On the global distribution of sporadic sodium layers

Limb-scanning measurements of the dayglow radiance at 589 nm, made with the OSIRIS spectrometer on the Odin satellite, are used to retrieve Na layer profiles with sufficient vertical resolution (∼2 km) to observe sporadic Na layers (SSLs). A two-year data base (2003–2004) has been analyzed to determine the global distribution of the SSL occurrence frequency (at 0600 and 1800 hrs local time). This shows that SSLs are much more prevalent in the southern hemisphere, with a particularly active region extending from South America at latitudes greater than 40°S to the Antarctic peninsula. The global average SSL occurrence frequency is about 5%. By analyzing the occurrence of SSLs in successive limb scans, it appears that most SSLs have a horizontal extent of less than ∼300 km. However, on one occasion during this 2-year study, two apparently very large SSLs, extending about 70° in latitude, were observed.