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Ann. Geophys., 26, 1109-1119, 2008
www.ann-geophys.net/26/1109/2008/
© European Geosciences Union 2008
Optical studies of noctilucent clouds in the extreme ultraviolet
J. Hedin1, J. Gumbel1, M. Khaplanov1, G. Witt1,2, and J. Stegman1
1Department of Meteorology Stockholm University, 10691 Stockholm, Sweden
2Department of Earth Sciences, Hebrew University, Jerusalem, Israel
Abstract. In order to better understand noctilucent clouds (NLC) and their sensitivity
to the variable environment of the polar mesosphere, more needs to be
learned about the actual cloud particle population. Optical measurements are
today the only means of obtaining information about the size of mesospheric
ice particles. In order to efficiently access particle sizes, scattering
experiments need to be performed in the Mie scattering regime, thus
requiring wavelengths of the order of the particle size. Previous studies of
NLC have been performed at wavelengths down to 355 nm from the ground and
down to about 200 nm from rockets and satellites. However, from these
measurements it is not possible to access the smaller particles in the
mesospheric ice population. This current lack of knowledge is a major
limitation when studying important questions about the nucleation and growth
processes governing NLC and related particle phenomena in the mesosphere. We
show that NLC measurements in the extreme ultraviolet, in particular using
solar Lyman-α radiation at 121.57 nm, are an efficient way to
further promote our understanding of NLC particle size distributions. This
applies both to global measurements from satellites and to detailed in situ
studies from sounding rockets. Here, we present examples from recent
rocket-borne studies that demonstrate how ambiguities in the size retrieval
at longer wavelengths can be removed by invoking Lyman-α. We discuss
basic requirements and instrument concepts for future rocket-borne NLC
missions. In order for Lyman-α radiation to reach NLC altitudes,
high solar elevation and, hence, daytime conditions are needed. Considering
the effects of Lyman-α on NLC in general, we argue that the
traditional focus of rocket-borne NLC missions on twilight conditions has
limited our ability to study the full complexity of the summer mesopause
environment.
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