An enhanced concentration of Hydroxyl molecules (chemical symbol OH) occurs in a thin layer near the mesopause (the boundary between the mesosphere and thermosphere, about 87 kilometres above the earth’s surface). The molecule is a product of the Hydrogen-Ozone reaction (H + O3 → OH* + O2), an exothermic (hot) process which results in an ‘excited’ form of the OH molecule.
The process by which the excited OH molecule radiates energy is a fortuitous property that allows us to measure the temperature of this remote part of the middle atmosphere. A multitude of vibrational and rotational states in excited OH, cascade to lower energy states by emitting light (photons), resulting in an emission spectrum that extends over a wide wavelength range (500–5000 nm), but most strongly in bands in the infra-red (1600 and 2800 nm). These are the brightest emissions in the night sky (by far) but are beyond the visible spectrum range (400–700 nm) that we can see.
However, optical instruments can ‘see’ these emissions and we have been measuring parts of this spectrum using spectrometers (remotely from the ground at Davis station), continuously
Accumulated spectra of the OH(6–2) band around 840 nm, at three levels of auroral activity are shown here. An oxygen emission line at 844 nm is used to gauge the level of auroral activity in each spectrum.
P-branch lines (of P, Q and R that describe different transitions within the excited OH molecule) seen in the graph at the top right are used to measure atmospheric temperature. The ratio of intensities of one line to another, coupled with known line parameters and constants allow for the temperature calculation (see analysis method details).
Routine, automated observations have provided a record of mesopause region temperature for more than 20 years and allow for long-term trend and variability studies and comparisons with satellite and model observations.
The Hydroxyl airglow project is supported through the Australian Antarctic Science (AAS) Programme.
The project title is “Mesospheric OH Climate Assessment (MOHCA)” and the investigators are:
- Dr J. French, Australian Antarctic Division (Chief Investigator)
- Dr. A Klekociuk, Australian Antarctic Division, and international collaborators Dr. Frank Mulligan and Prof. Bob Lowe.
The spectrometer is maintained and operated by the Antarctica in the Global System atmospheric staff at Davis station.