Far Ultraviolet Imaging of the Earth’s Thermosphere and Ionosphere 1. Summary The thermosphere-ionosphere (T-I) system is the interface between the Earth’s atmosphere and the space environment. It is here that the Sun’s most energetic radiation and the high-energy charged particles from the magnetosphere are absorbed. It is also here that waves propagating from the lower atmosphere finally dissipate. Determining how the T-I system responds, on a global scale, to variations in these forcings is essential to our physical understanding of how the space environment is coupled to our atmosphere. To understand the response to external and internal processes that drive the thermosphere-ionosphere, the spatial and temporal variations of the system should be measured simultaneously over the entire globe. Far ultraviolet (FUV) spectral imaging from appropriately spaced geostationary orbits accomplishes this, providing full disk images of atmospheric temperature and composition during the daytime and electron densities in the F2 region of the ionosphere at night. The ideal mission would use three imagers, operating simultaneously in orbits separated by ~120 degrees of longitude. This technique should be validated by initially flying a single imager. This document provides the scientific background for these missions (Section 2), their important practical applications (Section 3), some examples of the science that these missions would provide (Section 4) and an overview (Sections 5-8). 2. Scientific background Most space missions that have investigated the Earth’s T-I system have relied on measurements made along the track of a low Earth orbit (LEO) satellite, but spatial-temporal ambiguity is inherent to such measurements. Full-disk imaging of physical parameters such as temperature and composition, at a cadence of 1-2 hours, will remove this ambiguity. These observations will revolutionize our understanding of the Earth’s upper atmosphere, just as global imaging has revolutionized our understanding of the troposphere. Full-disk, FUV imaging provides an excellent means of obtaining such global measurements of the T-I system. The atmosphere at lower altitudes absorbs FUV light, providing a dark background for light emitted from the thermosphere. During the day, the ratio of emissions from atomic oxygen and molecular nitrogen provides composition information; during the night, imaging of the atomic oxygen emission provides ionospheric density and structure. Through moderate-resolution spectral imaging, daytime temperatures can also be obtained from the rotational distribution of the molecular nitrogen bands. High-altitude imagers such as DE-1 and IMAGE, as well as LEO spacecraft such as TIMED, have demonstrated the value of FUV imaging, and daytime temperature measurements have been successfully demonstrated using FUV data from the ARGOS satellite in LEO. Performing FUV spectral imaging from geostationary orbit would enable continuous global measurement of these key parameters. Scientific and societal benefits would be maximized using three imagers separated by ~120 degrees of longitude around the equator, providing continuous imaging of the entire Earth. The observations made by these imagers would also provide context for LEO and ground-based observations. However, before a multiple imager mission is attempted, a single imager should be flown to demonstrate the scientific benefits of this concept. 1 Heliophysics Decadal Survey FUV Imaging of Earth’s T-I System 3. Expected scientific benefits The T-I system consists of a weak plasma subsumed in a more dense neutral gas. This mixture of plasma and neutral gas is unusual, as sometimes the plasma drives the neutral gas, while at other times the neutrals drive the plasma. It has proved difficult to understand the T-I region as a weather-like system because most space-based measurements, being from LEO, have an inherent spatial-temporal ambiguity. High inclination orbits observe at a roughly constant local time, whereas low inclination orbits only provide information at a ~90 minute cadence in a limited latitude range. The result of these samplings is an understanding of the T-I system as essentially a set of climate states rather than as a fully interacting weather system. Ground-based data provide important information about the T-I system, but are not sufficient to move our understanding of the region from a climate-like state to a weather-like state. Comprehensive numerical models describing the T-I system have been developed, but they require global measurements for validation and improvement. FUV imaging observations of the entire Earth at a cadence of approximately one hour can provide the data necessary to achieve a comprehensive, weather-like understanding of the T-I region. Such observations would provide unprecedented information about coupling of the T-I system to the lower altitudes. FUV imaging will measure atmospheric tides across the entire dayside. A numerical model of thermospheric temperature changes, as viewed from a geostationary orbit, is shown in Figure 1. Such measurements have not been previously obtained for any region of the atmosphere, because the background signals mask tides in the troposphere and there is no method available to image them in the middle atmosphere. A simulation of the temperatures retrieved from observations using a FUV spectral imager is shown in Figure 2. FUV images provide the ability to separate migrating tides from non-migrating tides so that their temporal evolution can be determined directly, rather than being inferred from slowly changing observations at constant local times. This will allow the effects of planetary waves and other variability on the tides to be determined. FUV imaging from geostationary orbit will also advance our understanding of how the T-I system responds to forcing from above. A key objective is a comprehensive study of the effects of geomagnetic storms on the thermosphere. Tantalizing glimpses of the large-scale changes due to these storms are seen in images from DE-1, IMAGE, and LEO satellites like TIMED. These data show evolving regions where molecular nitrogen densities increase relative to atomic oxygen, but they have not allowed separation of longitudinal and universal time effects during geomagnetic storms on a regular basis. Even less is known about the evolution of temperature during geomagnetic storms. Optical observations of temperatures have been limited to measurements from the AE and DE-2 satellites and some ground-based observations using interferometers. Some information has also been gleaned by observing satellite drag, but this can be difficult to interpret. A limitation of in situ observations, another important source of temperature measurements, is the difficulty in distinguishing storm effects from local time or longitudinal variation. The data from full disk FUV imaging will enable comprehensive analysis of the global changes that occur during storms. A numerical model of such thermospheric temperature changes, as viewed from a geostationary orbit, is shown in Figure 3. 2 FUV Imaging of Earth’s T-I System A second objective is to better understand the effects of variable solar extreme ultraviolet (EUV) radiation on the T-I system. Solar EUV ionizes neutral atoms and ions or is absorbed as part of the process of dissociating molecules in the Earth’s upper atmosphere. The question of whether thermospheric temperature variations are consistent with our knowledge of the ionization and dissociation processes and the variations in input solar radiation can be determined using global observations of neutral temperature. These observations will also enable the effects of solar flares on the thermosphere to be determined. Until now, our understanding of flare effects has been limited by our ability to separate spatial and temporal effects. Full disk observations of the thermosphere will allow these variations to be uniquely determined. FUV imaging will also increase our understanding of ionospheric bubbles, a phenomenon of both scientific and Space Weather customer interest. These bubbles result from the Rayleigh- Taylor instability mechanism operating on the bottom of the F2 layer. The resulting instabilities expand vertically and along magnetic field lines, becoming bubbles. They occur after sunset within the equatorial ionosphere and are particularly prevalent between dusk and midnight. Our present understanding of bubbles is limited by the available observations, which are from either ground-based measurements or along the track of LEO satellites. These techniques have produced a wealth of information about individual bubbles, but they are not suitable for comparative bubble studies during the same set of geophysical conditions. Imaging bubbles from geosynchronous orbit will permit these comparative studies to be performed, allowing for the separation of longitudinal and universal time effects on bubbles, and providing a better understanding of the effects of geomagnetic activity (and penetration electric fields) on the generation of ionospheric bubbles. 4. Societal benefits FUV imager observations from geostationary orbit have important practical applications. For example, this technique will enable observations of low-latitude ionosphere bubbles across the whole disk within 15 minutes or less. This will allow, for the first time, the study of their evolution over the entire visible disk. Such observations will not only aid in understanding their spatial and temporal evolution, but will also