Satellite-borne limb scanning UV spectrometer for thermospheric remote sensing

A concave grating Wadsworth spectrometer designed to scan the UV limb of the earth was flown on a Defense Department meteorological satellite to obtain measurements of atmospheric emissions in the 85-395-nm wavelength range as a function of height above the solid earth. The instrument field of view was 0.14 x 3.8 degrees corresponding to 6 km in the vertical and 230 km in the horizontal at the limb. The scanning motion was controlled by a momentum compensated dc-torque motor mechanism that panned the line of sight across the limb corresponding to tangent altitudes of 80-480 km. A set of three photon counting detectors, each viewing a separate exit slit, provided simultaneous coverage of the wavelength bands of 85-120 nm (EUV), 110-163 nm (far UV), and 290-395 nm (UV) at a wavelength resolution of 0.4, 0.8, and 1.2 nm, respectively. A separate photometric channel isolated the atmospheric sodium doublet at 589.0-589.6 nm. The grating position and instrument view angle were controlled by digital circuitry operating on hardwired and uplinked command instructions. The operating modes included a variety of scanning and fixed wavelength and view angle operations. A description of the instrument and several examples of the data are presented. These include the dayglow emissions from thermospheric oxygen and nitrogen that form the basis of a thermospheric density determination, auroral enhancements observed in these emissions and in hydrogen Ly-alpha, and nighttime sodium emissions.