Co-ordinated IMAGE satellite and ground-magnetometer observations of a
cross-phase reversal at a steep  plasmapause

Z.C. Dent, I.R. Mann, F.W. Menk, J. Goldstein, and L.G. Ozeke


For a few hours during the local morning of the 14th May 2001 the cross-phase
peak observed using European sector ground-based magnetometer station pairs with
mid-points at L = 3.16 and L = 3.34 changed polarity from positive to negative. 
All other BGS, SAMNET and IMAGE station pairs examined showed a positive
cross-phase peak throughout the day. The IMAGE satellite made an excellent close
conjunction with these ground-based magnetometer arrays on this day, and data
from the RPI instrument shows a very steep plasmapause in this region during
this UT. IMAGE EUV results show that the plasmapause moved outward through the
day, passing through the region of observed negative cross-phase peak. The
observed negative cross-phase  peak is theoretically expected at a plasmapause
with a gradient steeper than r^-8, but is rarely observed. The observations
also infer the presence of an enhanced heavy ion population outside the
plasmapause in the inner plasmatrough on  this day. We discuss the possibility
that refilling through the day reduced the gradient of the initially steep
plasmapause, eventually removing the negative cross-phase peak.

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Presentation, Fall A.G.U. Meeting, San Francisco, CA, 13-17 December 2004