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Modeling composite and fluffy grains: The effects of porosityRecent studies of interplanetary and interstellar dust provide evidence that cosmic dust grains are fluffy, composite objects, highlighting the need for models of the electromagnetic scattering by these grains. Effective medium theory (EMT) with Mie-type series solutions has been used to explore the effects of porosity which would be important in composite dust particles. While this indirect approach is both flexible and computationally efficient, it is not necessarily a good approximation. The need for EMT and its rather restrictive assumptions may be circumvented through a direct computation of the scattering properties via finite element methods, such as the discrete dipole approximation (DDA). Recently, the utility of the DDA method has been advanced significantly through improvements in theory, in numerical algorithms, and in computer hardware. Extensive calculations with the DDA method are used here to examine more directly the effects of porosity. A particular emphasis is placed upon developing a valid methodology. For both solid and porous targets we establish both numerical and physical convergence properties over the range of size parameter that is required for our study. DDA cross sections for grains with a range of porosity are compared to those computed by the EMT/series expansion technique to examine the applicability of several mixing rules, including two extensions of the Bruggeman rule. We show that for particles with Rayleigh vacuum inclusions, the extension proposed by Rouleau & Martin is quite successful. We also investigate the effects of larger, non-Rayleigh vacuum inclusions for various levels of porosity and find that they can be significant.
Document ID
19950033886
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Wolff, Michael J.
(Univ. of Wisconsin, Madison, WI United States)
Clayton, Geoffrey C.
(Univ. of Colorado, Boulder, CO United States)
Martin, P. G.
(California Inst. of Tech., Pasadena, CA United States)
Schulte-Ladbeck, Regina E.
(Univ. of Pittsburgh, Pittsburgh, PA United States)
Date Acquired
August 16, 2013
Publication Date
March 1, 1994
Publication Information
Publication: The Astrophysical Journal, Part 1
Volume: 423
Issue: 1
ISSN: 0004-637X
Subject Category
Astrophysics
Accession Number
95A65485
Funding Number(s)
CONTRACT_GRANT: NAS5-2677
CONTRACT_GRANT: NAGW-2338
Distribution Limits
Public
Copyright
Other

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