Optical Estimation of Auroral Ion Upflow: Theory

This work presents a systematic analysis of optical emissions related to auroral ion upflow. Optical intensities and field‐aligned ion transport are computed for a set of monoenergetic incident electron beams using a combined fluid‐kinetic model. The kinetic portion models the energetic particle transport with a multiple stream approach and provides ionization, excitation, and heating rates to an eight‐moment fluid model of the ionosphere, which then calculates the resulting ion upflow. The analysis is used to develop a technique for estimating upward ion flux from photometric measurements at five discrete wavelengths: 427.8 nm, 557.7 nm, 630.0 nm, 732 nm, and 844.6 nm. The procedure involves (1) estimating the incident particle spectrum by inversion of multiwavelength optical measurements in the magnetic zenith, (2) applying this incident spectrum to the fluid‐kinetic model to estimate the upflow response. The robustness of the procedure is demonstrated by inverting brightnesses computed for a known electron spectrum and then comparing upflow directly calculated from the known spectrum to the upflow calculated from the estimated spectrum. The inversion is found to provide a reliable estimate of the precipitating electron spectrum and ion upflow, even in the presence of realistic uncertainties in brightness. The technique represents a new tool for studying mass coupling between the magnetosphere and ionosphere. Potential applications range from upflow event studies to estimating the total amount of plasma entering the transition region during a substorm surge via fusion of optical data from multiple sensors