Umov effect for single-scattering agglomerate particles. E. Zubko , 1,2 G. Videen, 3 Yu. Shkuratov, 2 K. Muinonen, 1,4 and T. Yamamoto 5. 1 Department of Physics, University of Helsinki, Finland 2 Institute of Astronomy, Kharkov National University, Ukraine
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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
Dependence of polarization in comets on phase angle
Circumstances of polarimetric observations
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)
Shkuratov & Opanasenko, Icarus 99, 468-484 (1992)
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:
Here, S11(0) is the Mueller matrix element at back-scattering, k – wavenumber, and G – the geometric cross-section of the particle.
Method: Discrete Dipole Approximation (DDA)
Gains:(1) arbitrary shape and internal structure (2) simplicity in preparation of sample particles
ρ = 0.169
ρ = 0.236
ρ = 0.336
We study 21 (!) variousrefractive indices m:
1.4+0i1.4+0.0175i 1.4+0.02i 1.4+0.05i 1.4+0.1i
1.5+0i1.5+0.02i 1.5+0.05i 1.5+0.1i
1.6+0.0005i 1.6+0.02i 1.6+0.05i 1.6+0.1i 1.6+0.15i
Size parameter x=2r/ (r– radius of circumscribing sphere and – wavelength) is variedfrom 1 throughout 26 – 40 (depending on m).
(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 r–a. The power index a is varied from 1 to 4.
Note: this range is well consistent with in situ study of Comet 1P/Halley: 1.5a3.4 (Mazets et al., 1986)
McBride et al., MNRAS 289, 535-553 (1997)
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 A=0.034–0.079, that is well consistent with the expected value of A=0.05.
For the innermost coma studied by Giotto in 26P/Grigg-Skjellerup, the Umov effect reveals dramatically higher geometric albedo A=0.23.