Influence of dust shape and material composition on the solar F-corona

We investigate the influences of dust shape and material composition on the brightness and reddening of the solar F-corona. Silicate and carbon 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 highly absorbing particles near the Sun can cause a variation in the radial 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 around the Sun. Further analysis of the influence of the dust size distribution on the F-coronal reddening shows that a drastic change of the size distribution is unlikely. (C) 1998 Elsevier Science Ltd. All rights reserved.