The application of ground-based optical techniques for inferring electron energy deposition and composition change during auroral precipitation events

Passive ground-based optical techniques have been used for over 15 years to study the effects of auroral energy deposition on the composition of the lower thermosphere. These techniques typically involve measuring bright auroral emissions that depend on the number density of atomic oxygen [O] and molecular nitrogen [N2] as well as the energy flux and energy spectrum of the precipitating auroral particles. Here we review the application of these techniques to the composition problem. In particular, we present data obtained from photometric observations in Alaska and Greenland in the 1980s and 1990s. While some of these results are available in the existing literature, some are new and have not been presented before. Recent results from Alaska obtained in support of the TIMED satellite are also presented. The data as a whole suggest that the ratio of column integrated [O] to [N2] above a given site decreases as the total energy fluence from both auroral particle and Joule heating increases. However, the dependence is complicated by effects such as the prior history of disturbance and by horizontal transport due to neutral winds; it is not yet determined if composition responds linearly to energy fluence even in an average sense.