**AAS Meeting #194 - Chicago, Illinois, May/June 1999**

*Session 44. Computation and Data Analysis*

Display, Tuesday, June 1, 1999, 10:00am-7:00pm, Southwest Exhibit Hall
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## [44.09] On the Size Parameter for Thermally-Emitting Particles

*D.K. Lynch, S. Mazuk (The Aerospace Corp.)*

The conventional means of quickly determining the scattering
regime of a particle is the size parameter X = 2\pi*
a*/\lambda where *a* is the particle radius and
\lambda is the wavelength of light being scattered. As
arguments to the Bessell functions in Mie theory, however, X
is multiplied by m = N_{p}/N where N_{p} = n +ik is the
complex index of the particle and N is the index of the
surrounding medium (= 1.0 for a vacuum). The quantity mX is
complex (mX = 2\pi*a*/\lambda +i2\pik*
a*/\lambda) For thermal emission from particles, we show
that the opacity parameter \Omega = 4\pik*
a*/\lambda= 2kX is a more accurate means of distinguishing
between ``small" and ``large" particles. Here ``large"
particles emit in a black body-like manner and ``small"
particles emit with spectral structure indicative of their
optical constants. \Omega is approximately equal to the
particle's mean optical depth and is equal to two times the
imaginary part of mX. \Omega can be derived from both
scattering theory and geometrical optics.

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