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.

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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 provide a short term forecast capability for determining communication and GPS outages over the visible disk.

Another practical application of an FUV imager is to provide sufficient data to drive assimilation models of the thermosphere and ionosphere. Successful assimilation models of the ionosphere have been developed and are currently used by the USAF for situational awareness. However, a shortage of thermospheric data has prevented these models from being deployed for the neutral atmosphere. A set of FUV imagers would provide an ideal data set for these assimilation models, enabling improved electron and neutral density specification and forecasts. In turn, better electron density forecasts would allow more reliable estimation of the likelihood of communication and navigation disruption, while better neutral density forecasts would improve estimates of the risks associated with space debris in LEO.

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FUV Imaging of Earth’s T-I System

5. Methodology

The ideal FUV spectral imager would have two channels. One channel would perform routine scanning of the whole observable disk in the far ultraviolet, while the other channel would be used to either improve the overall sampling rate or for executing specialized campaigns. An example of the latter would be imaging small regions for bubbles in order to study their temporal evolution at very high cadence.

The FUV imager can also scan the limb to gather information about the height variations of O2 density using stellar occultations. Bright stars are occulted sufficiently often to permit these observations to be made almost hourly, providing valuable data for understanding how composition evolves during geomagnetic storms and flares.

The imager can be hosted on commercial communications satellites. Such satellites are capable of providing the data stream in real time, which makes the data ideal for either direct application or for inclusion in assimilation models. Approximate costs are given in Section 7.

6. Straw-man mission

A proof-of-concept of the mission can be undertaken with a single imager. One imager can study, over most of one hemisphere, the spatial and temporal variations during geomagnetic storms as well as the effects of changing EUV radiation. A single imager would also observe upwardly propagating tides and enable the separation of their migrating and non-migrating components. In addition, it would allow bubble evolution to be studied on a global-scale, permitting an understanding of their temporal and spatial evolution that has not been possible previously. Flying three imagers would provide global observations and the more complete data set that assimilation models need to drive our theoretical understanding of the upper atmosphere.

7. Cost estimate

The total mission cost for one imager would be approximately $50 million (FY11 dollars), including accommodation costs, instrument development and the science investigation. This includes the approximately $10 million cost per imager for accommodation on commercial communication satellites. Most of the remaining costs are for the development and fabrication of the imagers. Flying a complete mission, consisting of three identical imagers, would reduce the cost per imager.

8. Why are these studies important?

FUV imaging of the Earth’s thermosphere-ionosphere from geostatonary orbit will provide breakthrough science. This poorly-understood region has characteristics that are part plasma and part neutral. Properly understanding how the neutral and charged elements interact is only possible if the temporal and spatial variations can be measured simultaneously, allowing the region to be treated as a weather regime rather than a set of climate states. Furthermore, data from dedicated LEO missions, flown in conjunction with the imagers, could be more readily placed into a global context.

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Calculated Images of Temperatures & Temperature Changes Due to Tides

K K

Temperatures Calculated using TIEGCM (a) Temperature Difference (b) With Tides TIEGCM With-Without Tides

• Temperatures and densities in the thermosphere are calculated using the TIEGCM model with and without tides from the GSWM model • For lines-of-sight from geostationary orbit, temperatures and densities from the TIEGCM

results are combined with emission profiles derived using AURIC to calculate the N2 emission temperature that would be seen from geostationary orbit (a, shown on the left). • Line-of-sight temperatures approximately represent the temperatures at 160 km and tides produce ~30 K perturbations in the calculated temperatures. Temperature change due to tides is shown on the right (b). Storm-time increases can be several times larger. Figure 1 5 Retrieved Temperatures from Simulated Observations

K • Simulated observation of tidal effects in the thermosphere. Observations are simulated by first calculating synthetic LBH spectra, running the spectra through an instrument simulator to account for instrumental effects (with instrument sensitivity derated by factor of 2) to generate simulated data, and finally retrieving temperatures from the simulated data. Retrieved Temperatures from Simulated Observations With Tides

Figure 2

6 Storm-time Increases in Neutral Temperatures

• Same as Figure 1b, but for storm-time K temperature changes. Differences between the neutral temperatures, storm-time – quiet-time, are shown. • Temperatures and densities in the thermosphere are calculated using the TIEGCM model with and without the storm. Then, for lines-of-sight from geostationary orbit, temperatures and densities from the TIEGCM results are combined with emission profiles derived using AURIC to calculate the

N2 emission temperature that would be observed.

Figure 3

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