Parviainen P, Bohren C F, Mäkelä V
Appl Opt. 1994 Jul 20;33(21):4548-51. doi: 10.1364/AO.33.004548.
Near-forward scattering by cloud droplets is known to give rise to colored rings, centered on the Sun or the Moon, which are called the corona. Because of the spherical shape of the droplets, the corona can be circular. A Finnish amateur astronomers' network has found a corona, in a seemingly cloudless sky, with a regular vertically elliptical shape. The aspect ratio of these ellipses changes with the altitude of the Sun or the Moon. Some brightening in the coronas has also been reported. Because of observations of high pollen concentrations at the time of occurrence of these coronas, we propose that some coronas can be explained as a result of scatteringbybirch pollen grains, which are more or less spheroidal. To explain other observed coronas, pollen grains with more complicated shapes, such as pine and spruce pollen grains, must be invoked. Our analysis is limited to spheroidal grains, for which the Fraunhofer theory gives analytical expressions of simple form. The more complicated shapes require numerical simulations or laboratory experiments, which we have not done.
众所周知，云滴的近前向散射会产生以太阳或月亮为中心的彩色光环，即日冕。由于液滴呈球形，日冕可以是圆形的。芬兰一个业余天文学家网络在看似无云的天空中发现了一个日冕，其形状呈规则的垂直椭圆形。这些椭圆的纵横比随太阳或月亮的高度而变化。也有报道称日冕中有一些亮度增强现象。由于在这些日冕出现时观测到高花粉浓度，我们认为一些日冕可以解释为桦树花粉粒散射的结果，桦树花粉粒或多或少呈球形。为了解释其他观测到的日冕，必须考虑形状更复杂的花粉粒，如松树和云杉花粉粒。我们的分析仅限于球形颗粒，对于球形颗粒，夫琅禾费理论给出了简单形式的解析表达式。形状更复杂的颗粒需要进行数值模拟或实验室实验，而我们尚未进行此类实验。
