Untangling daytime ionospheric remote sensing: RAIDS contributions to solving an old problem

The Remote Atmospheric Ionospheric Detection System (RAIDS) has been obtaining routine measurements of the OII 83.4 nm airglow from its location aboard the Japanese Experiment Module-Exposed Facility on the International Space Station since October 2009. This emission feature is produced in the lower atmosphere via the photoionization and excitation of atomic O in the lower thermosphere, but resonantly scatters through the ionosphere thereby allowing the measure of an altitude profile of this emission to reveal the characteristics of the daytime ionosphere. Although promising in concept, it has proven difficult to extract information from this emission in part due to the complexity of the physics and the lack of appropriate constraints on the initial source of photons from the lower thermosphere. Occasionally, RAIDS simultaneously measures OII 83.4 as well as the OII 61.7 nm feature that is produced by the same mechanism but is optically thin to the ionosphere, thereby providing a measure of this initial source region. The combination of these measurements in conjunction with forward models and ground-truth coincident ionosonde measurements provides the means to probe the response of these emission features to changes in solar zenith angle, local time, and geographic location. The ISS provides a unique platform to test underlying model assumptions since its precession allows sampling all local times over the course of one month, and its lower orbiting altitude creates a unique opportunity to compare emission profiles with the ionosphere both above and below the location of the sensor. We will present a summary of the new information that RAIDS has provide to resolve key questions about the basic physics behind the 83.4 nm emission feature. These new data represent a crucial step toward the development of remote sensing techniques to use this emission to routinely characterize the evolution of the global daytime ionosphere as part of the near-Earth space weather system.