Radar observations of negative-ion photodetachment at sunrise in the auroral-zone mesosphere

Physics

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Auroral Zones, Mesosphere, Negative Ions, Photodissociation, Radar Data, Sunrise, Atmospheric Chemistry, Electron Density (Concentration), Molecular Ions, Solar Radiation, Time Dependence

Scientific paper

During the winter half of the year, the large 50 MHz MST radar at Poker Flat, Alaska, records strong echoes from the mesosphere when energetic-particle precipitation is occurring. Echoes in the 70-75 km height range are mainly confined to the daytime, and often show a sharply defined sunrise onset at a solar zenith angle near 94 deg. Models of the time-varying atmospheric transmission during the sunrise period and of the time-dependent ion chemistry of the mesosphere are used to simulate the growth of electron concentrations during sunrise as a result of photodetachment from negative ions, chiefly CO3(-) and NO3(-). The balance between the concentrations of these two ions is critically dependent on the concentration of nitric oxide, and this balance can be estimated from the sunrise onset times, since CO3(-) and its hydrates are destroyed efficiently by visible sunlight, while NO3(-) requires u.v. light. The analysis suggests that the NO concentration at 72 km in the auroral-zone fall and winter may exceed 10 to the 9th/cu cm when energetic-particle precipitation is present, but is highly variable in time.

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