Umov effect for single-scattering agglomerate particles

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Umov effect for single-scattering agglomerate
particles
E. Zubko,1,2 G. Videen,3 Yu. Shkuratov,2 K.
Muinonen,1,4 and T. Yamamoto5
1 Department of Physics, University of Helsinki,
Finland 2 Institute of Astronomy, Kharkov
National University, Ukraine 3 Army Research
Laboratory AMSRL-CI-EM, USA 4 Finnish Geodetic
Institute, Finland 5 Institute of Low Temperature
Science, Hokkaido University, Japan
May 8, 2012
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Polarimetry of Comets
Dependence of polarization in comets on phase
angle
Circumstances of polarimetric observations
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Umov Effect
The brighter powder, the lower its linear
polarization
N. Umov, Phys. Zeits. 6, 674-676 (1905)
Origin of the phenomenon depolarization due to
multiple scattering in regolith
N. Umov (1846-1915)
In 1960-1970, the qualitative law was quantified
log(Pmax) linearly depends on log(A)
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Umov Effect
Shkuratov Opanasenko, Icarus 99, 468-484 (1992)
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Umov Effect for Single-Scattering Particles
As was found in Zubko et al. (2011, Icarus, 212,
403 415), the Umov effect holds also for
single-scattering particles with size comparable
to wavelength. Therefore, it can be applied to
comets.
Geometric albedo A for single particles A(S11(0
)?)/(k2G) Here, S11(0) is the Mueller matrix
element at back-scattering, k wavenumber, and G
the geometric cross-section of the particle.
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Numerical Simulation of Light Scattering
Method Discrete Dipole Approximation (DDA)
Basic idea
Gains (1) arbitrary shape and internal
structure (2) simplicity in preparation of
sample particles
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Models for Cometary Dust Particles
sparse agglomerate agglomerated
debris pocked spheres
? 0.169 ? 0.236 ? 0.336
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Input Parameters for Simulation
We study 21 (!) various refractive indices m
1.20i 1.20.015i 1.3130i
1.3130.1i 1.40i 1.40.0175i
1.40.02i 1.40.05i 1.40.1i 1.50i
1.50.02i 1.50.05i 1.50.1i
1.60.0005i 1.60.02i 1.60.05i
1.60.1i 1.60.15i 1.70i
1.70.1i 1.8550.45i
Size parameter x2?r/? (r radius of
circumscribing sphere and ? wavelength) is
varied from 1 throughout 26 40 (depending on
m).
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Averaging of light-scattering characteristics
(1) Over particle shapes For each pair of x and
m, we consider minimum 500 particle shapes. (2)
Over particle size Size distribution is
considered to be a power law ra. The power index
a is varied from 1 to 4. Note this range is
well consistent with in situ study of Comet
1P/Halley 1.5?a?3.4 (Mazets et al., 1986)
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Application to whole Comet C/1996 B2 (Hyakutake)

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Application to whole Comet C/1996 B2 (Hyakutake)

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Application to whole Comet C/1996 B2 (Hyakutake)

m a A m a A
1.20i 1.50.05i 2.2 0.036
1.20.015i 1.50.1i
1.3130i 2.2 0.063 1.60.0005i 3.4 0.079
1.3130.1i 1.60.02i 3.1 0.067
1.40i 2.9 0.066 1.60.05i 2.6 0.048
1.40.0175i 2.4 0.046 1.60.1i
1.40.02i 2.3 0.044 1.60.15i
1.40.05i 1.0 0.021 1.70i 3.6 0.081
1.40.1i 1.70.1i 1.8 0.034
1.50i 3.2 0.070 1.8550.45i
1.50.02i 2.9 0.054 Whole comets 0.050
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Application to innermost coma in
26P/Grigg-Skjellerup
McBride et al., MNRAS 289, 535-553 (1997)
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Application to innermost coma in
26P/Grigg-Skjellerup
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Application to innermost coma in
26P/Grigg-Skjellerup
m a A m a A
1.20i 1.50.05i
1.20.015i 1.50.1i
1.3130i 1.60.0005i 2.1 0.224
1.3130.1i 1.60.02i 1.2 0.114
1.40i 1.60.05i
1.40.0175i 1.60.1i
1.40.02i 1.60.15i
1.40.05i 1.70i 2.4 0.238
1.40.1i 1.70.1i
1.50i 1.1 0.216 1.8550.45i
1.50.02i Inner coma 0.231
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Summary
Using the Umov effect, one can estimate albedo of
single-scattering dust particles. When this
technique is applied to whole Comet C/1996 B2
(Hyakutake), it yields the geometric albedo in
the range A0.0340.079, that is well consistent
with the expected value of A0.05. For the
innermost coma studied by Giotto in
26P/Grigg-Skjellerup, the Umov effect reveals
dramatically higher geometric albedo A0.23.