Velocity of the Solar Wind as Determined from Interplanetary Scintillations.

The "method of smooth perturbations" is used to derive the equations for interplanetary scintillation in a (statistically) spherically symmetric solar wind with constant wind velocity. The expressions, valid in the limit of small scintillation index, are used to discuss the relation between the radial wind velocity and the observed projected velocity of the scintillation pattern. It is found that the pattern velocity is systematically less than the wind velocity by a substantial fraction, depending on the radial variation of the density fluctuations. If the fluctuations n fall off as 1/r3 the correction is t8 percent, whereas if 1/r the correction is 57 percent. The effects may be even more severe if the solar wind is not spherically symmetric. Application to various observations is discussed, and it is concluded that the case 1/r2 accounts well for discrepancies between direct and radio determinations of the solar-wind velocity.