Solar Cycle Variations of the F Corona Brightness Resulting from the Interaction of Dust Grains with CMEs

The density of interplanetary dust increases sunward to reach its maximum in the F corona, where its scattered white-light intensity dominates that of the electron K corona above about 4 Rs. We consider the effects of interactions between the dust and the particles and fields of coronal mass ejections (CMEs). The dominant forces, with and without CMEs, acting on the dust close to the Sun are calculated for dust grain radii ranging from 0.01 to 100 microns. Dust grain orbits are then computed to compare the drift rates from assumed grain injections at 5 Rs to lower orbits for periods of minimum and maximum solar activity, where a simple CME model is adopted to distinguish the two periods. The CMEs result in significantly shorter drift times of the large (> 3 microns) dust grains, hence faster depletion rates and lower dust grain densities, at solar maxima. This would explain a relatively strong (> 30%) solar cycle variation of the near infrared brightness close to the dust plane of symmetry. While trapping the smallest of the grains, the CMEs also help scatter in latitude the grains of intermediate size (0.1 to 3 microns). The consequences for the optical brightness should be a time variation correlated to the solar cycle, not to exceed 10% at high latitude with a better isotropy reached at solar maxima. Limits on the dust size spectra are set from the basic features of the optical and infrared brightness distributions and variations.