George H. Goedecke Paul M. Pellegrino Michael De Antonio
Dept. of Phys., NMSU, Las Cruces, NM 88003-0001
Harry J. Auvermann
Army Res. Lab., WSMR, NM 88002-5501
The first Born approximation has often been used to predict acoustical scattering by the quasistatic temperature fluctuations and velocities associated with atmospheric turbulence. For this approximation, only terms linear in these disturbances need be retained in the wave equation. Terms in this first-order wave equation involving spatial derivatives of the flow field have often been dropped. It is shown that retaining these terms yields a Born scattering amplitude equal to cos((theta)) times that obtained when they are dropped, where (theta) is the scattering angle. It is also shown that this Born scattering amplitude is identically zero in the forward direction for any solenoidal flow velocity field that goes to zero faster than r[sup -3] as r->(infinity). Analytic expressions and numerical results for the first Born differential and total cross sections are obtained for a model localized turbule. The model employs nonuniform rotation about an axis, as modulated by an axisymmetric Gaussian, plus commensurate temperature variation.