Abstract

The spectroscopy of the upsilon = 1 -> upsilon = 0 fundamental vibration-rotation band of nitric oxide (NO) in Earth's atmosphere is examined in depth in order to further assess the long-running dataset of infrared radiative cooling rates in the thermosphere from the SABER instrument on the NASA ... Show moreThe spectroscopy of the upsilon = 1 -> upsilon = 0 fundamental vibration-rotation band of nitric oxide (NO) in Earth's atmosphere is examined in depth in order to further assess the long-running dataset of infrared radiative cooling rates in the thermosphere from the SABER instrument on the NASA TIMED satellite. The fundamental band at 5.3 mu m is shown to be almost solely responsible for the cooling by NO. The distribution of line strength in this band and the concentration of NO in the atmosphere are such that NO is remarkably transparent in Earth's atmosphere. Every fundamental band photon emitted in the nadir direction by NO in the thermosphere has a nearly 100% chance of hitting the Earth's surface before being absorbed by another NO molecule. The mean free paths of these photons exceed 5000 km. Vertical optical depths of the strongest NO absorption lines in 2 km thick layers in the thermosphere are less than 10-4 even during geomagnetically disturbed conditions. Consequently, nearly all of the radiation emitted by thermospheric NO escapes to space or to the lower atmosphere. Radiative excitation of thermospheric NO by upwelling infrared radiation ("earthshine") from the troposphere is accurately assessed using measurements of infrared spectra made by the IASI instruments on the METOP satellites. Earthshine and solar excitation of NO are shown to compete with collisional excitation of NO by atomic oxygen below 110 km in polar regions and below 115 km in tropical regions. Therefore, NO 5.3 mu m energy loss rates currently derived below similar to 115 km from measurements made by the SABER instrument on the NASA TIMED satellite are not representative of radiative cooling. Consequently, the current values of SABER-derived daily global power radiated from the thermosphere by NO are 5% to 15% too large. Approaches to further improve the SABER radiative cooling dataset are presented. Show less