This project aims to develop an accurate technique for determining the atomic oxygen density in the mid-latitude thermosphere by combining modern instrumentation with state of the art modeling. The method will combine optical and radar measurements to constrain a forward model of a key thermospheric O-atom emission, the twilight airglow at 8446 A, and in turn use the constrained model to develop an [O] estimation scheme suitable for application at other mid-latitude locations. The 8446 A emission has long been considered an ideal candidate for [O] remote sensing owing to its relatively simple emission model, and the project will acquire an unprecedented set of 8446 A spectral data under various observational conditions at two distinct mid-latitude facilities: Millstone Hill (MH) Observatory in Massachusetts and Arecibo Observatory (AO) in Puerto Rico. Additional parameters derived from nested incoherent scatter radar, Fabry-Perot interferometer, and photometer measurements will not only serve as additional forward model constraints, but also help assess the validity of current model assumptions, specifically with regard to the role of secondary sources of 8446 A production. Together with a thorough quantification of model parameter dependencies, the constrained forward model will be used to develop a novel inverse-theoretical technique to estimate thermospheric [O] from measured 8446 A brightness at mid-latitudes. Quantification of neutral atomic oxygen, the dominant constituent in the Earth's thermosphere between 200 - 600 km, is important for several reasons. In this region, its resonant charge exchange with O+, the principle ion in the F-region ionosphere, plays a vital role in both the momentum and energy exchange between the thermosphere and ionosphere. Similarly, its charge exchange with H+ has long been recognized as an important influence on ion transport between the ionosphere and plasmasphere. Owing to this strong chemical coupling, the accuracy of many fundamental aeronomical calculations -- such as the derivation of transport coefficients, neutral wind speeds, energy deposition rates, chemical reaction rates, or photochemical emission brightnesses -- hinges on accurate specification of [O]. Thus, current uncertainties in thermospheric composition and density limit the understanding of the coupled thermosphere-ionosphere system, both with regard to its climatological variability as well as its response to impulsive forcing from above and below. The development of a new, ground-based capability of measuring thermospheric [O] will benefit both of these central priorities of the NSF CEDAR program. One graduate student will be trained in this Aeronomy-related area with support from the project. The student will gain familiarity with acquisition and analysis of ISR spectra as well as that of optical SHS, FPI, and photometer data. Available internal funds for undergraduate research support, together with the strong involvement of AO in the Research Experiences for Undergraduates (REU) program, presents another opportunity to introduce undergraduate students to aeronomy as well.
I. Introduction and Application As part of its Thermospheric Coupling and Dynamics (TCD) program, Scientific Solutions, Inc., has built and deployed a new spectrometer -- called a Spatial Heterodyne Spectrometer (SHS) -to observe neutral oxygen emissions near 844.6 nm at the Millstone Hill Observatory in Westford, Massachusetts. Oxygen is the most abundant atom between 250 and 500 km altitude, and contributes significantly to Space Weather, the dynamic conditions in the upper atmosphere which greatly affect the operation of satellites serving a wide variety of ground activities, including cell phone communication and GPS operation. The analysis of oxygen emission spectra from the atmosphere is vital to the effort to fully understand and model Space Weather, and mitigate its deleterious effects on everyday activities. The SHS is a novel type of unscanned Fourier-transform interferometer. At a given resolving power, it is capable of greater etendue at a smaller size compared to grating and prism spectrometers. Conventional interferometers, such as Michelsons and Fabry-Perots, also have this advantage. However, since the SHS requires no scanning mechanisms to operate at full resolving power, it can be built much more robust than conventional interferometers, and has relatively loose tolerances for the flatness of critical optics. Moreover, of particular importance in this project, it can, on a tabletop, be quickly and easily reset to look at wavelength regions several hundred angstroms apart. Thus, one instrument can make successive observations – at effectively the same resolving power – of several different airglow lines from a single small work space over several hours and days. II. Science Neutral oxygen, the dominant neutral species between 250 and 500 km ([Lancaster, 1997]), plays a vital role in the physics of the thermosphere. Despite its great importance, measurements of the density of the neutral oxygen, [O], are not extensive. Most analysis has employed semi-empirical models like the Mass Spectrometer Incoherent Scatter (MSIS) theory, to predict neutral composition based on input parameters including altitude, local time, and solar activity. While useful, these models depend on statistical averages, making them less accurate for studying phenomena which vary relatively quickly with time, such as magnetic storms and responses to coronal mass ejections. To have as complete an understanding of the thermospheric system as possible, it is absolutely crucial to understand the effect such impulsive phenomena have on redistributing the system’s momentum and energy. A forward model, incorporating aspects of Incoherent Scatter Radar (ISR) and other input, would be hugely preferable to current predictions. New ground-based airglow observations of neutral oxygen density [O] are vital to this effort. Its measurement, incorporated into a forward model of twilight airglow emissions, can be used to constrain theoretical relations between ion and neutral parameters. This will be the goal for the neutral oxygen density measurements obtained from this project. The instrument for this project will be a Spatial Heterodyne Spectrometer (SHS), which is similar to the more common Michelson interferometer. It will measure neutral oxygen density by detecting the emissions at 844.6 nm due to a process called Bowen fluorescence. The SHS analyzes the emissions by imaging an interference pattern created by the input light. The imaging detector is an electronic array detector similar to the pixel array in an electronic camera. The initial problem with this detection was that the electronic noise generated by the array detector in normal operation swamped the relaitvely weak target signal. To detect the signal required gathering in more of it by widening the field of view of the instrument. This field widening involved inserting wedge-shaped prisms into the instrument. However, the wedges, like bad lenses, also degraded the imaging of the system, so that the interference pattern necessary for analysis could not be detected. The solution to this was to reconfigure the SHS into a different mode, called the "differential system." This involved procuring slightly different optical elements, but also allowed for field-widening prisms which were not so severely wedged, solving the imaging problem. Figuring the exact parameters of the new components for the differential system, and modeling the imaging of the system to show that it did not degrade the system imaging in the same way as the previous prisms had, were the key tasks of the third year of this project. Once the system is modified, it will be able to make regular daysky measurements and become a productive instrument for the aeronomical science community. The tractable problems with noise and imaging have been worked out, and now simply have to be implemented to bring the instrument to viewing 844.6-nm emissions from the sky within six months.
