Secondary Emission From Clusters Composed of Spherical Grains

Dust grains or their clusters can be frequently found in many space environments-interstellar clouds, atmospheres of planets, tails of comets, or planetary rings are only typical examples. Space dust grains are formed by various processes and their shapes are complex. These grains are exposed to electrons with different energies, and thus, they can acquire positive or negative charge during this interaction. We present a systematic study of well-defined systems-clusters consisting of different numbers of small spherical (1 μm) grains and such objects can be considered as examples of real irregularly shaped space grains. The charges acquired by investigated objects as well as their secondary emission yields are calculated using the secondary emission model. We have found that: 1) the charge and surface potential of clusters exposed to the electron beam are influenced by the number of grains and by their geometry within a particular cluster; 2) the model results are in an excellent agreement with the experiment; and 3) there is a large difference between charging of a cluster levitating in the free space and that attached to a planar surface. The calculation provides a reduction of the secondary electron emission yield of the surface covered by dust clusters by a factor of 1.5 with respect to the yield of a smooth surface.