Variations of Lower Thermospheric FUV Emissions Based on GOLD Observations and GLOW Modeling.

Here we compare the global-scale morphology of Earth's the Far-Ultraviolet (FUV) emissions observed by NASA's Global-scale Observations of Limb and Disk (GOLD) mission to those modeled using the Global Airglow (GLOW) code with atmospheric parameters provided by Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM). The O 5S oxygen (135.6 nm) and N2 Lyman-Birge-Hopfield (LBH) emissions are observed over the Western hemisphere every 30 min by the GOLD instrument. The FUV brightness of the thermosphere-ionosphere is expected to vary in systemic ways with respect to geophysical parameters, solar energy input from above, and terrestrial weather input from below. In this paper we examine the O 5S oxygen emission and the N2 LBH emission brightnesses with local time, latitude, season, tides, geomagnetic activity, and solar activity based on GOLD observations and GLOW modeling. Early GOLD observations indicate that the model effectively reproduces the brightness variations with local time and latitude but is biased low in magnitude. However, the TIEGCM is unable to accurately represent the extraordinary nighttime equatorial ionization anomaly observed by GOLD. It is also expected from these results that the signal from geomagnetic storms may obscure tidal signals.

Introduction

Characterization of Earth's ionosphere, and the thermosphere to which it is coupled, is critical for communications and navigational technologies (Knipp et al., 2016). The lower and middle thermosphere and ionosphere (approximately 100 to 400 km) are influenced from both above (e.g., solar inputs) and below (e.g., atmospheric waves and tides). Due to these influences, this region experiences a wide range of dynamic variability, on a large spectrum of spatial scales and temporal scales.

Remote sensing is one of the most practical ways to gain information about the temperature and composition over the necessary spatial and temporal scales required for interpreting the dynamics that impact modern communications and navigational technologies. The Far‐Ultraviolet (FUV), spanning approximately 110–180 nm, is an ideal range to study the weather of this region. When the Earth is viewed from space, the Earth's lower atmosphere is opaque in the FUV, providing a dark background for FUV variations in the thermosphere. Also, the FUV contains spectral signatures of constituents of the lower thermosphere (O and N2), allowing the derivation of temperature and composition.

On the dayside, at 135.6 nm, an atomic oxygen emission line is created by photoelectron impact excitation and radiative recombination of O+, as well as the photoelectron impact excitation of molecular nitrogen generates the LBH emissions. On the nightside, FUV captures the emissions from the F‐region recombination of O+. The solar spectrum that produces the FUV emissions exhibits significant geophysical variability (Paxton et al., 2003) to which the dayglow and nightglow are sensitive (Conway et al., 1988; Strickland et al., 1995).

The technology for observing the Earth in the FUV has been maturing for some five decades. The OGO‐4 ultraviolet spectrometer measured nightglow at tropical latitudes, including the 1304‐ and 1356‐A lines of atomic oxygen (Barth & Schaffner, 1970); additional observations of the aurora were also made in the FUV (Chubb & Hicks, 1970). A far‐ultraviolet camera/spectrograph experiment was operated on the lunar surface during the Apollo 16 mission; this gave the spatial distributions and relative intensities of emissions from atomic hydrogen, atomic oxygen, molecular nitrogen, and other species—Some observed spectrographically for the first time (Carruthers & Page, 1972). Nadir views of the airglow and aurora were made on the polar orbiting S3‐4 satellite in the late 1970s (Huffman et al., 1980). Some of the first global images were made by Spin‐scan Auroral Imaging (SAI) on DE1 (Frank, 1998). Observations at 3.5A resolution using an Fastie‐Ebert monochromator were made from a sounding rocket in the 1980s (Eastes et al., 1985). The technology was pushed forward with the launch of the V5 ultraviolet auroral imager aboard the Viking spacecraft in 1986, which had improved spatial and temporal resolution (Anger et al., 1987). Additional improvements in spatial and temporal resolution were made with the Freja program of the 1990s (Murphree et al., 1994) and the Ultraviolet Imager for the Global Geospace Sciences (Torr et al., 1995) of the International Solar‐Terrestrial Physics program. At the same time, modeling demonstrated the spatial extent and short‐term temporal stability of brightness perturbations in the FUV (Nicholas et al., 1997). Additionally, new models of the emissions in the FUV by Earth's atmosphere by the dominant constituents (Eastes, 2000) were developed. A FUV instrument was flown aboard the NASA Imager for Magnetopause to Aurora Global Exploration (IMAGE) mission that could image Doppler shifted H Lyman alpha, OI 1356‐A, and the N2 LBH emissions (Mende et al., 2000). The Special Sensor Ultraviolet Spectrographic Imager (SSUSI) and the Global Ultraviolet Imager (GUVI) brought advances in the use of a full image cubes (two‐dimensional detectors and full FUV wavelength coverage) and large instantaneous FOV (Paxton et al., 2003). Using a forward model, GUVI is able to derive O, N2, and O2 on the limb (Christensen et al., 2003; Meier et al., 2015; Strickland et al., 2004). On the disk, GUVI derives O/N2 using the algorithm described by Strickland et al. (1995). Current FUV sensor includes the SSUSI‐Lite sensor (Paxton et al., 2017), the Global‐scale Observations of Limb and Disk (GOLD) (Eastes et al., 2017) instrument launched in January 2018, and the Ionospheric Connections Explorer (ICON) FUV instrument (Mende et al., 2017) launched in 2019.

The GOLD instrument was launched aboard the SES‐14 communication satellite on 25 January 2018 and began its observational operations in October of 2018. The mission is located in a geostationary orbit over 47.5°W longitude. GOLD is a far‐ultraviolet (FUV) imaging spectrograph. From the radiances at different spectral bands, thermospheric temperature and composition can be derived. The equatorial ionospheric structure and plasma density are also provided (Eastes et al., 2017). Here we describe the FUV emissions predicted to be observed by GOLD and some actual GOLD observations for comparison. In this paper, we use the Global Airglow (GLOW) model to calculate the 135.6 nm and LBH emissions which are observed in thermosphere by the GOLD mission. The atmosphere itself is modeled by the TIEGCM. GOLD has a unique vantage point from geostationary orbit that will be able to separate the spectrums of spatial and temporal variability.

Methodology

GOLD is an imaging spectrometer, and the returned image cubes (one spectral and two spatial dimensions) contain information about the geographic position, time, and wavelength of atmospheric emissions. Two models are used in this study along with early observations from GOLD. The Global Airglow (GLOW) model calculates the Far‐Ultraviolet and other atmospheric emission features for a given atmosphere (Solomon, 2017). The upper atmosphere used as input to GLOW is modeled by the TIEGCM.

TIEGCM

The Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) is a self‐consistent, numerical model of Earth's upper atmosphere (Richmond et al., 1992; Qian et al., 2014). Here we use the TIEGCM‐ICON version at a 2.5° resolution. This version of TIEGCM was developed in support of the ICON mission and is described more thoroughly by Maute (2017) and Greer et al. (2018).

GLOW

The Global Airglow (GLOW) model calculates various FUV emissions of the upper atmosphere (Solomon, 2017). Of particular interest here are the Lyman‐Birge‐Hopfield (LBH) bands of molecular nitrogen and the atomic doublet at 135.6 nm that are observed by the GOLD mission. Please see Solomon (2017) and references therein for more details. Model codes are freely available at the HAO/NCAR website (https://www2.hao.ucar.edu).

Figure 1 compares GOLD observations with GLOW calculations for 15 December 2018 at 9 UT for both the 135.6 nm oxygen emission (O 5S) and the LBH emissions. F10.7 was 72 Solar Flux Units (SFU), and Kp index was 0. For the GOLD observations, the wavelengths around the 135.6 nm spectral emission peak are summed to obtain the O 5S feature brightness, while the wavelengths between 141 and 153 nm, excluding the 149.3 nm atomic nitrogen feature, are summed to find the LBH bands emission brightness. For this GLOW calculation, the atmospheric state and dynamics were modeled by TIEGCM, which was run for 15 December 2009 (solar minimum conditions) with F10.7 at 70 SFU, and all other solar indices set to low solar activity. Next the emission information was processed for the geometry of an instrument located in geostationary orbit. Lastly, the absorption by O2 along a line of sight is included in the model calculation. The terminator is the division between night and day on the Earth, and the dawn terminator is seen on the right side of each disk image. The 15‐min difference between the time of the upper disk and lower disk scans can be seen in each GOLD image, although it is more pronounced in the O 5S data. There is some minor auroral activity in the Northern Hemisphere in the GOLD observations that is not seen in calculated brightnesses from GLOW for this date and time because TIEGCM was run without auroral activity inputs. Several stars can be seen in the lower left corner of the GOLD observations. The remainder of this paper examines the FUV emissions in both the model and the observations, in detail, as well as comparing them.

Figure 1

Panels display GOLD observations and TIEGCM/GLOW modeling for 15 December 2018 at 9 UT for both the 135.6 nm oxygen emission (O 5S) and the LBH short bands emissions. F10.7 was 72 SFU, and Kp index was 0. The color scale is in Rayleighs and saturates at 250 R (500 R) in the O 5S (LBH) emission.

Results and Discussion

The general characteristics and evolution of the FUV emissions over a day need to be characterized in order to address more in‐depth studies of the effect of waves, tides, and other phenomena. Here we address some of the calculated characteristics of O 5S and LBH emissions.

Shown in Figure 2 are the GOLD observations (solid line) and GLOW calculations (dotted line) of the 135.6 oxygen emissions, O 5S, for 15 December 2018 at 12 UT and 16 UT at three different row positions on the detector. The three positions on the detector roughly correspond to 23°N, the equator, and 23°S latitude. The globes on the left show the location of the data on the disk, while in the plots on the right, the x‐axis is longitude and the y‐axis is emission brightness (R). The GOLD observations are the average (median) of three adjacent rows on the detector. The nadir point of the satellite is at −47.5°E longitude. In the 12 UT panels the terminator is on the left side of the disk, while at 16 UT the terminator is now on the right side. At a given universal time, the longitudinal variation also corresponds to the local time variation (or solar zenith angle variation) across the disk of the Earth. This local time change with longitude can be seen in the variation in the brightness modeled and observed across the disk of the Earth. The variation of brightness as a function of longitude of the observations and model match very well. However, the magnitude of the variations in the observations and model are different. At 12 UT, the differences range from less than 10% to nearly 25% at the southern latitude. At 16 UT, the magnitude differences are even greater: at the equator and northern hemisphere latitude of up to 60% different and up 100% different at 23°S latitude.

Figure 2

Panels display GOLD observations (solid line) and GLOW modeling (dotted line) for 15 December 2018 at 12 UT and 16 UT of the 135.6 oxygen emission (O 5S).

There are several possible reasons for the differences between the modeled brightnesses and the GOLD observations. The modeled brightnesses depend on the composition of the atmosphere, which in turn depend on the neutral dynamics, temperatures, O2 density, and dissociation rates of the models. The selected North and South latitudes are where the tidal effects can be significant. Disparities between the Horizontal Wind Model (HWM07, Drob et al., 2008), which drives the lower boundary of the TIEGCM in these simulations, and the actual propagating tides may have a strong effect at these latitudes. It is also known that the TIEGCM is better tuned to the real atmosphere in the northern hemisphere (Maute, 2017); this may account for some of the latitudinal difference. For the GLOW calculations, the emission cross sections for Oxygen OI 135.6 nm and the LBH band system have uncertainties as high as 30% (Meier et al., 2015; Solomon, 2017), which could also introduce a systematic bias.

Figure 3 is the same as Figure 2 but for the LBH emission; these are for again solar minimum and geomagnetically quiet conditions. The brightness drops off faster (than the 135.6 nm emission) at large solar zenith angles; this is because the LBH emission requires photoelectron impact on the nitrogen molecules. Where there is less sunlight (at higher solar zenith angles), there is less nitrogen emission. Many of the same factors that result in differences between the GLOW calculation and O 5S observations also affect the LBH bands, and again, there is uncertainty in the nitrogen cross section of up to 25% (Meier et al., 2015; Solomon, 2017).

Figure 3

Panels display GOLD observations (solid lines) and GLOW modeling (dotted lines) for 15 December 2018 at 12 UT and 16 UT for the LBH band emissions.

Additional factors that may influence the longitudinal and local time variability of the FUV emissions include the terminator, the limb of the Earth, tides, geomagnetic latitude, and the equatorial ionospheric anomaly (Appleton anomaly). Both the terminator and the limb have been shown in the previous figures; the dayside limb is typically brighter than at the subsolar point. As the instrument views off nadir toward the limb, the path through the atmosphere becomes longer and contains more emitters of FUV light.

Figure 4 shows the annual cycle of LBH emissions (O 5S not shown) calculated by GLOW (solid line) for solar minimum, geomagnetically quiet conditions compared to the first year of GOLD observations (dotted line). Emissions at 23°N, the geographic equator, and 23°S are shown. The equator (red) has maxima at the equinoxes and minima at the solstices. The 23°N location (blue) exhibits a peak in June and a minimum in December, while the 23°S location (green) displays a maximum in January and a minimum in June. The 23°S peak is slightly stronger than the 23°N peak. The early GOLD observations are suggestive of GLOW calculations being correct in shape, if not magnitude, as explained around Figures 2 and 3.

Figure 4

LBH emission brightness at 15 UT throughout the year for 23°N (blue), 0°N (red), and 23°S (green). The TIEGCM model F10.7 parameter was set to 70 SFU, and Kp index was set to 0. The solid line is the model while the dotted line is early GOLD observations.

A well‐known and prominent feature seen in FUV emissions of Earth is the Equatorial Ionization Anomaly (EIA). It appears in the FUV as two crests of oxygen emission at about ±15° latitude from the geomagnetic equator. The processes that govern the development of the EIA involve the upward ExB drift of plasma near the equator (e.g., Balan et al., 2018; Lin et al., 2007) which produces the crests of ionization. Figure 5 compares the EIA simulated by TIEGCM and the nightglow emissions calculated by GLOW with observations from GOLD on 15 November 2018 at 23 UT. The O 5S emission is a proxy for plasma in the EIA. The geomagnetic equator (Burrell et al., 2018) is shown as a green line. The EIA is more readily identifiable in November than on 15 December 2018 because the EIA is stronger, as is typical nearer equinox (Liu & Watanabe, 2008). At night, the EIA is seen in GOLD observations as two crests of emission that extend across the Atlantic Ocean until local times near midnight, sometimes even into the morning hours. However, while an EIA is present in GLOW calculations, it is less bright, the crests are less distinct, the symmetry between crests is lacking, and the plasma crests do not extend as far over the Atlantic. The difference between the observations and the GLOW calculations is due to the TIEGCM model that generates the EIA. That TIEGCM does not properly represent that the nighttime EIA is a known problem of the model. One possible reason that TIEGCM is not properly representing the nighttime EIA is that the model is not lofting the plasma high enough. If the plasma is not lofted high enough, it experiences recombination more quickly resulting in lower plasma densities in the EIA.

Figure 5

O 5S emission signal at 23 UT showing the equatorial ionospheric anomaly (EIA) for 15 November. F10.7 was 73 SFU and Kp index was 1. The geomagnetic equator is plotted as a green line.

During quiet solar and geomagnetic activity conditions, tides are a strong source of variability of the thermosphere, up to 60–80% of the variability (Gasperini et al., 2015). TIEGCM's lower boundary is forced by HWM07 (Drob et al., 2008) to introduce migrating and non‐migrating tides. However, by turning these tides off we can examine the impact they have on the FUV emissions of the thermosphere.

Figure 6 shows the contribution of the migrating and non‐migrating (both diurnal and semi‐diurnal) tides to the O 5S and LBH emission brightnesses (R) and the effective temperature (K) at 12 UT and 19 UT. The effective temperature is a weighted column integrated temperature given by where this is a function of the viewing geometry angles Χ and Ψ, and T(s) is temperature along the line of sight, η(s) is the emission contribution function along the line of sight, and B(Χ,Ψ) is the emergent brightness at a specific observation angles. Teff takes into account observational geometry and any O2 absorption. Without use of equation (1), an estimate of the effective temperature can be obtained by taking the absolute temperature at the altitude peak of the emission feature (near 155 km). This is because the temperature at this emission peak contributes the most to the weighted column integrated effective temperature.

Figure 6

Contribution of migrating and non‐migrating tides in TIEGCM to the O 5S and LBH emission brightnesses (Rayleighs) and the effective temperature (K) at both 12 UT and 19 UT.

Evident in Figure 6 is the anticorrelation of O 5S and LBH emissions (see over Africa at 12 UT and the Eastern Pacific at 19 UT). This anticorrelation is expected due to differential advection of O and N2 in the lower thermosphere (Greer et al., 2018; Hickey et al., 2015). Without the diurnal and semidiurnal migrating and non‐migrating tides, up to 50 R of the O 5S emission and up to 20 R of the LBH emission variability is missing. In addition, the tides induce a variability of ±50 K in the effective temperature. The structure in Teff is reminiscent of the diurnal eastward wave number 3 (DE3) tide. This tide has significant consequences for the thermosphere and ionosphere (e.g., Immel et al., 2006; Pedatella et al., 2012). Other tides that appear to be critical for this region and the FUV emissions are diurnal Westward wave number 2 (DW2), diurnal Eastward wavenumber 2 (DE2), and diurnal Westward wavenumber 5 (DW5).

The change in brightnesses for the O 5S and LBH emissions (R) and the effective temperature change (K) at 12 UT and 19 UT between a geomagnetic quiet and a geomagnetic active day (Kp = 7) at solar minimum for a 15 December day is shown in Figure 7. The effect of geomagnetic activity is some 10 times greater than that of the tides. According to models, this level of geomagnetic activity is expected to contribute a variability of greater than 800 R to the O 5S signal and more than 320 R to the LBH signal. Additional energy input from the geomagnetic storm elevates the effective temperature by 300 K or more and increases the variability of the FUV emissions, especially in the high latitudes.

Figure 7

The change in brightnesses for the O 5S and LBH emissions (Rayleighs) and the effective temperature change (K) at 12 UT and 19 UT between a geomagnetic quiet day and a geomagnetic active (Kp = 7) 15 December day. The green line is the geomagnetic equator.

Figure 8 shows the spatial gradients in O 5S and LBH emission and effective temperature for solar maximum conditions. The structure of the emissions across the disk is similar to solar minimum although it is nearly a kR brighter. The effective temperature of the thermosphere is also hotter, by some 250 K. The transition across the terminator is more abrupt at solar maximum. The greater gradients at the terminator may induce solar terminator waves around the June solstice (Forbes et al., 2008; Liu et al., 2009). As we move into solar maximum, we can expect a brighter disk as viewed by GOLD.

Figure 8

The change in brightnesses for the O 5S emission (Rayleighs), LBH emission (Rayleighs), and the effective temperature (K) at between a solar maximum day (F10.7 = 210) and a solar minimum day (F10.7 = 70) for 15 December at 12 UT and 19 UT.

Conclusions

Using the models TIEGCM and GLOW, the brightness of the Earth's disk in the O 5S 135.6 nm oxygen emission and the LBH band emission from geostationary orbit showed expected variability in local time, latitude, season, tides, geomagnetic activity, and solar activity. The model trends match the observations well, if not the absolute magnitude. However, the modeling failed to adequately portray the extraordinary nighttime EIA, especially its distinct crests that extend well into the low latitude nightside. The calculated brightness of the O 5S emission was shown to vary by ±45 R due to tides alone. Increasing solar activity significantly increased the brightness of the disk, while increased geomagnetic activity generated brightness changes of up to ±400 R. Future observations will contribute to improving models, and eventually, forecasts of ionospheric weather anomalies impacts on communications and navigation.