INFLUENCE OF DUST SHAPE AND MATERIAL COMPOSITION ON THE SOLAR F-CORONA

We investigate the influences of dust shape and material composition o n the brightness and reddening of the solar F-corona. Silicate and car bon are considered as the F-coronal dust material to represent low and high absorbing materials, respectively. The observational data of the visible and near-infrared F-coronal brightness are well reproduced by our model of porous grains. We find that a change in the amount of hi ghly absorbing particles near the Sun can cause a variation in the rad ial slope of the near-infrared F-coronal brightness, as well as of the coronal reddening. Hence a temporal variation of coronal features may be connected to a variation of the dust cloud composition rather than to the variation of a solar dust ring, which may or may not exist aro und the Sun. Further analysis of the influence of the dust size distri bution on the F-coronal reddening shows that a drastic change of the s ize distribution is unlikely. (C) 1998 Elsevier Science Ltd. All right s reserved.