RETRIEVAL OF LOCAL INTERPLANETARY DUST EMISSIVITY BY ASTRO-F

Abstract

This is a proposal to probe local part of the interplanetary dust (IPD) cloud complex and retrieve mean volume emissivity of the local IPDs at mid-infrared wavelengths. This will be done by monitoring, with Infrared Camera (IRC) aboard the ASTRO-F, the annual modulation of the zodiacal emission. In pointing mode of the ASTRO-F mission the spacecraft can make attitude maneuvering over approximately <TEX>${\pm}1^{\circ}$</TEX> range centered at solar elongation <TEX>$90^{\circ}$</TEX> in the ecliptic plane. The attitude maneuvering combined with high sensitivity of the IRC will provide us with a unique opportunity observationally to take derivatives of the zodiacal emission brightness with respect to the solar elongation. From the resulting differential of the brightness over the <TEX>${\pm}1^{\circ}$</TEX> range, one can directly determine the mean volume emissivity of the local IPDs with a sufficient accuracy to de-modulate the annual emissivity variations due to the Earth's elliptical motion and the dis-alignment of the maximum IPD density plane with respect to the ecliptic. The non-zero eccentricity (<TEX>$e_{\oplus}$</TEX>= 0.0167) of the Earth's orbit combined with the sensitive temperature dependence of the Planck function would bring modulations of amplitude at least <TEX>$3.34\%$</TEX> to the zodiacal emission brightness at mid-infrared wavelengths, with which one may determine the IPD temperature T(r) and mean number density n(r) as functions of heliocentric distance r. This will in turn fix the power-law exponent <TEX>$\delta$</TEX> in the relation <TEX>$T(r) = T_o(r/r_o)^{-\delta}$</TEX> for the dust temperature and v in <TEX>$n(r) = n_o(r/r_o)^-v$</TEX> for the density. We discuss how one may de-couple the notorious degeneracy of cross-section, density, reference temperature <TEX>$T_o$</TEX> and exponent <TEX>$\delta$</TEX>.