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| Titel |
Circular-polarization ratios for aggregates of spherical particles |
| VerfasserIn |
A. Virkki, K. Muinonen, A. Penttilä |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250070002
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| Zusammenfassung |
| A basic strategy for observing a small solar-system object using radar is to measure the
distribution of echo power in time delay and Doppler frequency for a circularly polarized
transmitted wave, in the same and opposite senses of circular polarization. The measurement
can be repeated for differing orientations and plane-of-sky directions of the object. The
circular-polarization ratio μ is the ratio of the echo power in the same circular-polarization
state (SC) to that in the opposite circular-polarization state (OC). The ratio μ is often the
most important physical observable with the radar technique, as it provides the best
indications for wavelength-scale complexity of the surface. At the typical transmitter
frequencies of 2380Â MHz or 8495Â MHz, the wavelengths are 12.6 cm or 3.5 cm,
respectively.
We model electromagnetic scattering from closely-packed random aggregates of spheres
imitating the structure of an asteroid’s regolith. Both scattering and absorption of the
electromagnetic wave are treated. The Multiple-Sphere T -Matrix Method computer software
(MSTM; D. W. Mackowski and M. I. Mishchenko, JQSRT 112, 1282, 2011) is utilized to
study how different parameters affect the circular-polarization ratio, e.g., the size distribution
and electric permittivities of the spherical particles forming the different aggregates. Our
primary goal is to see if the computed circular-polarization ratios can be linked to the
observational data of asteroids detected with radar.
The results of the simulations show striking structure for the circular-polarization ratio as
a function of the size parameter and the electric permittivity of the medium. Also differences
between aggregates of monodisperse and polydisperse spheres clearly exist: the aggregates
consisting of polydisperse spherical particles, and hence, showing more complex
structure and surface, result in circular-polarization ratios higher than the aggregates of
monodisperse spherical particles, probably due to the increased significance of multiple
scattering. Most importantly, the simulations show how the variations of the different
parameters affect the ratio, indicating reasons for the variations in the observed
data.
We have simulated circular-polarization ratios for aggregates of monodisperse
particles, and are currently initiating simulations for aggregates of polydisperse
spheres. |
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