宇宙科学特別研究 I

偏光撮像装置 “HOPS” で探る金星上層ヘイズ

2014 年 1 月 21 日
総合研究大学院大物理科学研究科宇宙科学専攻
所属研究室 佐藤毅彦研究室

学籍番号 20121151
榎本孝之

目次

1
概要Venus has very thick cloud of which optical depth is large enough to obscure the ground invisible light. Hansen and Hovenier [1974] analyzed the polarization data obtained by ground–based
observations during 1960’, based on the multiple light scattering theory including polarization eﬀect,
and proposed that the cloud particles of Venus are composed of H2 SO4 droplets whose eﬀective
radius and eﬀective variance are 1.05µm and 0.07 respectively. This fact was verified again byOrbiter Cloud Photopolarimeter (OCPP) onboard Pioneer Venus Orbiter (PVO). OCPP can map
spatially-variable polarization of reflected sunlight from Venus, and it was newly found that many
sub micrometer sized particles “hazes” were distributed mainly on polar regions. In addition, theoptical depth of the haze layer had declined exponentially during the mission period [Sato et al.,1996]. Similar trend of the SO2 abundance was also reported from PVO [Esposito, 1985]. Thevariation of the optical depth of the hazes has not been studied since PVO, so it is not clear that
there is a similar correlation between them.
To monitor the distribution of the Venusian upper hazes, we developed an imaging-polarimetry
system “HOPS” (Hida Optical Polarimetry System) and performed observations by attaching it to
the 65cm refracting telescope at Hida Observatory of Kyoto University. As HOPS provides pixelby-pixel polarization map, polarization in an arbitrary area can later be obtained just by summingup the corresponding pixels for comparison with previous measurements. This is the biggest advantage of imaging polarimetry against the aperture measurements. Additionally, there were nomonitoring research through the ground based observations since PVO, so our study provides thelatest information about microphysical properties of hazes continuously.
The observations were carried out at solar phase angles around 39◦ (Jul., 2013), 56◦ (Aug., 2013),58◦ (Oct., 2012), 85◦ (Aug., 2012) and 129◦ (May, 2012) at 4 selective wave lengths 438nm, 546nm,650nm and 930nm; 546nm and 930nm data can be compared with PVO data. HOPS is a “twobeam type” polarimetry instrument which enables high accurate measurements against variable
atmospheric conditions. The eﬀect of variable atmospheric transparency, non-uniformity of sensitivities over the CCD pixels and diﬀerent throughputs of two beams can be corrected througharithmetic operations in image processing. However, the time variable image blurring due to atmospheric turbulence remains source of errors. The estimated errors using artificial polarization maps,
whose polarization degrees are 5% at polar regions, are around ±1% (relative error: ±20%). The
position angle of the fast axis of a half–wave retarder in HOPS were determined by observing standard polarized and unpolarized stars. The polarization data of actual observations were correctedby considering the orientation of the Sun–Venus plane and instrument rotator, and this angle.
We compared the data with PVO data at wavelength 930 and 546nm directly. The clear diﬀerences
appear at polar regions in 930nm. The neutral point of our data is found to be at around 75◦
while the point of PVO is around 40◦ . This diﬀerence may indicate the diﬀerent situation of thedistribution of hazes. In addition, several days of data taken in a single observing run (the solar
phase angle can be considered the same) indicate possible oscillation of polarization. Previously,
Nagata et al. [1984] reported oscillation of polarization, in infrared, with a period of 4.5 days. The
cause may be either changes of aerosol properties or changes of low-latitude boundary of the haze.
Because the date at phase angle 129 exhibits a large north-south asymmetry (the northern polar
haze appears much more confined near the pole than the south), such oscillating polarization may
possibly be a manifestation of longitudinal variation of the haze boundaries.
We developed a radiative transfer calculation code using Adding–Doubling model [de Haan et al.,1987, Hovenier et al., 2004] to analyze the polarization data. This calculation code introduces“Super Matrix” for integrating several matrix products for the purpose of good eﬃciencies of thenumerical integration processes. The accuracy of the code has been verified for model atmospheres
including particles whose scattering properties are isotropic [van de Hulst, 1980], Henyey–Greenstein目次

2functions [van de Hulst, 1980], Rayleigh scattering and Mie scattering [de Haan et al., 1987]. The
accuracy is perfect for non–conservative isotropic scattering atmosphere and conservative and non–conservative Henyey–Greenstein functions. However, for conservative isotropic scattering semi infinite atmospheres the relative error of the calculations are around 0.02%. Additionally, the errorsof the calculation is less good (max ∼ 3%) for the atmosphere including Mie scattering particles.
These errors must be reduced in the future, but the current accuracy is good enough comparedwith the errors caused by atmospheric turbulence during observations.
The comparisons of the theoretical polarization degrees with observation data are performed by
calculating the averages of sum of squared residuals (SSR) for three wavelengths 930nm, 650nm and546nm. We treated cloud particles eﬀective radius reﬀ,c , eﬀective variance veﬀ,c , refractive index
nr and haze particle eﬀective variance veﬀ,h as fixed parameters, and haze particle eﬀective radiusreﬀ,h and optical depth τh as free parameters, respectively. The resultant parameters for northern
polar region are reﬀ,h = 0.22µm, τh = 0.09 and for southern polar region reﬀ,h = 0.20µm, τh = 0.05
at 930nm. The optical depth is smaller compared with the initial observations of PVO τh = 0.25
but comparable with those observed during the declininig phase. Such declination of the abundanceof SO2 is also observed by Venus Express orbiter [Marcq et al., 2012], so our results are consistentwith the report of the correlation with it.