Correlative Motion of two IMAGE/LENA Hydrogen Rate Peaks Observed in the Low-
and Mid-Latitude Dayside Magnetosheath Direction

S. Taguchi, M.R. Collier, T.E. Moore, M. Fok


Recent studies using IMAGE/LENA observations have shown that significant
brightening between the direction of the Sun and the Earth under certain
conditions is the result of solar wind ions charge-exchanging with geocoronal
neutral hydrogen atoms in the flow in the magnetosheath, and that the
brightening tends to increase when the subsolar standoff distance is reduced.
The inward motion of the magnetopause is caused by the increase of the solar
wind dynamic pressure and that of a southward component of IMF, and the latter
also causes the cusp entry region to move equatorward. We test if LENA can
monitor the magnetopause inward/outward motion and the cusp
equatorward/poleward motion simultaneously to identify dynamical features of
these two types of motion. First, from the intervals of IMAGE noon-midnight
orbits, we surveyed for events when the magnetopause was inside of
geosynchronous orbit and at the same time either GOES 8 or GOES 10 observed
magnetosheath magnetic field near the subsolar point. We also required that
LENA hydrogen counts from the sun direction during a selected interval are not
correlated with the variations of the ACE/EPAM flux data for ions with
energies between 47 and 65 keV/e so as to exclude a possibility that the count
enhancements are due to suprathermal ions penetrating collimator. LENA
observations taken at Z=4 Re in the dayside magnetosphere on April 11, 2001
show the continuous and correlated motion of two hydrogen peaks coming from
the mid- and low-latitude sheath direction. When a sphere with a radius of
geosynchronous orbit (=6.6 Re) is assumed around the Earth, the line of sight
of these mid- and low-latitude enhancements intersects the sphere at 5-6 Re
and a few Re away from the equatorial plane, respectively. We show how these
motions can be related to the dynamics of the magnetopause, and their
characteristics are discussed in terms of solar wind data measured by ACE and
sheath magnetic field data from GOES 8/10.

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Fall 2002 Meeting of the American Geophysical Union
San Francisco, CA, USA, 6-10 December 2002