Monitoring the high-altitude cusp with low-energy neutral atom imager: 
Simultaneous observations from IMAGE and Polar     

S. Taguchi, S.-H. Chen, M.R. Collier, T.E. Moore, M.-C. Fok, K. Hosokawa, 
and A. Nakao

The Low Energy Neutral Atom (LENA) imager on the IMAGE spacecraft in the dayside
magnetosphere can detect neutral particles that are emitted in the magnetosheath
flow. During a period of dynamic pressure of 4-6 nPa and IMF BZ of Ð5 to 3 nT on
April 12, 2001, LENA on IMAGE at (XGSM, YGSM, ZGSM)~(4 RE, 0 RE, 6 RE) observed
significant emission in the direction of the high-latitude magnetosheath.
Detailed analyses have revealed that the high-latitude sheath emission consists
of two parts: the stable emission at the higher-latitudes, and the
lower-latitude emission that occurs on and off. During the interval of this
event, the Polar spacecraft was located at somewhat lower latitudes than IMAGE
in similar noon meridian, and the plasma observations with the Thermal Ions
Dynamic Experiment showed that the entry of the cusp ions happens in concurrence
with the appearance of the lower-latitude LENA emission. This coincidence
strongly suggests that the cusp ions flowing earthward charge-exchange with the
hydrogen exosphere. For the higher-latitude emission, its stability suggests
that the source is associated with the structure persistently existing, which is
consistent with the recent result showing that the sheath flow in the cusp
indentation can create neutral atom emissions. Comparison between the LENA
emission and ACE solar wind suggests that the lower-latitude LENA emission
occurs during the southward tilting of dawnward IMF, indicating that this
emission is associated with the earthward ion flux along the newly reconnected
field lines. Hence, this unique event for the simultaneous observations strongly
suggests that LENA monitors the entry of the ions in the cusp, which is
triggered by the southward tilting of IMF, and that the significant flux of the
cusp ion entry occurs equatorward of, and separately from the cusp indentation.   

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Journal of Geophysical Research, in press, 2005