The orientation parameter of dust grains in three special cases

The orientation parameter F = < cos2 v — 1/3 > has been obtained by solving a Fokker‐Planck equation taking into account a two parameter model of collision and Jones and Spitzer's correction to Davis and Greenstein's interpretation of grain alignment by magnetic relaxation. Three cases have been considered: 1 Small values of the ratio δ of the collision relaxation time to the magnetic relaxation time, any elongation and magnetic anisotropy. 2 Nearly spherical grains, with small magnetic anisotropy, any value of δ. 3 Oblate spheroids (or flat cylinders) with very high magnetic anisotropy, any value of δ. The results found in the first case show that the parameter F is very sensitive to the model of collision, particularly for oblate spheroids. The effects of increasing field and grain temperature are discussed briefly on the basis of the results for nearly spherical grains. Unexpected negative values of the orientation parameter for small and moderate δ obtained in the third case exclude the possibility of alignment by diamagnetic relaxation in graphite.