt angle increases or when IMF BX
increases the effective dipole tilt.
Key words. Magnetospheric physics (magnetopause,
cusp and boundary layers; magnetospheric configuration and dynamics; solar
wind-magnetosphere interactions)
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