Impact of Solar Proton Events and Planetary Wave Activity on Noctilucent Clouds

Abstract

Noctilucent clouds (NLCs), also known as Polar Mesospheric Clouds (PMCs) are a rare phenomenon observed at polar latitudes during the summer season. These optically thin clouds are situated in the mesopause region at about 83 km altitude. We study the possible impact of solar proton precipitation in the Earth's atmosphere and the behaviour of the noctilucent clouds during the solar proton event. For this purpose we use SBUV/2 (Solar Backscatter Ultraviolet) NLC observations that cover the last 30 years. The proton fluxes measured by GOES (Geostationary Operational Environmental Satellite) cover the same time period. A statistical investigation between solar proton events (SPEs) and a possible depletion of NLCs is aimed at in this work. This is done for both hemispheres separately. It is shown that solar proton events are well correlated with the depletion of NLCs. The majority of SPEs lead to significant reductions in the observed NLC residual albedo and occurrence rate time series. The extraction of the SPE forcing on the NLCs is disturbed by other forcings such as planetary wave activity, causing large scale perturbations in meridional wind circulation which can occur simultaneously. We assume, that therefore the reduction of NLCs albedo and occurrence rate during SPEs due to an increase of temperature in the mesopause region is a combination of several forcings and effects which contribute to the total variation in NLC signal. The last part of the work is dedicated to the possible influence of planetary waves such as the 2-day wave and 5-day wave on the mesopause temperatures at mid and polar latitudes. The planetary wave (PW) signatures retrieved from the SBUV/2 and SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) NLC data are in good agreement with the observed PW signatures in temperature data from MLS (Microwave Limb Sounder). Moreover, since the NLC perturbation is mainly triggered by temperature in the mesopause region, temperature increases at polar latitudes are compared with the peak amplitudes of the PWs activity at mid-latitudes. The 2-day wave activity at mid-latitudes coincides with the temperature pulses at polar latitudes for the seasons 2004/05, 2005/06 and 2006/07. For the seasons 2007/08 and 2008/09 the 5-day wave activity at mid-latitudes is responsible for the temperature pulses at polar latitudes. Peak 2-day wave amplitudes are anti-correlated with the peak 5-day wave amplitudes at mid-latitudes. Comparison of peak amplitudes at polar latitudes show that the peak 5-day wave amplitudes are higher at polar latitudes in comparison to the 2-day wave peak amplitudes at these latitudes except for the January 2005 in the southern hemisphere. A combined forcing on temperature due to the SPE and 2-day wave activity in the summer polar mesopause region can be assumed to be responsible for the warming and massive depletion of NLCs for mid January 2005 in the southern hemisphere.