INTERFERENCE FILTER SPECTRAL IMAGING OF TWILIGHT O+(P-2-D-2) EMISSION

A spectral imager specifically designed to measure the O+(P-2-D-2) emission in the thermosphere during twilight has been constructed and tested in Toronto (43.8 degrees N, 79.3 degrees W), and found to show promise for long-term and campaign-mode operations. A modification of the mesopause oxygen rotational temperature imager (MORTI), it consists basically of a narrow-band interference filter (0.14 nm bandwidth) to separate wavelengths as a function of off-axis angle, a lens to focus the spectrum into a series of concentric rings, and a focal plane array (CCD) to record the spectral images in digital form. The instrument was built two fields of view, one for the zenith and one for 20 degrees above the horizon, movable to track the azimuth of the Sun, in order to provide appropriate data for inversion. Data gathered during June 1991 provided measurements of the column-integrated emission rate with a precision of about 3%. An atomic oxygen profile was deduced that showed good agreement with that predicted by the MSIS-90 model atmosphere. Geomagnetically induced variations of the O+ lines, calcium spectra resulting from meteor showers, and OH nightglow were also observed.