从HST观测得到的土星环的对立和倾斜效应
The opposition and tilt effects of Saturn's rings from HST observations
摘要：
造成土星环的相对增亮的两个主要因素是：i）由于相干后向散射和/或表面阴影隐藏而导致的粒子的内在增亮，以及ii）减少的粒子间阴影。我们利用哈勃太空望远镜观测不同的仰角B解开这些贡献。我们假设本征贡献与B无关，因此相位曲线的任何B依赖性都是由于粒子间阴影，它对所有颜色的作用也必须类似。我们构建了一个网格的动力学/光度模拟模型，以适应海拔依赖的部分反对增亮。消除建模的粒子间分量产生的内在贡献的反对效果：对于B和A环，它几乎完全是由于相干后向散射; C环，粒子内阴影隐藏的贡献也可能存在。根据我们的模拟，粒子间阴影效应的宽度大致与B成正比。这是因为当B减小时，散射主要来自稀薄的低填充因子的上环层，而在较大的$B$中，致密的内部部分是可见的。粒子间阴影的仰角依赖性也解释了大部分B环倾斜效应（亮度随仰角增加）。通过比较不同滤光片下倾斜效应的大小，我们发现，当B -> 26 ^o时，多重散射最多可以解释10%的亮度增加，而剩下的20%的亮度增加是由于不同程度的粒子间阴影。中间A环的负倾斜效应很好地解释了相同的自重力尾流模型，占所观察到的A环的方位角亮度不对称。
英文摘要：
The two major factors contributing to the opposition brightening of Saturn's rings are i) the intrinsic brightening of particles due to coherent backscattering and/or shadow-hiding on their surfaces, and ii) the reduced interparticle shadowing. We utilize the Hubble Space Telescope observations for different elevation angles B to disentangle these contributions. We assume that the intrinsic contribution is independent of B, so that any B dependence of the phase curves is due to interparticle shadowing, which must also act similarly for all colors. We construct a grid of dynamical/photometric simulation models to fit the elevation-dependent part of opposition brightening. Eliminating the modeled interparticle component yields the intrinsic contribution to the opposition effect: for the B and A rings it is almost entirely due to coherent backscattering; for the C ring, an intraparticle shadow hiding contribution may also be present. Based on our simulations, the width of the interparticle shadowing effect is roughly proportional to B. This follows from the observation that as B decreases, the scattering is primarily from the rarefied low filling factor upper ring layers, whereas at larger $B$'s the dense inner parts are visible. The elevation angle dependence of interparticle shadowing also explains most of the B ring tilt effect (the increase of brightness with elevation). From comparison of the magnitude of the tilt effect at different filters, we show that multiple scattering can account for at most a 10% brightness increase as B -> 26^o, whereas the remaining 20% brightening is due to a variable degree of interparticle shadowing. The negative tilt effect of the middle A ring is well explained by the the same self-gravity wake models that account for the observed A ring azimuthal brightness asymmetry.
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