Hydroxyl airglow temperature observations
The concentration of a chemical constituent called Hydroxyl (whose chemical symbol is OH) is much greater in a thin layer approximately 87 km above the earth’s surface. Known as the Hydroxyl airglow layer, it allows us to measure the temperature in this remote part of the middle atmosphere. This is made possible by the faint infra-red emissions that emanate from this layer during the hours of darkness (airglow) and our ability to "decode" information stored in these emissions.
Although faint, emissions from the 8 km thick OH layer are the brightest in the night sky. However, because they are in the infra-red part of the light spectrum, they are not visible to the naked eye.
Temperature measurements are obtained using a spectrometer, an instrument that measures how much of each colour is present in the light it receives. Colour is described using a parameter called wavelength. The light we see with our eyes is in the wavelength range 400 (violet) to 700 (red) nanometers (a nanometer is one thousand-millionth of a metre). A measurement of the intensity of light versus its wavelength (colour) is called a spectrum and some spectra of part of the Hydroxyl airglow emission is given below. Wavelengths from 837 to 856 nanometers (nm) are shown.
Spectra of the OH(6-2) at three levels of auroral activity. Auroral activity affects the intensity of one of the Oxygen (O) lines.
The light is emitted when a "vibrationally excited" form of the OH molecule is made from the reaction of hydrogen with ozone. In this vibrationally excited state, the molecule is unstable and quickly achieves a more stable state by radiating energy as light. This light is emitted at certain discrete wavelengths as shown in the spectra above. The intensity of the emissions at these wavelengths is related to the temperature. The emissions are labelled with a capital letter and numbers that describe the mechanics of their formation. Here, it is the P1 emission lines that are used to measure atmospheric temperature. The line marked "OI" indicates how much auroral activity was present at the time of the measurement. This portion of the spectrum is often termed the OH(6-2) spectrum.
The complexities of the analysis method are described in more detail later. However, it is the intensity of the lines in the spectrum relative to each other that provides the temperature. The task of the spectrometer is to measure these spectra as quickly, reliably and accurately as possible.
The Hydroxyl airglow project is supported by Antarctic Science Advisory Committee funding. The project name is "Long-term monitoring of the mesopause region via hydroxyl emissions at Davis" and the investigators are:
Dr G. Burns, Australian Antarctic Division (Chief Investigator)
Dr J. French, Australian Antarctic Division.
The spectrometer is maintained and operated by the Space and Atmospheric Sciences staff at Davis.