RETRIEVAL OF LOCAL INTERPLANETARY DUST EMISSIVITY BY ASTRO-F

초록

This is a proposal to probe local part of the interplanetary dust(IPD) cloud complex and retrievemean 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. Inpointing mode of the ASTRO-F mission the spacecraft can make attitude maneuvering over approx-imately 1o range centered at solar elongation 90' in the ecliptic plane. The attitude maneuveringcombined with high sensitivity of the IRC will provide us with a unique opportunity observationallyto take derivatives of the zodiacal emission brightness with respect to the solar elongation. From theresulting dierential of the brightness over the ±1' range, one can directly determine the mean volumeemissivity of the local IPDs with a sucient accuracy to de-modulate the annual emissivity variationsdue to the Earth's elliptical motion and the dis-alignment of the maximum IPD density plane withrespect to the ecliptic. The non-zero eccentricity (e = 0.0167) of the Earth's orbit combined with thesensitive temperature dependence of the Planck function would bring modulations of amplitude at least3.34% to the zodiacal emission brightness at mid-infrared wavelengths, with which one may determinethe IPD temperatureT(r) and mean number density n(r) as functions of heliocentric distancer. Thiswill in turn x the power-law exponent in the relationT(r) = To(r=ro)for the dust temperatureand in n(r) = no(r=ro)for the density. We discuss how one may de-couple the notorious degeneracyof cross-section, density, reference temperatureTo and exponent .

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