Modeling Optical Emission Intensities of Rapid Small-Scale Aurora

Abstract:

Auroral transitional line emissions have often been used to infer the fluence and characteristic energy of precipitating electrons. A common practice is to use emissions from an allowed transition to infer absolute particle flux since the odds of quenching before photon emission are negligible. Characteristic energy is then determined by line emission intensity ratios between a forbidden and allowed transitions: The intensity of a forbidden transition will increase with altitude since the probability for quenching drops with decreasing density. Bright metastable lines such as 630.0 nm O(1D) -> O(3P) and the 557.7 nm O(1S) -> O(1D) are often used with a prompt line such as 427.8 nm N₂⁺(1N) to determine characteristic energy. With the advances in scientific cameras, narrow-band filtered video of pulsing aurora up to 32 fps are now in use. The question then becomes, if the transitional lifetimes of the metastable species are significantly greater (or even comparable to) the aurora pulsing period, how can the ratio technique be used to determine the characteristic energy of the precipitating electrons? Once it is realized that the quoted lifetimes are average values, we note that there will be a fraction of photons that are emitted before the species is quenched. With this study, we present results from the GLOW model for different metastable species to determine the optimal combination of lines that would be helpful in determination of precipitating electron characteristics in pulsing aurora up to 100 Hz. Enabling technology and optimal configurations will be presented, along with suggested applications for linking different optical signatures with their corresponding precipitating electron distribution shape.