Neutral Thermosphere Research Articles

Ionospheric irregularities at Jupiter observed by JWST

Jupiter’s upper atmosphere is composed of a neutral thermosphere and charged ionosphere. In the latter, the dominant molecular ion H3+ emits in the near-infrared, allowing for the remote exploration of the physical properties of the upper atmosphere. However, the Jovian low-latitude ionosphere remains largely unexplored because H3+ emissions from this region are faint and spectrally entangled with bright neutral species, such as CH4. Here, we present James Webb Space Telescope H3+ observations of Jupiter’s low-latitude ionosphere in the region of the Great Red Spot, showing unexpected small-scale intensity features such as arcs, bands and spots. Our observations may imply that the low-latitude ionosphere of Jupiter is strongly coupled to the lower atmosphere via gravity waves that superimpose to produce this complex and intricate morphology.

Asymmetric Ionospheric Jets in Jupiter's Aurora

AbstractSimultaneous infrared observations of and H2 emissions from Jupiter's northern aurora using the Near Infrared Spectrograph at Keck Observatory were used to measure the ionospheric and thermospheric wind velocities. ions supercorotate near the dawn auroral oval and subcorotate across the dusk sector and in the dawn polar region relative to the planetary rotation rate, broadly in agreement with past observations and models. An anticyclonic vortex is discovered in H2 flows, closely matching the mean magnetospheric subcorotation when the observed magnetospheric flows are averaged azimuthally. In comparing ion and neutral winds, we measure the line‐of‐sight effective ion drift in the neutral reference frame for the first time, revealing two blue‐shifted sunward flows of ∼2 km/s. Observed and H2 emissions overlap with predictions of the Pedersen conductivity layer, suggesting two different regions of the ionosphere: (a) a deep layer, where neutral forces dominate the thermosphere and symmetric breakdown‐in‐corotation currents can close, and (b) a higher layer, where the observed effective ion drift allows dawn‐to‐dusk Pedersen currents within the upper atmosphere, in turn closing asymmetric currents within the magnetosphere. This ionospheric structure aligns well with recent Juno observations of Jupiter's aurora. The detected thermospheric vortex implies the driving of neutral flows by the momentum from the magnetosphere within the thermosphere and deeper in the atmosphere to potentially 20 mbar. Jovian neutral thermosphere might bridge the gap between current observations and modelings and perhaps be significant to the dynamics of aurora on Earth and other outer planets.

Effects of the 10 September 2017 Solar Flare on the Density and Composition of the Ionosphere of Mars: Significance of Thermospheric Changes

AbstractThe impact of solar flares on the upper atmosphere of Mars presents an opportune environment for investigating the ionosphere's response to the rapid and high‐energy dynamics of our Sun, with implications for characterizing atmospheric escape and evolution. The ionosphere of Mars is strongly influenced both by the levels of ionizing solar irradiance it receives and the associated behavior of the neutral thermosphere within which it is embedded. Here, we characterize density and composition changes in the topside ionosphere of Mars due to the X8.2‐class solar flare on 10 September 2017 from MAVEN spacecraft observations roughly 90 min after the onset of flare irradiance at Mars, by which time the neutral thermosphere had experienced significant thermal expansion. By contrasting altitude and pressure‐based representations of the ionospheric changes during the flare with the neutral thermospheric changes in both representations and basic photochemical expectation, we interpret and quantify the ionospheric behavior through a lesser explored lens of the neutral thermosphere's response to a flare. We find that the high‐altitude ionospheric density enhancements during this flare are larger than many identified from other flares, and that the neutral thermosphere's expansion accounts for a majority of the cumulative ion density changes observed. The ion density and compositional changes exhibit corresponding behavior with the neutral thermosphere's changes from both thermal expansion and photochemical effects. The identified characteristics of the ionosphere's response to a solar flare provide observational benchmarks for assessing and aiding numerical modeling efforts in accurately reproducing the behavior of the environment of Mars.

Dynamical Heating in the Martian Thermosphere

AbstractDynamical heating and cooling are prominent features of planetary atmospheres resulting in thermospheric structures on Venus, Earth and Mars. The purpose of this study is to determine the location and amplitude of localized heating regions in the Martian thermosphere, confirm that they occur in regions of wind convergence and compare the observed dynamical heating with that predicted by a global thermospheric model. This investigation uses several years of data from the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) mission including observations made by the Neutral Gas and Ion Mass Spectrometer (NGIMS) as well as the Extreme Ultraviolet Monitor (EUVM). Specifically, the analysis focuses on several years of horizontal wind, temperature, and composition data. EUVM measurements provide a solar forcing context for the neutral thermosphere data sets and aid in the statistical analysis. Statistical results are compared with two versions of the Mars Global Ionosphere Thermosphere (M‐GITM) global circulation model: one that includes gravity wave parametrization and a version without gravity wave effects. Data analysis indicates that heating features exist around 2–3 and 17–18 local solar time. These locations coincide with regions of converging winds and are in better agreement with M‐GITM when a gravity wave parametrization is included in the model. An oscillation in the observed wind field also results in convergence and a density enhancement near 15 local time. While a similar oscillation is reproduced by the model, the amplitude is much lower than observed and may be a result of modeled zonal winds that are too low.

Neutral Composition Information in ICON EUV Dayglow Observations.

Since the earliest space‐based observations of Earth's atmosphere, ultraviolet (UV) airglow has proven a useful resource for remote sensing of the ionosphere and thermosphere. The NASA Ionospheric Connection Explorer (ICON) spacecraft, whose mission is to explore the connections between ionosphere and thermosphere utilizes UV airglow in the typical way: an extreme‐UV (EUV) spectrometer uses dayglow between 54 and 88 nm to measure the density of O+, and a far‐UV spectrograph uses the O 135.6 nm doublet and N2 Lyman‐Birge‐Hopfield band dayglow to measure the column ratio of O to N2 in the upper thermosphere. Two EUV emission features, O+ 61.6 and 83.4 nm, are used for the O+ retrieval; however, many other features are captured along the EUV instrument's spectral dimension. In this study, we examine the other dayglow features observed by ICON EUV and demonstrate that it measures a nitrogen feature around 87.8 nm which can be used to observe the neutral thermosphere.

Inferring thermospheric composition from ionogram profiles: a calibration with the TIMED spacecraft

Abstract. We present a method for augmenting spacecraft measurements of thermospheric composition with quantitative estimates of daytime thermospheric composition below 200 km, inferred from ionospheric data, for which there is a global network of ground-based stations. Measurements of thermospheric composition via ground-based instrumentation are challenging to make, and so details about this important region of the upper atmosphere are currently sparse. The visibility of the F1 peak in ionospheric soundings from ground-based instrumentation is a sensitive function of thermospheric composition. The ionospheric profile in the transition region between F1 and F2 peaks can be expressed by the “G” factor, a function of ion production rate and loss rates via ion–atom interchange reactions and dissociative recombination of molecular ions. This in turn can be expressed as the square of the ratio of ions lost via these processes. We compare estimates of the G factor obtained from ionograms recorded at Kwajalein (9∘ N, 167.2∘ E) for 25 times during which the Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED) spacecraft recorded approximately co-located measurements of the neutral thermosphere. We find a linear relationship between G and the molecular-to-atomic composition ratio, with a gradient of 2.55±0.40. Alternatively, using hmF1 values obtained by ionogram inversion, this gradient was found to be 4.75±0.4. Further, accounting for equal ionisation in molecular and atomic species yielded a gradient of 4.20±0.8. This relationship has potential for using ground-based ionospheric measurements to infer quantitative variations in the composition of the neutral thermosphere via a relatively simple model. This has applications in understanding long-term change and the efficacy of the upper atmosphere on satellite drag.

Effects of the 10 September 2017 Solar Flare on the Density and Composition of the Thermosphere of Mars

AbstractThe effects of solar flares on the upper atmosphere of Mars are large but poorly constrained by observations. These effects are an important aspect of the response of Mars to space weather events and may also influence the escape of volatiles from Mars, particularly in the solar system's early history. Here we report the effects of the X8.2 flare on 10 September 2017 on the density and composition of the thermosphere of Mars. This analysis uses neutral number densities of He, O, N2, CO, Ar, and CO2 from the MAVEN Neutral Gas and Ion Mass Spectrometer (NGIMS). From these observations, we investigate how the enhanced solar irradiance during the flare produced changes in the neutral upper atmosphere of Mars due to atmospheric heating and photochemistry. Flare‐produced photochemical changes in the neutral thermosphere of Mars have been previously implied but not quantified. We find that at fixed altitudes, the number densities of all species barring He increase and the O/CO2 ratio decreases by ∼15–45%, indicating thermal expansion. However, when viewed at fixed total number densities, the densities of CO2 and Ar decrease, and the O/CO2 ratio increases by up to a factor of ∼3. The photodissociation of CO2 is one photochemical process that produces changes resembling those identified. The quantified changes will help to constrain the shifting chemical makeup of the upper atmosphere due to these impactful flare events and may aid modeling of flare behavior and the impact of flares on the evolution of the Martian climate.

Tidal Wave-Driven Variability in the Mars Ionosphere-Thermosphere System.

In order to further evaluate the behavior of ionospheric variations at Mars, we investigate the Martian ionosphere-thermosphere (IT) perturbations associated with non-migrating thermal tides using over four years of Mars Atmosphere and Volatile Evolution (MAVEN) in situ measurements of the IT electron and neutral densities. The results are consistent with those of previous studies, namely strong correlation between the tidal perturbations in electron and neutral densities on the dayside at altitudes ~150–185 km, as expected from photochemical theory. In addition, there are intervals during which this correlation extends to higher altitudes, up to ~270 km, where diffusive transport of plasma plays a dominant role over photochemical processes. This is significant because at these altitudes the thermosphere and ionosphere are only weakly coupled through collisions. The identified non-migrating tidal wave variations in the neutral thermosphere are predominantly wave-1, wave-2, and wave-3. Wave-1 is often the dominant wavenumber for electron density tidal variations, particularly at high altitudes over crustal fields. The Mars Climate Database (MCD) neutral densities (below 300 km along the MAVEN orbit) shows clear tidal variations which are predominantly wave-2 and wave-3, and have similar wave amplitudes to those observed.

Electron Temperature Response to Solar Forcing in the Low‐Latitude Martian Ionosphere

AbstractElectron temperatures are a key parameter for understanding the ionosphere‐thermosphere system. This study uses in situ data from the MAVEN mission to investigate the level of coupling between the Martian ionospheric electrons and the neutral thermosphere below the altitude of 190 km. Such coupling is responsible for significant diurnal variations and dawn/dusk asymmetries in both the neutral and plasma portions of the ionosphere‐thermosphere system. Earlier theoretical studies suggested that neutral species controlled the electron temperature by collisional cooling thus implying a diurnal trend in electron temperatures due to the diurnal variability of the neutrals. This hypothesis is evaluated across the dayside ionosphere in the present work. Observations of neutral densities confirm that neutral‐electron collisions are the primary process controlling the cooling of electrons in the Martian ionosphere below ~190 km. The consequence of this is that electron temperatures can be anticorrelated with the extreme ultraviolet (EUV) irradiance due to enhanced electron cooling brought about by an expansion of neutral densities. The results also indicate that in the afternoon sector, neutral densities are less responsive to changes in EUV. This might be associated with enhanced winds leading to dynamic cooling that mitigates the thermal expansion of the thermosphere in the afternoon region. Subsequently, the electron temperature as a function of EUV is relatively uncorrelated in the afternoon sector.

First Evidence of Persistent Nighttime Temperature Structures in the Neutral Thermosphere of Mars

AbstractUsing two Mars years of data collected by the Neutral Gas and Ion Mass Spectrometer on the Mars Atmosphere and Volatiles EvolutioN spacecraft, we reconstruct the local solar time structure of the Martian equatorial thermosphere for the dawn and dusk sectors. The results indicate the presence of several persistent features near the dusk and dawn terminators appearing in the neutral temperature and in the O, Ar, N2, and CO2 densities. The dusk temperature features include a minimum at the terminator surrounded by two local maxima with amplitudes between 20 and 40 K. A nighttime temperature enhancement occurs at a local solar time of 4–5 hr and has an amplitude between 50 and 100 K relative to the surrounding temperatures. The observed enhancements are interpreted to be a result of either nighttime dynamical heating caused by converging and downwelling winds or of a terminator wave originating in the lower atmosphere.

Ionospheric Electron Content During Solar Cycle 23

AbstractThe solar minimum period between solar cycles 23 and 24 has generated extensive study, in part because the changes in the upper atmosphere and ionosphere were so dramatic. Thermospheric density during 2008–2009, at the benchmark 400‐km altitude, was shown to be approximately 30% lower than the previous solar minimum during 1996, in observations and in model simulations. Ionospheric densities were also estimated to be lower, by 10% to 20%. However, earlier analyses using data from the global network of Global Navigation Satellite System receivers did not find any significant reduction in global mean total electron content. Recent work reexamining those data using a limited set of receivers, but with consistent processing, identified a 19% reduction (Emmert et al., 2017, https://doi.org/10.1002/2016JA023680). In this study, we compare model simulations of the thermosphere‐ionosphere system during the 2008–2009 solar minimum to its predecessor and estimate a 16% reduction in global mean total electron content. This is consistent with the neutral thermosphere density change seen in model simulations and in satellite drag observations. In the model simulations, the primary driver is a 10% reduction in solar extreme ultraviolet irradiance, with a smaller contribution from lower geomagnetic activity, and a very small decrement due to increase in atmospheric carbon dioxide.

Thermospheric Expansion Associated With Dust Increase in the Lower Atmosphere on Mars Observed by MAVEN/NGIMS

AbstractWe present for the first time the dramatic variations in atmospheric composition and density at high altitudes from 170 to 220 km in Mars' neutral thermosphere in response to dust increases in the lower atmosphere, observed by the in situ Neutral Gas Ion Mass Spectrometer onboard the Mars Atmosphere and Volatile EvolutioN satellite. The observations reveal that CO2, Ar, N2, CO, and O densities all increase up to ~200% compared to the longer‐term running median densities. The density increases are seen throughout this altitude region, and the relative variations are seen to be stronger at higher altitudes. Density increases indicating the solar extreme ultraviolet influence are also seen. This study is consistent with that during increased dust load, the whole atmosphere expands and rises, and other processes may also be involved. The results agree with the general circulation model predictions, providing observational evidence for how dust increases affect different atmospheric species in the thermosphere.

Variations in Ionospheric Peak Electron Density During Sudden Stratospheric Warmings in the Arctic Region

AbstractThe focus of the paper is the ionospheric disturbances during sudden stratospheric warming (SSW) events in the Arctic region. This study examines the ionospheric behavior during 12 SSW events, which occurred in the Northern Hemisphere over 2006–2013, based on vertical sounding data from DPS‐4 ionosonde located in Norilsk (88.0°E, 69.2°N). Most of the addressed events show that despite generally quiet geomagnetic conditions, notable changes in the ionospheric behavior are observed during SSWs. During the SSW evolution and peak phases, there is a daytime decrease in NmF2 values at 10–20% relative to background level. After the SSW maxima, in contrast, midday NmF2 surpasses the average monthly values for 10–20 days. These changes in the electron density are observed for both strong and weak stratospheric warmings occurring at midwinter. The revealed SSW effects in the polar ionosphere are assumed to be associated with changes in the thermospheric neutral composition, affecting the F2‐layer electron density. Analysis of the Global Ultraviolet Imager data revealed the positive variations in the O/N2 ratio within the thermosphere during SSW peak and recovery periods. Probable mechanisms for SSW impact on the state of the high‐latitude neutral thermosphere and ionosphere are discussed.

Diurnal Variations in Global Joule Heating Morphology and Magnitude Due To Neutral Winds

AbstractIn the polar ionosphere, variations in Joule heating are significantly controlled by changes in plasma convection, such as that brought about by changes in the interplanetary magnetic field. However, another important consideration when calculating Joule heating is the velocity difference between this plasma and the neutral thermosphere colocated with the ionosphere. Neutral wind data are often difficult to obtain on a global scale; thus, Joule heating has often previously been calculated assuming that neutral velocities are small and can therefore be neglected. Previous work has shown the effect of neutral winds on Joule heating estimations to be more significant than originally thought; however, the diurnal variations of the neutrals due to changes in solar pressure gradients and Coriolis forces have yet to have their impact on Joule heating assessed. We show this universal time effect to be significant in calculating Joule heating and thus can differ significantly from that calculated by neglecting the neutrals. In this study, we use empirical models for the neutral wind, conductivities, and magnetic field to create Northern Hemispheric patterns of Joule heating for approximately 800,000 individual plasma convection patterns generated using data from the Super Dual Auroral Radar Network. From this, a statistical analysis of how Joule heating varies in morphology and magnitude with universal time is shown for differing seasons and levels of geomagnetic activity. We find that neutral winds do play a significant role in the morphology and total energy output of Joule heating.

Observations of storm time midlatitude ion‐neutral coupling using SuperDARN radars and NATION Fabry‐Perot interferometers

AbstractIon drag is known to play an important role in driving neutral thermosphere circulation at auroral latitudes, especially during the main phase of geomagnetic storms. During the recovery phase, the neutrals are known to drive the ions and generate ionospheric electric fields and currents via the disturbance dynamo mechanism. At midlatitudes, the precise interplay between ions and neutrals is less understood largely because of the paucity of measurements that have been available. In this work, we investigate ion‐neutral coupling at middle latitudes using colocated ion drift velocity measurements obtained from Super Dual Auroral Radar Network radars and neutral wind velocity and temperature measurements obtained from the North American Thermosphere Ionosphere Observing Network (NATION) Fabry‐Perot interferometers. We examine one recent storm period on 2–3 October 2013 during both the main phase and late recovery phase. By using ion‐neutral momentum exchange theory and a time‐lagged correlation analysis, we analyze the coupling time scales and dominant driving mechanisms. We observe that during the main phase the neutrals respond to the ion convection on a time scale of ∼84 min which is significantly faster than what would be expected from local ion drag momentum forcing alone. This suggests that other storm time influences are important for driving the neutrals during the main phase, such as Joule heating. During the late recovery phase, the neutrals are observed to drive the ion convection without any significant time delay, consistent with the so‐called “neutral fly wheel effect” or disturbance dynamo persisting well into the late recovery phase.

Sources of variability in equatorial topside ionospheric and plasmaspheric temperatures

Jicamarca measurements of electron temperatures at high altitudes (500–1500km) from the last solar minimum routinely show variations of hundreds of Kelvin from day-to-day. Possible sources of these variations are explored using the SAMI2-PE is another model of the ionosphere including photoelectron transport (SAMI2-PE) model, which includes a multistream photoelectron transport model. Changes to the electric fields, meridional winds, and thermospheric densities can all change the electron densities and temperatures at high altitudes. The high altitude electron temperatures are primarily determined by a balance between heating from photoelectrons which travel up the field lines and thermal diffusion which carries heat back down the field lines. The winds and electric fields will change the altitude and densities of the off-equatorial F-region peaks, especially on the field lines connected to the equatorial arcs. The densities and temperatures in the plasmasphere will self consistently adjust themselves to achieve diffusive equilibrium with the off-equatorial F-regions. Furthermore, decreases in the density and/or altitude of the F-region makes it easier for photoelectrons to escape to high altitudes. These connections between the equatorial plasmasphere, the off-equatorial F-regions, and the neutral thermosphere suggest that high altitude measurements at Jicamarca could be used to study thermospheric variability.

Possibilities of improving the TD88 atmospheric total density model

In this paper we have examined possibilities for preserving and improving the total density model of the Earth?s neutral thermosphere TD88 (Sehnal and Posp?silov? 1988) via modelling differences between TD88 and NRLMSISE-00 (Picone et al. 2002), which is used as a control model. It is shown that these residuals can be approximated with polyharmonic function. Starting from this we have developed the mathematical model of the residuals to identify their origin and possibilities to improve the TD88 model itself.

Broglio Drag Balance for neutral thermosphere density measurement on UNICubeSAT

The nanosatellite UNICubeSAT is described, carrying a Broglio Drag Balance Instrument for neutral thermosphere density in situ measurements. The aim of the mission is to contribute to the development of accurate thermosphere models, achieving in situ, real time measurements of atmosphere density, that could be exploited for global atmosphere model validation and accurate short term (1–3 days) real time space weather forecasts. The satellite is inexpensive and swarms could be easily launched operating as a distributed sensor network to get simultaneous in situ local (not orbit averaged) measurements in multiple positions and orbit heights. The nanosatellite is based on the Cubesat standard architecture, weighing about 1 kg for 1-L volume. Atmospheric drag force is measured by the displacement of light plates exposed to the incoming particle flux seen by the spacecraft, applying the original three dimensional Broglio Drag Balance concept to a single nanosatellite axis. The instrument concept and its relation to the satellite bus is depicted, showing that many long term potential measurement error sources and biases can be removed in data processing if the spacecraft is spin stabilized. The expected accuracy in density measurements is 20%. The instrument cost is a fraction of that of accurate accelerometers. The onboard systems are based on commercial off the shelf components, in accordance with the short lifetime typical of aeronomy satellites.

Numerical simulations of thermospheric dynamics: divergence as a proxy for vertical winds

Abstract. A local scale, time dependent three-dimensional model of the neutral thermosphere was used to test the applicability of two previously published empirical relations between thermospheric vertical wind and velocity divergence, i.e., those due to Burnside et al. (1981) and Brekke (1997). The model self-consistently solves for vertical winds driven by heat and momentum deposited into the neutral atmosphere by high latitude ion convection. The Brekke condition accurately mimicked the overall "shape" of the three-dimensional model vertical wind field although, as written, it consistently overestimated the vertical wind magnitude by a factor of approximately 5/3, for the heating scenarios that we considered. This same general behavior was observed regardless of whether the forcing was static or rapidly changing with time. We discuss the likely reason for the Brekke condition overestimating the magnitude of our vertical winds, and suggest an alternative condition that should better describe vertical winds that are driven by local heating. The applicability of the Burnside condition was, by contrast, quite variable. During static heating, both the magnitude and the sign of the model vertical winds were predicted reliably at heights above those of maximum energy and momentum deposition per unit mass. However, below the thermal forcing, the Burnside condition predicted vertical winds of the wrong sign. It also introduced significant artefacts into the predicted vertical wind field when the forcing changed suddenly with time. If these results are of general applicability (which seems likely, given the way these relations are derived) then the Burnside condition could usually be used safely at altitudes above hmF2. But it should be avoided below this height at all times, and even at high altitudes during periods of dynamic forcing. While the Brekke condition (or our modified version of it) could likely be used in all circumstances, there are few experimental scenarios for which this would be useful. This is because evaluation of the Brekke condition would not usually be possible unless the vertical wind was already known in advance.

TD-88Up: Upgraded neutral Earth's thermosphere total density TD-88 model

Improved constants of the total density TD-88 model for the Earth's neutral thermosphere are calculated. The model is fully functional within the height range of 200 to 500 km, with fixed values of the mean solar flux and geomagnetic index. The control data of the atmospheric air density are derived from the aeronomical NRLMSISE-00 model which was used as the reference one. The upgraded TD-88, named TD-88Up, model is obtained by the extended LSQ method with varying all model parameters.