Space-borne observations of meteoric metal layers in the upper atmosphere.

The upper mesosphere/lower thermosphere (MLT) is an important transition region. However, it remains poorly understood relative to other parts of the Earth’s atmosphere, largely due to a lack of observations. Metal species, produced by meteoric ablation act as useful tracers of upper atmospheric dynamics and chemistry. Of these meteoric metals, K has long been an enigma. Limited lidar data at extra-tropical latitudes shows that the K layer displays a semi-annual seasonal variation rather than the annual pattern seen in other metals such as Na and Fe. This is a rather surprising feature as both Na and K are Group 1 alkali metals and, thus, should exhibit similar behaviour.

The aim of this thesis was to produce the first near-global K retrieval which could be used to evaluate this unusual behaviour, as well as providing a new dataset with which to test our understanding of the MLT region. The K retrieval uses dayglow measurements of K at ~770 nm from the Optical Spectrograph and InfraRed Imager System (OSIRIS) instrument on-board the Odin satellite. This retrieval is shown to be capable of retrieving K number density profiles with a 2 km vertical resolution and a typical peak layer error of ±15%. It is shown to compare well with the limited available lidar data.

A first near-global look at the global K layer is presented, which shows that the unusual semi-annual seasonal behaviour is global in extent. The OSIRIS data is used to validate the National Center of Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model (WACCM) modelled K layer; showing good overall agreement and providing support for a new K chemistry scheme which is included in the model. Both OSIRIS and WACCM datasets are used to examine the response of the Na and K metal layers to the 11-year solar cycle. Unlike Na, K shows an anti-correlation with the 11-year solar cycle. The associated temperatures appear to be the predominant source of this anti-correlation. Finally, the response of the WACCM modelled K, Na and Fe layers is examined with respect to longer-term (50-year) changes within the MLT region. K is the only metal to demonstrate a pronounced response to the recent cooling temperature trend.