Seasonal variation of solar wind ENA as a measure of interstellar gas
in the inner solar system

M.R. Collier (1), T.E. Moore (1), K.W. Ogilvie (1), D. Chornay (1), 
J. Keller (1), S.A. Fuselier (2), J.M. Quinn (3), P. Wurz (4), 
M. Wuest (5), K.C. Hsieh (6) 

1 NASA/Goddard Space Flight Center
2 Lockheed Martin Advanced Technology Center
3 University of New Hampshire
4 University of Berne
5 Southwest Research Institute
6 University of Arizona

The Low Energy Neutral Atom (LENA) imager, launched on the Imager for
Magnetopause to Aurora Global Exploration (IMAGE) spacecraft in March
of  2000, detects neutral atoms with energies as low as 10~eV up to the
range of solar wind above 1~keV. Because LENA has low sensitivity to
light and it looks directly at the Sun every spin when the Sun is in
the field of view, it has observed a neutral component of the solar
wind that results when solar wind ions charge exchange with
interstellar neutrals, dust and the Earth's geocorona.

In this study, we examine long term variations, over a period of about
a year, in the intensity of the Sun pulse. Initial results suggest the
count rate reached a maximum between June and July of the year 2000
with a long, low count-rate period stretching from mid-November through
at least early March. The peak may be due to the solar wind interaction
with interstellar neutrals.

Charge exchange between the solar wind and the interstellar neutrals
(creating a neutral solar wind beam) is maximum in June when the Earth
is upstream of the Sun in the interstellar flow and is minimum when the
Earth is downstream of the Sun in the interstellar flow. Other
possibilities will also be examined.