Campaign helps reveal secrets of upper atmosphere

26 March 2010

The first flight of an instrument on a high-altitude aircraft has shown that it can provide insight into the complexities of the upper atmosphere. This is crucial for the development of a candidate ESA mission aiming to provide vital clues on how atmospheric chemistry and climate are linked.

Scientists and engineers involved in the development of one of the candidates for ESA’s seventh Earth Explorer mission called PREMIER, short for Process Exploration through Measurement of Infrared and millimetre-wave Emitted Radiation, are very pleased with the results from the maiden flight of an upgraded instrument that has been developed to test the PREMIER mission concept.

If selected, the PREMIER mission would advance our understanding of the processes that link trace gases, radiation and chemistry in the upper troposphere and lower stratosphere. The radiative effects of water and clouds are at a maximum in this region. It is also a region characterised by small-scale processes that have not been studied by previous missions.

In support of the development of the mission concept, two innovative limb sounding instruments that operate in the infrared and microwave region of the electromagnetic spectrum have been built, one of which is an instrument called Marschals developed by Rutherford Appleton Laboratory in the UK. It is the Marschals instrument, short for (Millimetre-wave Airborne Receivers for Spectroscopic Characterisation in Atmospheric Limb Sounding) that has just been taken for its maiden voyage high up in the atmosphere.

PREMIER measuring concept
PREMIER measuring concept

The airborne campaign was designed to test the instrument and simulate how this kind of limb sounding would work from space. The instrument was flown on an M-55 Geophysica aircraft – one of the very few aircrafts that can fly up to 21 km above the surface of Earth, which is twice the height of a commercial plane.

The flight took place over Kiruna in Sweden and clearly demonstrated that the sophisticated Marschals instrument can offer a different way of characterising trace gases, thin cirrus clouds and temperature in the mid to upper troposphere.

Brian Moyna from Rutherford Appleton Laboratory said, "Marschals is the first limb sounder to be explicitly designed and built for the purpose of sounding the composition of the upper troposphere and lower stratosphere. A particular attribute of millimetre-wave measurements is their comparative insensitivity to ice clouds."

Preparing the Marschals instrument
Preparing the Marschals instrument

The campaign is very important for the development of the PREMIER mission, which is currently undergoing feasibility study. If selected, it would quantify the processes that control the composition of the mid to upper troposphere and lower stratosphere, which equates to 5–25 km above the surface of Earth.

This region is particularly important for climate because the greenhouse effect, due to clouds and gases, is amplified at these altitudes. Recent research has revealed that the stratosphere has cooled and the troposphere has become warmer over the last few decades. The likely cause is the depletion of stratospheric ozone and increased concentrations of carbon dioxide in the troposphere. Through the processes of absorption, emission and scattering, greenhouse gases and aerosols interact with electromagnetic radiation and, hence, influence weather and climate.

However, the abundance and distribution of trace gases and aerosols is controlled by complex chemical reactions and dynamic processes that are currently not well understood. One of the main challenges in climate research, which the PREMIER mission aims to address, is to improve our understanding of these atmospheric processes and how they are linked to climate.

The PREMIER mission concept would observe thermal emissions from the limb, or edge, of Earth’s atmosphere. Limb sounders look horizontally through the atmosphere towards space so that information on temperature trace gases and water vapour at different altitudes can be gathered.

Measured ozone signal

The first test flight of the improved Marschals instrument has shown how this concept could reveal the complex make up of the upper atmosphere. For example, a full scan yielded a high but flat signal from the dense humid lower layers of the atmosphere, and at higher altitudes the spectral lines of gases such as ozone start to emerge from the lower water vapour background. From the amplitudes and shapes of these features scientists are able to determine the amount and the distribution of the respective gases in the atmosphere.

PREMIER is one of three missions concepts, one of which will eventually be selected for implementation and thus lead to the launch of ESA's seventh Earth Explorer mission around 2016.

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