Physics
Scientific paper
May 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agusmsh24a..03g&link_type=abstract
American Geophysical Union, Spring Meeting 2009, abstract #SH24A-03
Physics
2124 Heliopause And Solar Wind Termination, 2126 Heliosphere/Interstellar Medium Interactions, 2151 Neutral Particles (7837), 2194 Instruments And Techniques
Scientific paper
A sun-pointed Earth-orbiting spinning spacecraft offers a convenient cost-efficient platform for global imaging the heliosphere in fluxes of energetic neutral atoms (ENAs). A single-pixel ENA instrument pointed normally to the spin axis images a full swath in the sky perpendicular to the ecliptic plane during each spacecraft rotation. As the Earth revolves around the Sun, the spacecraft spin axis is precessed to maintain its sun-pointed direction. Consequently, the instrument achieves the full 4π coverage of the sky in six months. The recently launched NASA's Interstellar Boundary Explorer (IBEX) mission implements such an observational approach to remotely probe the solar wind and pickup proton populations beyond the termination shock in the heliospheric sheath. The opposing forces of the sun's gravity and solar radiation pressure determine the energy change and survival probabilities of heliospheric ENAs along their trajectory to an observation point near 1 AU. ENA imaging simulations usually consider energetic neutral atoms approaching the Sun on radial trajectories. The observation geometry that is perpendicular to the sun-pointed spinning axis is unique because, for atomic hydrogen, the instrument detects H ENAs at their distance of the closest approach to the Sun. The images obtained under such observational conditions would differ markedly from those obtained for radial ENA trajectories. In addition, the 30-km/sec velocity of the observational platform with respect to the sun leads to velocity aberration effects and energy shifts that increase with the decreasing ENA energies. These effects of non-radial observational geometry are predictable. We present here a theoretical and modeling approach to account for specifics of ENA global imaging from a sun-pointed spinning spacecraft. We focus on the energy range of one ENA sensor on IBEX, IBEX-Hi, from 200-6000 eV. Quantitative analysis shows that the image-forming aberration effects are typically confined within the instrument imaging pixel (6 deg) while the combined modifications of ENA energies due to spacecraft motion and inward transport from the heliospheric sheath are generally small and easily corrected, even at the lowest energies measured by IBEX-Hi.
Funsten Herb
Gruntman Mike
Roelof Edmond
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