First study of the global Nickel and Aluminium Layers in the upper atmosphere (NIALL)

The edge of the Earth's atmosphere is approximately 100 km above the surface, in a region known as the mesosphere/lower thermosphere (MLT). This part of the atmosphere is subject to high energy inputs from above in the form of extreme UV radiation and energetic particle precipitation, and a roughly equal amount of energy from breaking atmospheric gravity waves which propagate up from the lower atmosphere. The MLT also acts as a filter of waves that propagate from the troposphere into the ionosphere, which has important implications for space weather. Furthermore, energetic solar protons and electrons from the radiation belts produce highly reactive species in the MLT, which can then be transported down into the stratosphere, affecting the ozone layer and impacting on tropospheric climate. The MLT is also extremely sensitive to climate change, due to the cooling effect of increasing greenhouse gases such as CO2, ozone depletion in the stratosphere, and changes to the large-scale atmospheric circulation. However, it is a difficult region in which to make direct measurements, because it is more than 40 km higher than altitudes reached by research balloons or aircraft, and is at least 100 km lower than short-lived satellite orbits. Rocket-borne measurements do provide direct access, but are unsuitable for sustained global measurements.

Fortunately, the ablation of cosmic dust particles entering the atmosphere from space deposits metal atoms such as Na and Fe in layers around 90 km altitude. These layers can be observed with lasers from the ground (lidar) and by satellite-borne spectrometers, providing detailed information about the chemistry and physics (wind, temperature, gravity waves) of the region. There is increasing evidence that accurate simulations of changes to the Earth's climate require models with a well resolved and accurate stratosphere and mesosphere, and so metal species in the upper atmosphere offer a unique way of observing this region and of testing the accuracy of climate models.

The purpose of this proposal is to make the first ever study of Ni and Al chemistry in the MLT. The Ni layer has recently been observed for the first time: it is much broader than the well-studied layers such as Na and Fe, and the concentration of Ni atoms is more than 10 times higher than expected based on its cosmic abundance. These very unexpected features need to be understood, since there is the clear potential to develop lidar observations of the Ni layer as a probe of the entire MLT from 70 to 115 km.

Aluminium makes a very interesting contrast with Ni. The Al-O bond is so strong that it is very likely there is a substantial layer of the AlO radical in the MLT. This species has a strong optical absorption in the green part of the visible spectrum, and so there is the exciting prospect of making lidar observations of AlO and developing an accurate temperature probe over the full range of mesospheric temperatures.

The project will involve first making a series of experimental studies of key neutral and ion-molecule reaction rates in the gas phase, in order to understand the unique characteristics of the Ni layer and the likely concentration of the AlO layer. At the same time, we will use a novel instrument to simulate the ablation of Ni and Al from micron-sized fragments of meteorites such as Allende and Murchison. From this a model will be developed which predicts the injection rates of these elements into the MLT as a function of location and season.

The chemistry of Ni and Al, together with their meteoric ablation rates, will then be placed into a global chemistry-climate model. Of particular interest will be to see how the Ni and AlO layers are predicted to respond to perturbations caused by major solar storms, the 11-year solar cycle, and climate change in the MLT over the past 70 years and projected forward to 2100.

The proposed research will be of societal and economic benefit in a number of ways:

1. Educational benefits to the wider public on the question of anthropogenic versus natural climate change. The mesosphere/lower thermosphere (MLT) is a particularly sensitive region, both to anthropogenic influences (increased greenhouse gases, stratospheric ozone depletion) and solar influences (strength and frequency of the solar cycle and longer-term changes in solar activity). It is one region of the atmosphere where there is an unambiguous climate change signal which is easy to observe - noctilucent clouds. Auroral emissions in the lower thermosphere are the most visual manifestation of space weather. The NIALL project also involves understanding the cometary sources of interplanetary dust and the impacts of meteors in the atmosphere, and there is a great deal of public interest in meteor showers and cometary missions (e.g. Rosetta). Linking these different phenomena together provides a natural forum for public lectures and debate on the relative significance of solar versus anthropogenic influence on climate.

2. Satellite operators are already able to fly satellites for longer periods in lower orbits as a result of the reduced drag caused by thermal contraction in the middle atmosphere. A validated chemistry-climate model of the MLT can be used to predict future trends. The satellite re-entry region is 90-140 km, and a precise knowledge of the local characteristics of the MLT is needed to determine the position and required change in velocity to de-orbit a satellite. Similarly, satellite launch operators need to know the small-scale, local fluctuations in density above 80 km to calculate accurately the aerodynamic forces acting on the launcher. The high temporal/spatial resolution provided by lidar observations of the metal layers, such as the Ni layer measurements proposed in NIALL, could become an important tool for these operations.

3. Several aerospace companies in Europe and North America (e.g. EADS Astrium, Northrop Grumman, Virgin Galactic and XCOR) are currently building sub-orbital "space planes" which are designed to fly through the MLT to around 110 km. The environmental impact of the huge quantities of water vapour, carbon soot particles and metals which these re-usable vehicles will inject into the mesosphere will need to be assessed through a properly validated whole atmosphere chemistry-climate model.

4. ESA and NASA are both considering space-borne metal resonance lidar missions to monitor the mesosphere. Payload designers will require access to a global model of meteoric metals when formulating lidar performance requirements and spacecraft orbital parameters.

5. The metallic ions produced by meteoric ablation are the major constituents of sporadic E layers. These layers have a significant effect on ground-to-satellite and over-the-horizon radio transmissions. Understanding the processes which control the distribution of metallic ions in the lower thermosphere, and hence predicting sporadic E occurrence, is important to many industrial and governmental organizations.

6. Numerical weather forecasting organizations (e.g. the UK Met Office) are starting to extend their operational forecast and climate models into the thermosphere. One reason is that a well-constrained mesosphere is now considered to be an important element of climate modelling due to the impact of middle atmospheric chemistry. There is also increasing evidence for an improved accuracy of weather forecasting, particularly if mesospheric data from satellites such as Aura-MLS and SABER is assimilated. Another reason for developing high-top models is for space weather prediction, since there is a clear impact in the thermosphere of upward-propagating waves from the lower atmosphere. The NIALL project will provide calibration/validation data both for satellite remote sensing and high-top model development.