Solar-induced 27-day variations of polar mesospheric clouds from the AIM SOFIE and CIPS experiments

Polar Mesospheric Cloud (PMC) observations from the Solar Occultation for Ice Experiment (SOFIE) and the Cloud Imaging and Particle Size (CIPS) experiment are used to investigate the response of PMCs to forcing associated with the 27-day solar rotation. We quantify the PMC response in terms of sensitivity values. Analysis of PMC data from 14 seasons indicate a large seasonal variability in sensitivity with both correlation and anti-correlation between PMC properties and Lyman-alpha irradiance for individual seasons. However, a superposed epoch analysis reveals the expected anti-correlation between variations in solar Lyman-alpha and variations in PMC ice water content, albedo, and frequency of occurrence. The PMC height is found to significantly correlate with 27-day variations in solar Lyman-alpha in the Southern Hemisphere (SH), but not in the Northern hemisphere (NH). Depending on instrument and property, the time lag between variations in PMC properties and solar Lyman-alpha ranges from 0 to 3 days in the NH and from 6 to 7 days in the SH. These hemispheric differences in PMC height and time lag are not understood, but it is speculated that they result from dynamical forcing that is controlled by the 27-day solar cycle.