This graph shows SCIAMACHY NLC occurrence rates at different latitudes in the southern hemisphere during the 2004/2005 NLC season, together with the ionisation caused by the precipitating solar protons. At the time of the SPE, the NLC occurrence rate in both southern latitude bands decreased rapidly. A mechanism for SPE-induced NLC depletion is proposed by Becker and von Savigny (2010) using model simulations with a General Circulation Model (GCM). It suggests that a polar mesopause warming is caused by the SPE-driven catalytic ozone loss in the middle mesosphere, followed by different stages of dynamic processes, and finally leading to a reduced upwelling above the pole, i.e. reduced adiabatic cooling. The January 2005 event is not the only event where NLC depletion has been detected. Rahpoe et al. (2010) demonstrated that a depletion of NLC also occurred during some of the other strong SPE in the last three decades.
Solar proton events are rather intermittent and irregular events mainly occurring during solar maximum. They are not the only cause for NLC variability linked to solar impacts seen in SCIAMACHY observations. A 27-day solar cycle signature in NLC was identified for the first by Robert et al. (2010). Maxima in solar activity associated with the 27-day solar cycle – quantified for example using the MgII index described below (Skupin et al. 2004) – coincide with minima in the NLC occurrence frequency. Using MLS (Microwave Limb Sounder) observations of middle atmospheric temperatures, a 27-day solar cycle signature in the polar mesopause temperature was identified as the immediate cause of the apparent 27-day signature in NLC.
Another important driver for variability in NLC are planetary wave signatures. The most important of these signatures are the quasi-2-day-wave and the quasi-5-day-wave. Both of them are caused by instabilities of the summer mesosphere jet and occur intermittently during the NLC seasons in both hemispheres. SCIAMACHY NLC observations, again in combination with MLS temperature observations, showed for the first time, that the quasi 5-day-wave signatures in NLC are caused by similar wave signatures in mesopause temperatures (von Savigny et al. 2007b).
Apart from the detection and mapping of NLC, SCIAMACHY observations also permit the estimation of the NLC particle size. For wavelengths below about 310 nm, the multiple scattering and surface reflection components to the limb signal are negligible. In single scattering approximation, the spectral exponent of the NLC scattering spectrum can be related to the NLC particle size assuming for example Mie theory and the refractive index of H2O ice (von Savigny et al. 2004a). NLC particle sizes of 40-50 nm were determined from a distance of about 3300 km. The SCIAMACHY NLC size dataset currently presents the most comprehensive satellite dataset of NLC particle sizes. The derived particle sizes are in good agreement with independent observations (von Savigny and Burrows 2007, von Savigny et al. 2009).
Mesospheric Ozone and the October/November 2003 Solar Storm
Highly energetic protons ejected from the Sun during phases of high coronal activity, such as solar flares or solar coronal mass ejections, reach the Earth with the solar wind, ionise the atmosphere and lead to the formation of HOx and NOx in the mesosphere and upper stratosphere. Both families participate in catalytic O3 destruction cycles, with HOx being more efficient above about 50 km and NOx below about 50 km. Consequently, enhanced O3 destruction is expected after strong solar proton events.