Cusp Dynamics and Ionospheric Outflow 

Fuselier, S. A., S. B. Mende, T. E. Moore, H. Frey, S. B. Petrinec, H. L. Collin, 
E. S. Claflin, M. R. Collier

The ionospheric footpoints of the EarthÕs magnetospheric cusps are relatively
narrow regions in invariant latitude that map magnetically to the dayside
magnetopause.  Monitoring the cusp reveals three important aspects of magnetic
reconnection at the magnetopause.  Continuous cusp observations reveal the
relative contributions of quasi-steady versus impulsive reconnection to the
overall transfer of mass, energy, and momentum across the magnetopause.  The
spatial extent of the cusp is related to the length of the merging line.  This
length, along with the local mass transfer rate, determines the total mass
transfer across the magnetopause at any instant in time.  Finally, the location
of the cusp is used to determine where magnetic reconnection is occurring on the
magnetopause.  Of particular interest is the distinction between anti-parallel
reconnection, where the magnetosheath and magnetospheric field lines are
strictly anti-parallel and component merging, where the magnetosheath and
magnetospheric field lines have one component that is anti-parallel.

Traditionally, the location of the cusp footpoint has been monitored by
ground-based observations. The IMAGE mission has provided the first monitoring
of magnetosheath proton precipitation in the cusp from high altitudes.  The
observations from this imager indicate that the cusp is continuous over long
periods of time (up to several hours).  Thus, quasi-steady reconnection plays an
important role in the overall plasma transfer across the magnetopause.  The
spatial extent of the cusp is small when the interplanetary magnetic field (IMF)
is directed northward and is relatively large when the IMF is directed
southward.  Thus, the merging line opens magnetosheath field lines over a
relatively small area of the magnetopause for northward IMF and a much larger
area of the magnetopause for southward IMF.  Finally, the location of the
reconnection site indicates that anti-parallel merging is occurring during
northward IMF while component merging must be occurring during southward IMF.

The ionospheric footpoint of the cusp is also a region of relatively intense
ionospheric outflow.  Since other regions of the auroral oval are also regions
of outflow, one of the long-standing problems has been to determine the relative
contributions of the cusp and the rest of the auroral oval to the overall
ionospheric ion content in the EarthÕs magnetosphere.  The IMAGE spacecraft also
carries a unique neutral atom imager that images neutral atoms originating from
charge exchange of ionospheric ion outflow.  This imager has made the first
"images" of ionospheric outflow on timescales commensurate with the
magnetospheric substorm.  While the nature of ionospheric outflow has made it
difficult to resolve the long-standing problem of the cusp contribution to
ionospheric ions in the magnetosphere, the new neutral atom images from IMAGE
have provided important evidence that ionospheric outflow is strongly controlled
by solar wind input, is "prompt" in response to changes in the solar wind, and
has very narrow and distinct pitch angle structures and charge exchange
altitudes.

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To be presented at the Magnetospheric Imaging Workshop, 
Yosemite National Park, California, U.S.A., Feb. 5-8, 2002.