Nonrandomly-Oriented Targets

Some special cases (where the target orientation distribution is uniform for rotations around the x axis = direction of propagation of the incident radiation), one may be able to use DDSCAT.5a \ with appropriate choices of input parameters. More generally, however, you will need to modify subroutine ORIENT to generate a list of NBETA values of , NTHETA values of , and NPHI values of , plus two weighting arrays WGTA(1-NTHETA,1-NPHI) and WGTB(1-NBETA). Here WGTA gives the weights which should be attached to each (,) orientation, and WGTB gives the weight to be attached to each orientation. Thus each orientation of the target is to be weighted by the factor WGTAWGTB. For the case of random orientations, DDSCAT.5a chooses values which are uniformly spaced in , and and values which are uniformly spaced, and therefore uses uniform weights WGTB=1./NBETA When NTHETA is even, DDSCAT sets WGTA=1./(NTHETANPHI) but when NTHETA is odd, DDSCAT uses Simpson's rule when integrating over and WGTA= (1/3 or 4/3 or 2/3)/(NTHETANPHI)

Note that the program structure of DDSCAT may not be ideally suited for certain highly oriented cases. If, for example, the orientation is such that for a given value only one value is possible (this situation might describe ice needles oriented with the long axis perpendicular to the vertical in the Earth's atmosphere, illuminated by the Sun at other than the zenith) then it is foolish to consider all the combinations of and which the present version of DDSCAT is set up to do. We hope to improve this in a future version of DDSCAT.