Thermal Radiation from Interplanetary Dust.

A review of the classical radiometric eclipse observations along with the more modern coronal reddening measurements is presented. In a re-examination of the possible role of thermal emission by interplanetary dust in the interpretation of these measurements, a model for the emission of thermal radiation by the interplanetary dust cloud is introduced. A column integral is set up involving the usual exponential form of the distributions describing the dust cloud and a temperature distribution derived from a gray- body approximation. The integral is evaluated using the set of parameter values believed to best describe the dust. The results obtained from this dust-emission model are presented graphically and compared with the scattered corona. Thermal intensities which exceed the scattered intensities are predicted at wavelengths in the near infrared region. Experiments intended to isolate this thermal-emission component and to permit the location of the edge of the dust-free zone surrounding the sun are suggested. The model successfully explains Blackwell's infrared coronal reddening measurement which is well accounted for by the combined effect of diffraction reddening and thermal emission, but poorly explained by diffraction alone. Interpretation of the radiometric measurements, which indicated contradictory infrared excesses, is not significantly helped by the thermal-emission model.