Support for atmospheric origin of anomalous submillimetre wave spectral features

Abstract

A RECENT letter in Nature¹ showed a feature at a frequency of 8 cm⁻¹ in one of two spectra of the atmosphere. That the feature is an artefact follows from the authors' own statement that no true spectral observation was possible. The indication was found only in records from daylight hours and the proposal was made that it was caused by some solar process with an appropriate time variation coupling to the recording system in some unspecified way. We do not wish to comment on this process or on the authors' own evidence given to support it. But they also quote published results as giving further support to their hypothesis with the clear implication that features in the spectra of other workers may be due to the same kind of artefact. We contest this since it is easy to distinguish between a true spectral indication and an artefact, coming from temporal variations of source or absorption intensity, by varying the time rate of change of path difference given by the interferometer.