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1. Introduction Noctilucent clouds (NLC), also known as polar mesospheric clouds (PMC), are atmospheric phenomena usually occurring poleward of 50° during the summer season. The extremely low temperatures (~130 K) and enhanced water vapor content (~3 ppm) near the summer mesopause lead to large saturation ratios (~100) and consequently to the formation of water ice aerosols [Hervig et al., 2001]. It is still not clear what preexisting cores act as nuclei for the formation of NLC [Rapp and Thomas, 2006; Gumbel and Megner, 2009], if indeed heterogenous nucleation is required [Zasetsky et al., 2009]. The NLC particles usually grow up to typical sizes of several tens of nanometers [Gumbel et al., 2001; Karlsson and Rapp, 2006; Robert et al., 2009] by direct deposition of water vapor on their surface. An ensemble of such particles forms a layer of roughly 1 km in vertical extent at altitudes of about 83 km [Fiedler et al., 2003] and has an ice particle density on the order of 102 cm−3 at the cloud peak brightness [Baumgarten et al., 2007]. NLC particles are transported by winds and settle under the action of gravity, and eventually sublimate when reaching subsaturated regions. A typical NLC season lasts from beginning of June until the end of August in the Northern Hemisphere and from beginning of December until the end of February in the Southern Hemisphere [Thomas and Olivero, 1989]. NLC are considered to be indicators of the state of the mesosphere [Thomas and Olivero, 2001], and observing their temporal and spatial variation can tell us more about phenomena taking place in this remote region of the Earth's atmosphere. NLC are affected by many different atmospheric processes such as gravity waves [e.g., Gerrard et al., 2004; Chandran et al., 2009], planetary waves [e.g., Merkel et al., 2003; von Savigny et al., 2007; Merkel et al., 2008] and interhemispheric coupling [Karlsson et al., 2007, 2009]. The SBUV instruments on board various NOAA satellites have been an invaluable source of information, providing NLC properties since 1979. As a result of this lengthy data record, it was possible to assess the impact of the 11 year solar cycle on NLC activity, showing significant anticorrelation between the occurrence frequency and the Lyman-α irradiance in both hemispheres [DeLand et al., 2003], with a stronger anticorrelation in the Northern Hemisphere. Hervig and Siskind  confirmed these findings using the HALOE instrument data set and showed the variation in NLC properties to be a consequence of temperature and water vapor changes in the upper mesosphere. Moreover, once the effect of the solar activity is removed from the SBUV time series, a positive secular trend of up to 20% was observed over the last 27 years in both NLC occurrence and albedo [Shettle et al., 2009; DeLand et al., 2007]. It has been argued that this long-term change in NLC properties could be caused by an enhanced radiative cooling of the upper atmosphere due to a rise in greenhouse gas concentration as well as an increase in mesospheric water vapor concentrations [Olivero and Thomas, 2001; Grygalashvyly and Sonnemann, 2006], although most measurements do not support the hypothesis of a significant long-term temperature decrease near the mesopause [Lübken, 2000; Beig et al., 2003]. While the effect of the 11 year solar cycle on NLC is fairly well established, there is no peer-reviewed publication on the consequences of the quasi 27 day variation of the solar irradiance on NLC properties. This short-term UV flux variability, the result of the Sun's differential rotation, has a mean amplitude at 121.6 nm which corresponds to about 25% that of the 11 year solar cycle [Woods et al., 2000], and varies considerably with solar activity itself so that it is larger during solar maximum and vice versa. Because the magnitude of this signal is not negligible compared to that of the 11 year solar cycle, it could conceivably affect the state of the upper mesosphere and consequently, the formation of NLC. The difficulty of the task lies in the detection of this signal during the short NLC season. If one conserves only the season's core containing approximately 90% of the total NLC detections, about 70 days are left to detect a 27 day signal. Moreover, as described before, many other processes with no direct link to the solar irradiance impact NLC, and so the search for a connection between NLC and solar activity will invariably be affected by these. The proxy for NLC activity must therefore be chosen with care so as to reflect best the effect that the solar radiation could have on NLC and maximize the population sampled. Among the many alternatives of possible NLC properties, the daily occurrence frequency, averaged zonally and over a latitude range of 60°–80° was chosen as a good indicator of NLC activity. It has the advantage of being simple, easily retrieved with few assumptions made and available from both SCIAMACHY and SBUV. The NLC albedo, used in many SBUV studies [DeLand et al., 2007] could be employed as well, but because it is strongly correlated to the occurrence frequency for SBUV data, the conclusions drawn from a cross-correlation analysis based on either parameter should be similar. Moreover, SCIAMACHY albedo computation would require making assumptions on the particle size distribution of the NLC and would be quite sensitive to these assumptions because of the limb-viewing geometry. Other possible proxies of NLC activity would be the brightness peak altitude, the particle size and ice water content, which have the advantage of being real physical properties of NLC but usually require more assumptions to be made or cannot be measured accurately by either SCIAMACHY or SBUV. In this work, we investigate the effect of the 27 day solar flux variation on NLC occurrence frequency in both hemispheres using NLC data sets from the SCIAMACHY and SBUV instruments. Cross-correlation plots of solar Lyman-α irradiance and NLC occurrence frequency anomalies are presented for years 2002–2009 for SCIAMACHY and 1979–2006 for SBUV. Results obtained through a superposed epoch analysis of the solar forcing on NLC occurrence frequency are also shown in order to substantiate the relationship between these geophysical parameters. In connection with this analysis, we also examine MLS Aura mesospheric temperature and H2O volume mixing ratios for the summer seasons 2005–2007.