Physics
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmsa33a..02d&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #SA33A-02
Physics
0340 Middle Atmosphere: Composition And Chemistry, 1650 Solar Variability (7537), 7536 Solar Activity Cycle (2162)
Scientific paper
Ground-based noctilucent cloud (NLC) observations have been analyzed for the existence of a long-term relationship with solar activity for many years. Physical processes such as upper atmospheric heating by solar EUV radiation or photodissociation of water vapor suggest that an anti-correlation between solar activity and NLC occurrence should be observed. However, different studies found inconsistent results depending on the data set and time period considered. The availability of satellite polar mesospheric cloud (PMC) data sets beginning in the late 1970s improved our ability to address this question with uniform geographic coverage, data from both Northern and Southern Hemispheres, and quantitative brightness information. The merged PMC data sets from the SBUV and SBUV/2 instruments now cover more than two solar cycles, and show a clear solar cycle anti- correlation between solar Lyman alpha flux and both PMC occurrence frequency and brightness. The solar cycle response in occurrence frequency is significantly larger than the corresponding secular change over an 11-year period, whereas these two effects are more comparable in PMC brightness. Variations in amplitude between different latitudes and hemispheres are observed in the SBUV data. Consistent results have also been obtained from long single instrument PMC data sets acquired by SAGE II and UARS HALOE. The best statistical fit between solar activity and PMC behavior typically requires a phase lag between 0-1 years. Modeling of the PMC response to solar activity has evolved with time. Initial predictions suggested that there would be direct photochemical control through water vapor photodissociation, but later work shows that freeze-drying and phase changes of mesospheric H2O must also be considered. Recent analysis of PMC data from the SCIAMACHY and SBUV/2 instruments also demonstrates the presence of a 27-day solar rotational modulation signal in PMC occurrence frequency. Coordinated observations from the suite of instruments on the AIM spacecraft during the rising phase of solar cycle 24 will provide new insight into the connection between solar forcing and PMC variability.
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