Fig. 3-20 shows the occurrence rate of PSC in the southern hemisphere for September in the years 2002 to 2008. With the exception of 2002, the PSC occurrence rate at high southern latitudes is typically quite large. It is obvious that PSC are not symmetrically distributed around the South Pole, but the distribution is characterised by a wave-1 structure with a maximum in the South Atlantic sector and a minimum in the Australian sector. The low PSC occurrence rates in September 2002 are due to the anomalous mid-winter major stratospheric warming after 22 September, which caused PSC to disappear. In most cases there is a good correspondence between the detected PSC and their formation threshold of -78° C.
3.4 The Upper Atmosphere and Solar Activity
The upper atmosphere, i.e. the mesosphere and lower part of the thermosphere (MLT region), is still a relatively poorly explored region. The MLT region forms a transition between interplanetary space and the terrestrial atmosphere, both influenced by extraterrestrial impacts – e.g. solar radiation, solar wind, meteors or cosmic dust – as well as by impacts from the lower atmosphere. Thus, there are indications that effects of global climate change in the upper atmosphere can be detected rather early. Solar-terrestrial interactions can be studied, and at the same time, the impact of anthropogenic emissions on our atmosphere at remote altitudes can be investigated.
Noctilucent Clouds – NLC
Noctilucent Clouds, also referred to as Polar Mesospheric Clouds, are a high latitude summertime mesospheric phenomenon, even observable from ground. They occur at altitudes of about 83-85 km near the polar summer mesopause and consist of H2O ice particles with radii ranging from a few nm up to about 80-100 nm. NLC received a significant amount of scientific interest in recent years, since they may be early indicators of global change. This is because they react very sensitively to small changes in ambient conditions, particularly to changes in temperature and H2O abundance. The scattering properties of the NLC particles allow mapping of these high altitude clouds. Since they scatter solar radiation efficiently, they affect the measured limb radiance profiles significantly, especially in the northern hemisphere where scattering angles at polar latitudes are particularly small for SCIAMACHY limb observations.
SCIAMACHY observations of NLC have contributed in different ways to the current research on the polar summer mesopause. The main focus of these investigations was to improve the scientific understanding of the natural variability in NLC in order to better understand the role of NLC as indicators of global change. The natural variability is partially driven by the solar input and by dynamical processes such as planetary waves.
SCIAMACHY NLC observations were used to observe a depletion of NLC for the first time during a solar proton event (SPE, von Savigny et al. 2007a). Associated with such an event in January 2005, highly energetic solar protons precipitated into the Earth’s polar cap areas. The January 2005 SPE is included in the period covered by fig. 3-21.