First nightglow 844.6-nm observations with the Millstone Hill Spatial Heterodyne Spectrometer

Details First nightglow 844.6-nm observations with the Millstone Hill Spatial Heterodyne Spectrometer First nightglow 844.6-nm observations with the Millstone Hill Spatial Heterodyne Spectrometer

May 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmsa41c..01w&link_type=abstract

American Geophysical Union, Spring Meeting 2008, abstract #SA41C-01

Physics

0310 Airglow And Aurora, 0350 Pressure, Density, And Temperature, 0355 Thermosphere: Composition And Chemistry

Scientific paper

A Spatial Heterodyne Spectrometer (SHS) has been installed at Millstone Hill Observatory to detect mid-latitude 844.6-nm emissions from nightglow due to Bowen fluorescence, as a way of measuring thermospheric neutral oxygen density. Neutral oxygen is a dominant species in the F-region of the thermosphere, and new measurements are needed to make more accurate models of that region, models which better describe responses to impulsive events such as storms and coronal mass ejections. The SHS instrument at Millstone was originally designed with a field of view of one degree, taking observations with a time resolution of twelve minutes. While this provides an adequate signal-to-noise ratio (SNR) for line detection in spectrum obtained through the Fourier transform, it was found insufficient to detect the O I emission in nightglow above detector noise. Because of this, the SHS needed to be field-widened, to look at a field of view of 4.65 degrees. The field-widened SHS defeats detector dark noise, and reduces the exposure time to create the same SNR as the original instrument to two minutes. Field-widening the SHS requires inserting wedge prisms in each arm, and those prisms will arrive in early March 2008. Meanwhile, the SHS is being calibrated at 844.6 nm, using an oxygen spectrum tube (collimated through the SHS's five-angstrom line filter). The SHS prisms, once they arrive, will be inserted in the instrument between the gratings and beamsplitter at the proper angle, approximately 34 degrees from the system's optical axis. Once this is done, sky observations will commence in March 2007. It is expected that a significant fraction of the detected 844.6 light will be due to photoelectron (PE) impact excitation, though, as observations proceed into summer, Bowen fluorescence will come to dominate the 844.6 nightglow emissions. The two will be distinguished by characteristic line ratios between two of the three lines of the 844.6 triplet. Initial sky observations will be presented.

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