Inner Heliospheric Gas and Dust from Solar Wind Charge Exchange 

Collier, M.R., Moore, T.E., Ogilvie, K.W., Simpson, D., Fok, M., Chornay, D., 
Keller, J., Fuselier, S., Quinn, J., Wurz, P., Wuest, M., Hsieh, J., 
Tsurutani, B. 


The Low Energy Neutral Atom (LENA) imager, launched on the IMAGE spacecraft in 
March of 2000, detects neutral atoms with energies from 10 eV up to >1 keV. 
Because LENA has low sensitivity to light and looks directly at the Sun every 
spin during six months of the year, it has observed a neutral component of the 
solar wind (NSW) that results when solar wind ions charge exchange with 
interstellar neutrals, with dust and with the Earth's geocorona [Collier et al., 
JGR, 106, 24,893, 2001]. We examine long-term variations in the intensity of the 
counting rate from the solar direction (NSW). Results from year 2001, during 
which the instrument state remained constant, show a maximum in the count rate 
between June and July with a long, low count-rate period stretching from 
mid-November through early March. This annual modulation of solar wind energetic 
neutral atom flux at the Earth is interpreted as a pronounced variation of the 
neutral gas column density between the Sun and the Earth with season. This 
modulation is evidently dominated by interstellar neutral gas and the solar 
erosion of that gas in the galactic downstream region. It also contains a 
relatively constant contribution from inner solar system dust and relatively 
smaller variations produced by solar wind fluctuations and possibly structure in 
the dust population. The LENA observations place an upper limit on the column 
density of dust at 1 AU of less than 6x10^-19 cm^-1. Implications of the LENA 
data on the interpretation of observations of low frequency electromagnetic 
waves by Tsurutani et al. [GRL, 21, 633, 1994] will also be considered. 

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Presented at the 2002 Spring A.G.U. Meeting, 
Washington, D.C., U.S.A., 28-31 May 2002