The global mesospheric sodium layer observed by Odin/OSIRIS in 2004–2009

Abstract

The source of the mesospheric sodium layer is the daily ablation of 10–100tons of meteoric material in Earth’s atmosphere. Global studies of this layer yield important information about the chemistry and dynamics of Earth’s mesosphere and lower thermosphere (MLT). For nine years the Optical Spectrograph and Infra-Red Imager System (OSIRIS) on-board the Odin satellite has observed Earth’s middle atmosphere by limb measurements of scattered sunlight from the ultraviolet to the infrared. In its aeronomy mode, Odin performs limb scans during 15 near-polar sun-synchronous orbits each day. The current measurement programme provides scans up to 110km on about 300 days per year. Above 70km, Na D resonance scattering at 589nm results in a strong limb signal. Retrievals from this dayglow feature have provided a global database of the mesospheric sodium layer. We present an updated sodium climatology from the Odin mission, including latitudinal and seasonal dependence, and interannual variability. We find a weak seasonal variation at low latitudes and an annual variation at mid- and high-latitudes with a clear summer minimum. An interesting feature is an interhemispheric asymmetry in the global dataset with larger sodium abundances during fall in the northern hemisphere and during spring in the southern hemisphere.