Low-energy neutral atoms observed near the Earth

Abstract

During the summer of 2000 the LENA instrument on the IMAGE spacecraft collected a unique data set of low-energy, neutral atom emissions. The data suggests an ever-present flux of low energy (<50 eV) oxygen neutrals in the near-Earth environment, typically below 4000 km altitude. Because of two different patterns in the images made during a number of spacecraft perigee passes we infer at least two main sources for these particles. One (possibly from the auroral zone) produces a high degree of variability from one image to the next and creates more energetic particles, and another (possibly part of the hot oxygen geocorona) gives less variability within a sequence of images and lower energy particles. Overall, the measured neutral fluxes increase with increasing magnetic activity. During Southern Hemisphere winter the total neutral flux rates vary diurnally with peak intensity near 0740 UT and a minimum near 1940 UT. Near perigee, neutral fluxes are often seen coming from a broad range of directions so that a localized source, such as the auroral zone, alone cannot produce it. Processes affecting low-energy oxygen neutrals, such as gravitational deflection or spacecraft ram deflection, cannot broaden a localized source enough to match the data. Copyright 2003 bt the American Geophysical Union.