Footprints of Dayside Reconnection

T. E. Moore, M.-C. Fok, M. O. Chandler, Chen, and O. L. Vaisberg 


The hypothesis that reconnection occurs along a locus where merging fields are
antiparallel is both intuitively appealing and supported by a number of in
situ observations. The loci of antiparallel fields are curves lying on the
dayside magnetopause that radiate away from the cusps, equatorward for
southward Bz, poleward for northward Bz, and flankward for finite By with
east-west sense determined by the sign of By. Yet when reconnection commences
at any point along an antiparallel locus, its local X-line is near-normal to
that locus, and reconnection cannot extend far along the X-line and remain
antiparallel. If the reconnection rate drops precipitously for small
deviations from antiparallel fields, it must then be limited to one or more
short X-line(s) extending across the antiparallel locus, limiting the
transpolar potential that can be developed. If, instead, the reconnection rate
is proportionate to the magnitude of the antiparallel field component (the
reconnecting component) at any point, then reconnection will extend away from
the antiparallel locus along an X-line curve that we have integrated and which
extends for great distances across the magnetopause. Recent observations and
simulations suggest that the reconnection rate is insensitive to the angle
between the merging fields, or equivalently, to the "guide field" component.
We explore the consequences of this for the configuration of the X-line,
suggesting a synthesis of the antiparallel and component reconnection
hypotheses. An S-or Z-shaped X-line, depending on By, is the simplest general
case and consistent with the By dependence of ionospheric convection features.
Multiple X-lines form when more than one site initiates reconnection, as for
northerly Bz. The result of multiple active X-lines must be a region of
helical fields between any nearby pair of X-lines, forming an isolated region
of mixed internal and external plasmas. Newly linked flux tubes continue to be
peeled off from the X-lines on either side of the helical region. These
features are seen in certain MHD simulations and evidence of them has also
been found in Interball plasma observations.

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