This investigation will develop a novel inversion algorithm to be applied to three-dimensional time-dependent images of global electron density in order to quantify the physical processes that are driving the density distribution. Ideally, ionospheric scientists would obtain direct global measurements of the drivers of the ion continuity equation, namely the winds, electric fields, neutral and ion composition and densities, and neutral and ion temperatures. However, in practice there are so few ground-based instrument and satellite measurements that evaluation of the physical processes is not possible. In the past few years, two developments have occurred which offer a new way to approach the problem: significant advances have been made in applying tomographic methods to ionospheric imaging, and global ionospheric data with good time and spatial resolution, such as total electron content, are now routinely available through the internet. The research plan will start with consideration of the ion continuity equation. But rather than attempt a numerical solution, an algebraic matrix inversion will be done which will incorporate observed time rates of change of the electron densities. The work will be performed in several steps. The development of the algorithm occurs first, using electron densities from a first-principles model as a numerical simulation lab. Then an assimilative model will be used to construct three-dimensional time-dependent maps of electron densities. The inversion will be applied to these datasets, and the drivers will be derived over well-instrumented regions so that they may be compared with observations from radars and Fabry-Perot interferometers (FPIs). The final step is to apply the algorithm to observational datasets to derive the drivers of phenomena such as storm-time enhanced densities. If successful, this approach will yield direct estimates of the neutral winds, electric fields, production, loss, and diffusion terms. The goals of the research are consistent with the CEDAR Phase III Document which stated that progress is needed in identifying the interrelated processes governing the thermosphere/ionosphere response to high latitude energy inputs. Specifically, the investigation will focus on three main questions: What are the physical drivers of storm enhanced densities and the source plasma that appears to feed the densities? What are the causes of ionospheric variability such as patches? Can global three-dimensional time-dependent maps of electron density be used to constrain the ion continuity equation to derive estimates of the underlying physical drivers responsible for the observed electron density distributions? The results of the research could be used to estimate the drivers of mechanistic or assimilative models and to validate three-dimensional first principles models. The mathematical techniques developed in this project can be applied to any geophysical problem where a physical equation can be related to three-dimensional time-dependent maps. A graduate student will participate in the project part-time as well as an international collaborator, Dr. C. N. Mitchell at the University of Bath, United Kingdom.
