Optical studies of noctilucent clouds in the extreme ultraviolet

摘要

In order to better understand noctilucent clouds (NLC) and their sensitivityto the variable environment of the polar mesosphere, more needs to belearned about the actual cloud particle population. Optical measurements aretoday the only means of obtaining information about the size of mesosphericice particles. In order to efficiently access particle sizes, scatteringexperiments need to be performed in the Mie scattering regime, thusrequiring wavelengths of the order of the particle size. Previous studies ofNLC have been performed at wavelengths down to 355 nm from the ground anddown to about 200 nm from rockets and satellites. However, from thesemeasurements it is not possible to access the smaller particles in themesospheric ice population. This current lack of knowledge is a majorlimitation when studying important questions about the nucleation and growthprocesses governing NLC and related particle phenomena in the mesosphere. Weshow that NLC measurements in the extreme ultraviolet, in particular usingsolar Lyman-α radiation at 121.57 nm, are an efficient way tofurther promote our understanding of NLC particle size distributions. Thisapplies both to global measurements from satellites and to detailed in situstudies from sounding rockets. Here, we present examples from recentrocket-borne studies that demonstrate how ambiguities in the size retrievalat longer wavelengths can be removed by invoking Lyman-α. We discussbasic requirements and instrument concepts for future rocket-borne NLCmissions. In order for Lyman-α radiation to reach NLC altitudes,high solar elevation and, hence, daytime conditions are needed. Consideringthe effects of Lyman-α on NLC in general, we argue that thetraditional focus of rocket-borne NLC missions on twilight conditions haslimited our ability to study the full complexity of the summer mesopauseenvironment.