Dayside Merging for Northward IMF

S.-W. Chang, D. L. Gallagher, S. B. Mende, H. U. Frey, R. A. Greenwald, and P. T. Newell


Simultaneous observations from IMAGE, DMSP, and SuperDARN during intervals of positive
IMF Bz from Sep. 17 to Sep. 18, 2000 provide important evidence for magnetic merging at
the dayside magnetopause. IMF By was negative. Its magnitude was initially small and
later became comparable to the Bz component. The proton aurora observed by IMAGE
FUV/SI12 exhibited the typical auroral oval and a bright dayside auroral arc extending
from the oval to very high latitude. DMSP detected magnetosheath-like electron and ion
precipitations on magnetic field lines that mapped to the auroral arc. It suggests that
the observed auroral arc is the manifest of the cusp precipitations. The ionospheric
convection within the cusp aurora that was deduced from the SuperDARN radar observations
was sunward. The flow was most intense at the poleward edge of the cusp aurora and it
gradually decreased toward the oval. The polar cap convection is the typical two-cell
pattern with the cross-polar cap potential of the typical value for northward IMF. As
IMF By became more negative, cusp aurora moved toward the dawn side. These observations
are consistent with the high-latitude merging at the magnetopause but inconsistent with
the subcusp merging. The ionospheric footprint of the x-line is located at the poleward
edge of the cusp aurora. The estimated potential drop across the x-line is approximately
equal to the cross-polar cap potential. Thus the ionospheric convection for this event
is mainly driven by the magnetic merging at the magnetopause.

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Presented at the Magnetospheric Imaging Workshop, 
Yosemite National Park, California, U.S.A., Feb. 9 - 13, 2003.