A document by Krishnaprasad Chirakkil. Click on the document to view its contents.

The October 14, 2023 annular solar eclipse was visible from the US Pacific coast to Brazil’s east coast. NASA’s Global-scale Observations of Limb and Disk (GOLD) mission observed the first synoptic thermospheric temperature changes from a geo-stationary orbit above 47.5°W longitude between 17-20 UT during the eclipse. These daytime thermospheric changes were derived using GOLD’s disk FUV measurements. A significant decrease in the daytime disk temperatures (~ 100 K) was seen near the peak annularity compared to the day before (baseline). The temperature reduction’s spatial morphology is also like that of the eclipse shadow. Modelling simulations underestimate the eclipse-induced temperature decrease (expected ~ 30-40 K) by a factor of 2-3 compared to GOLD observations. These first of kind results provide new insight into the dynamic response of the coupled thermosphere and ionosphere system to transient solar events, including eclipses.

Observations of far-ultraviolet (FUV) dayglow by the Global-scale Observations of Limb and Disk (GOLD) mission provide an opportunity for quantifying the global-scale response of the thermosphere to solar extreme-ultraviolet (EUV) variability and geomagnetic activity. Relative temperature changes can be measured by monitoring changes in the rotational structure observed in molecular nitrogen Lyman-Birge-Hopfield (LBH) band emissions. We present a new technique for deriving neutral temperatures from GOLD FUV observations using optimal estimation fits to spectra containing LBH band emissions. We provide an overview of the theoretical basis for the GOLD Level 2 neutral temperature algorithm. Effects on derived neutral temperatures from instrument artifacts and particle background are reviewed. We also discuss GOLD Level 1C DAY and Level 2 TDISK data products and present representative examples of each. We show that neutral temperatures vary with local time, exhibit a strong seasonal dependence, and correlate strongly with geomagnetic and solar activity. Finally, we present results from a preliminary data product validation that show good agreement with coincident GOLD exospheric temperatures and predictions from a global reference atmospheric model.

Observations of far-ultraviolet (FUV) dayglow by the Global-scale Observations of Limb and Disk (GOLD) mission provide a new opportunity to monitor relative composition changes in the upper atmosphere as well as solar extreme ultraviolet (EUV) variability. Relative composition changes are quantified by ΣO/N2, the column density ratio of atomic oxygen to molecular nitrogen, while QEUV provides a measure of the solar EUV energy flux from 1 to 45 nm into the upper atmosphere. This spectral range provides the ionizing radiation which ultimately results in FUV airglow emission produced by photodissociation and photoelectron impact. The quantities ΣO/N2 and QEUV are derived from GOLD FUV observations through lookup tables that are constructed using a first-principles photoelectron transport model. The two FUV emissions used are O I 135.6 nm and the N2 Lyman-Birge-Hopfield (LBH) bands. We present an overview of the theoretical basis for the algorithms and practical considerations for application to GOLD data. The effects of uncertainties in electron impact cross sections, off-nadir viewing, and instrument artifacts are reviewed. We also discuss GOLD Level 1C DAY, Level 2 data products ON2 and QEUV, and present representative samples of each.

The Global-scale Observations of the Limb and Disk (GOLD) Mission images middle thermosphere temperature and the vertical column density ratio of oxygen to molecular nitrogen (ΣO/N2) using its far ultraviolet imaging spectrographs in geostationary orbit. Since GOLD only measures these quantities during daylight, and only over the ~140º of longitude visible from geostationary orbit, previously developed tidal analysis techniques cannot be applied to the GOLD dataset. This paper presents a novel approach that deduces two specified non-migrating diurnal tides using simultaneous measurements of temperature and ΣO/N2. DE3 (diurnal eastward propagating wave 3) and DE2 (diurnal eastward propagating wave 2) during October 2018 and January 2020 are the focus of this paper. Sensitivity analyses using TIE-GCM simulations reveal that our approach reliably retrieves the true phases, whereas residual contributions from tides assumed to be absent, the restriction in longitude, and random uncertainty can lead to ~ 50% error in the retrieved amplitudes. Application of our approach to GOLD data during these time periods provides the first observations of non-migrating diurnal tides in measurements taken from geostationary orbit. We identify discrepancies between GOLD observations and TIE-GCM modeling. Retrieved tidal amplitudes from GOLD observations exceed their respective TIE-GCM amplitudes by a factor of two in some cases.

Mars’ ultraviolet airglow has been used to study its upper atmosphere for over four decades. Identifying variations in emission features has provided information on composition, density and temperature. The Emirates Mars Ultraviolet Spectrometer onboard the Emirates Mars Mission observes Mars’ airglow at Far and Extreme UV wavelengths. Variations in disk emission features are studied, with a focus on O I 1304 Å, CO Fourth Positive Group and C I 1561 Å. All show variations with local time and emission angle as expected. Dawn-dusk asymmetry observed is attributed to local time differences in advection. Variations in the brightness of several dayglow features, including 1304 Å, with irregular shapes are noted in around 25% of the disk images. These display some local time and hemispheric asymmetry in their occurrence rates. Examination of their spatial structure, occurrence, and spectra suggests these are associated with variations in composition and photoelectron flux.

We report six years of observations of dayside temperatures of the middle and upper atmosphere of Mars made by the Imaging Ultraviolet Spectrograph instrument aboard the MAVEN spacecraft. Thermospheric temperatures show strong long-term variability associated with Martian season and solar cycle. Temperatures from both the Martian thermosphere and mesosphere show strong short-term variability indicating coupling from the lower atmosphere. The observed local time effect is strong in both upper and middle atmosphere temperatures. The thermosphere tends to be colder in the morning compared to the evening when temperatures are higher. Middle atmospheric temperatures show cooling during the dawn and dusk hours. Our analysis shows strong tidal activity during aphelion, whereas non-migrating tides are suppressed during perihelion, possibly due to increased dust activity. Observations during the deep minimum of solar cycle 24 reveal that thermospheric temperatures are highly variable with respect to local time if solar forcing, Mars-Sun distance, and spatial effects are removed. We will discuss these results in the context of coupling between the lower and upper atmosphere of Mars.

Travelling ionospheric disturbances (TIDs) and their neutral counterparts known as travelling atmospheric disturbances (TADs) are believed to play a central role in redistributing energy and momentum in the upper atmosphere and communicating inputs to other locations in the fluid. While these two phenomena are believed to be connected, they may not have a one-to-one correspondence as the geomagnetic field influences the TID but has no direct impact on the TAD. The relative amplitudes of the perturbations seen in the ionosphere and atmosphere have been observed but rarely together. This study reports results from a three-day campaign to observe TIDs and TADs simultaneously over a broad latitudinal region over the eastern United States using a combination of GOLD and a distributed network of ground based Global Navigation Satellite System (GNSS) receivers. These results demonstrate that GOLD and the ground-based total electron content (TEC) observations can see the atmospheric and ionospheric portions of a large-scale travelling disturbance. The phase difference in the perturbations to the GOLD airglow brightness, O/N2 and thermospheric disk temperature are consistent with an atmospheric gravity wave moving through this region. The ionospheric signatures move at the same rate as those in the atmosphere, but their amplitudes do not have a simple correspondence to the amplitude of the signal seen in the atmosphere. This campaign demonstrates a proof-of-concept that this combination of observations is able to provide information on TIDs and TADs, including quantifying their impact on the temperature and chemical composition of the upper atmosphere.

Accurate photoionization rates are vital for the study and understanding of ionospheres and may account for the discrepancy in electron densities and mismatched altitude profiles of current E-region models. The underestimation of electron density profiles could be mitigated by high-resolution cross sections that preserve autoionization lines which allow solar photons to leak through to lower altitudes. We present new ionization rates calculated with high-resolution (0.001 nm) O and N2 photoionization and electron impact cross sections, and a high-resolution solar spectrum as inputs to CPI’s Atmospheric Ultraviolet Radiance Integrated Code [AURIC, Strickland et al., 1999]. The new electron impact cross sections show little structure and have minimal effect on calculations of ionization rates. Results from AURIC with updated O and N2 cross sections indicate increased production rates up to ~40% in the E-region, specifically between 100–115 km. Likewise, production rates determined using the ionospheric photoionization rate code from Meier et al. [2007] also illustrate an increase in the O and N2 production rates (typically of more than 10%) when using the newly calculated cross sections. Additionally, we find that O and N2 dominate the volume production rates above 130 km while O2 is expected to be the main contributor from 95–130 km. AURIC model results that use the default data and model results with the new O and N2 cross sections both track very well with electron density profiles determined from Arecibo ISR observations. AURIC model results using the new cross section calculations are in better agreement with Arecibo observations at higher altitudes. Our current findings indicate that O2 plays a dominant role in photoionization production rates in the E-region. Therefore it is crucial to update ab initio ionospheric models with high-resolution photoionization cross sections.

The present investigation evaluates the assimilation of synthetic data which has properties similar to actual Global-scale Observations of the Limb and Disk (GOLD) level-2 (L2) and other conventional lower atmospheric observations. The lower atmospheric and GOLD L2 temperature (Tdisk) are assimilated in the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCMX) using Data Assimilation Research Testbed (DART). It is found that inclusion of the GOLD Tdisk improves the forecast root mean square error (RMSE) and bias by 5% and 71%. When compared to lower atmosphere only assimilation the improvements in RMSE and bias are 20% and 94%. An investigation of the global DW1 and local diurnal tidal characteristics shows that inclusion of the GOLD temperatures improves the DW1 by about 8% and diurnal tide by more than 17%. The percentage improvement in tides is higher at lower thermospheric altitudes. Considerable improvements in the model state are also seen at times and locations where there are no GOLD observations available. These results and the background data assimilation procedure are presented here, which demonstrates that GOLD thermospheric temperature is an excellent dataset which can be used for thermospheric assimilation studies and operational purposes.

Global-scale Observations of Limb and Disk (GOLD) disk measurements of far ultraviolet molecular nitrogen band emissions are used to retrieve temperatures (T$_{disk}$), which are representative of lower thermospheric altitudes. The present investigation studies the response of lower thermospheric temperatures to geomagnetic activities of varying magnitudes. In this study, it has been observed that T$_{disk}$ increases over all latitudes in response to enhanced geomagnetic activity. The increase in temperature is proportional to the strength of the geomagnetic activity and is greater at higher latitudes. Temperature enhancements vary from 10s to 100s of Kelvins from low- to mid-latitudes. Local time behavior shows that pre-noon enhancements in temperatures, during relatively stronger geomagnetic activities, are greater compared to afternoon, which can be attributed to the combined action of daytime dynamics and geomagnetic forcing. This study thus demonstrates the utility of GOLD T$_{disk}$ measurements investigating the effects of dynamical and external forcings in the thermosphere.

Accurate photoionization rates are vital for the study and understanding of planetary ionospheres. Previous model calculations of terrestrial photoionization rates lack sufficient spectral resolution to account for highly structured photoionization cross sections as well as the solar spectral irradiance. We present new photoionization rate calculations from CPI’s Atmospheric Ultraviolet Radiance Integrated Code [AURIC; Strickland et al., 1999] using high-resolution (0.01 Å) solar spectra and high-resolution (0.01 Å) atomic oxygen (O) and molecular nitrogen (N2) photoionization cross sections. Theoretical photoionization cross sections of O are determined utilizing the R-matrix plus pseudo-states (RMPS) approximation whereas N2 cross sections are determined using the R-matrix approximation. We include 34 high-resolution partial O state photoionization cross sections and 3 high-resolution partial N2 state photoionization cross sections with supplemental Conway [1988] tabulations for molecular oxygen and the remaining N2 states. We find that photoionization rates computed at 0.01 Å resolution differ substantially from rates computed using low-resolution cross sections, especially in the lower thermosphere below 200 km. Specifically, we find that ionization production rate ratios exhibit variations in altitude of more than ±40% between the high- and low-resolution cases. Past low-resolution calculations at various low spectral resolutions do not sufficiently account for or preserve the highly structured auto-ionization lines in the photoionization cross sections [Meier et al., 2007]. These features, in combination with high-resolution solar spectra, allow photons to penetrate deeper into the Earth’s atmosphere producing larger total ionization rates. These higher ionization rates may finally resolve data-model discrepancies in altitude profiles of electron densities due to the use of low-resolution photoionization cross sections in current E-region models.