Scholarly Works, Center for Space Science and Engineering Research (Space@VT)

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Research articles, presentations, and other scholarship

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The Sensitivity of Polar Mesospheric Clouds to Mesospheric Temperature and Water Vapor
Lee, Jae N.; Wu, Dong L.; Thurairajah, Brentha; Hozumi, Yuta; Tsuda, Takuo (MDPI, 2024-04-28)
Polar mesospheric cloud (PMC) data obtained from the Aeronomy of Ice in the Mesosphere (AIM)/Cloud Imaging and Particle Size (CIPS) experiment and Himawari-8/Advanced Himawari Imager (AHI) observations are analyzed for multi-year climatology and interannual variations. Linkages between PMCs, mesospheric temperature, and water vapor (H₂O) are further investigated with data from the Microwave Limb Sounder (MLS). Our analysis shows that PMC onset date and occurrence rate are strongly dependent on the atmospheric environment, i.e., the underlying seasonal behavior of temperature and water vapor. Upper-mesospheric dehydration by PMCs is evident in the MLS water vapor observations. The spatial patterns of the depleted water vapor correspond to the PMC occurrence region over the Arctic and Antarctic during the days after the summer solstice. The year-to-year variabilities in PMC occurrence rates and onset dates are highly correlated with mesospheric temperature and H₂O. They show quasi-quadrennial oscillation (QQO) with 4–5-year periods, particularly in the southern hemisphere (SH). The combined influence of mesospheric cooling and the mesospheric H₂O increase provides favorable conditions for PMC formation. The global increase in mesospheric H₂O during the last decade may explain the increased PMC occurrence in the northern hemisphere (NH). Although mesospheric temperature and H₂O exhibit a strong 11-year variation, little solar cycle signatures are found in the PMC occurrence during 2007–2021.
Global-GMDs: The global map of geomagnetic disturbances
Hu, Hongyi; Xu, Zhonghua (Elsevier, 2024-02)
To improve the understanding and monitoring the impacts of geomagnetic disturbances (GMDs) on power grids globally, the presented software, Global-GMDs, uses magnetic field measurements from geomagnetic observatories worldwide and Kriging method to generate global maps of GMDs. It provides better observational information during a solar storm to power grid operations and other crucial infrastructures. It can also help researchers to assess the GMDs prediction model by comparing with Global-GMDs maps and to get better understanding of physics mechanisms.
Global Variations in the Time Delays Between Polar Ionospheric Heating and the Neutral Density Response
Weimer, Daniel R.; Mehta, Piyush M.; Licata, R. J.; Tobiska, W. K. (American Geophysical Union, 2023-04)
We present results from a study of the time lags between changes in the energy flow into the polar regions and the response of the thermosphere to the heating. Measurements of the neutral density from the Challenging Mini-satellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) missions are used, along with calculations of the total Poynting flux entering the poles. During two major geomagnetic storms in 2003, these data show increased densities are first seen on the dayside edge of the auroral ovals after a surge in the energy input. At lower latitudes, the densities reach their peak values on the dayside earlier than on the night side. A puzzling response seen in the CHAMP measurements during the November 2003 storm was that the density at a fixed location near the "Harang discontinuity" remained at unusually low levels during three sequential orbit passes, while elsewhere the density increased. The entire database of measurements from the CHAMP and GRACE missions were used to derive maps of the density time lags across the globe. The maps show a large gradient between short and long time delays between 60 degrees and 30 degrees geographic latitude. They confirm the findings from the two storm periods, that near the equator, the density on the dayside responds earlier than on the nightside. The time lags are longest near 18-20 hr local time. The time lag maps could be applied to improve the accuracy of empirical thermosphere models, and developers of numerical models may find these results useful for comparisons with their calculations. The interaction of the solar wind with the Earth's magnetosphere causes varying levels of heating in the ionosphere. This heating is produced by auroral currents at high latitudes, which in tur n causes the density of the upper atmosphere to change. A topic of importance is to determine how rapidly the density can increase at different locations around the globe following a surge in the heating, which can be calculated from measurements of the solar wind velocity and embedded magnetic fields. This study used measurements of the atmospheric density on two satellite missions known as Challenging Mini-satellite Payload and Gravity Recovery and Climate Experiment. The results show that the density increases first near the poles, and much longer at lower latitudes, as expected. The time lags between changes in the energy input and the density response have been determined for the first time on a global scale. Maps of the time lags are derived. Near the equator the lags are shorter near local noon, and longer before local midnight. The time lag maps can used to improve empirical and numerical models of the thermosphere. More accurate models are needed for more precise predictions of the drag that satellites will encounter, and the subsequent changes in their orbits.
NRLMSIS 2.1: An Empirical Model of Nitric Oxide Incorporated Into MSIS
Emmert, J. T.; Jones, M.; Siskind, D. E.; Drob, D. P.; Picone, J. M.; Stevens, M. H.; Bailey, Scott M.; Bender, S.; Bernath, P. F.; Funke, B.; Hervig, M. E.; Perot, K. (American Geophysical Union, 2022-10)
We have developed an empirical model of nitric oxide (NO) number density at altitudes from similar to 73 km to the exobase, as a function of altitude, latitude, day of year, solar zenith angle, solar activity, and geomagnetic activity. The model is part of the NRLMSIS (R) 2.1 empirical model of atmospheric temperature and species densities; this upgrade to NRLMSIS 2.0 consists solely of the addition of NO. MSIS 2.1 assimilates observations from six space-based instruments: UARS/HALOE, SNOE, Envisat/MIPAS, ACE/FTS, Odin/SMR, and AIM/SOFIE. We additionally evaluated the new model against independent extant NO data sets. In this paper, we describe the formulation and fitting of the model, examine biases between the data sets and model and among the data sets, compare with another empirical NO model (NOEM), and discuss scientific aspects of our analysis.
Modeling geomagnetic induction in submarine cables
Chakraborty, Shibaji; Boteler, David H.; Shi, Xueling; Murphy, Benjamin S.; Hartinger, Michael D.; Wang, Xuan; Lucas, Greg; Baker, Joseph B. H. (Frontiers, 2022-10)
Submarine cables have become a vital component of modern infrastructure, but past submarine cable natural hazard studies have mostly focused on potential cable damage from landslides and tsunamis. A handful of studies examine the possibility of space weather effects in submarine cables. The main purpose of this study is to develop a computational model, using Python, of geomagnetic induction on submarine cables. The model is used to estimate the induced voltage in the submarine cables in response to geomagnetic disturbances. It also utilizes newly acquired knowledge from magnetotelluric studies and associated investigations of geomagnetically induced currents in power systems. We describe the Python-based software, its working principle, inputs/outputs based on synthetic geomagnetic field data, and compare its operational capabilities against analytical solutions. We present the results for different model inputs, and find: 1) the seawater layer acts as a shield in the induction process: the greater the ocean depth, the smaller the seafloor geoelectric field; and 2) the model is sensitive to the Ocean-Earth layered conductivity structure.
MSIS-UQ: Calibrated and Enhanced NRLMSIS 2.0 Model With Uncertainty Quantification
Licata, Richard J.; Mehta, Piyush M.; Weimer, Daniel R.; Tobiska, W. Kent; Yoshii, Jean (American Geophysical Union, 2022-11)
The Mass Spectrometer and Incoherent Scatter radar (MSIS) model family has been developed and improved since the early 1970's. The most recent version of MSIS is the Naval Research Laboratory (NRL) MSIS 2.0 empirical atmospheric model. NRLMSIS 2.0 provides species density, mass density, and temperature estimates as function of location and space weather conditions. MSIS models have long been a popular choice of thermosphere model in the research and operations community alike, but-like many models-does not provide uncertainty estimates. In this work, we develop an exospheric temperature model based in machine learning that can be used with NRLMSIS 2.0 to calibrate it relative to high-fidelity satellite density estimates directly through the exospheric temperature parameter. Instead of providing point estimates, our model (called MSIS-UQ) outputs a distribution which is assessed using a metric called the calibration error score. We show that MSIS-UQ debiases NRLMSIS 2.0 resulting in reduced differences between model and satellite density of 25% and is 11% closer to satellite density than the Space Force's High Accuracy Satellite Drag Model. We also show the model's uncertainty estimation capabilities by generating altitude profiles for species density, mass density, and temperature. This explicitly demonstrates how exospheric temperature probabilities affect density and temperature profiles within NRLMSIS 2.0. Another study displays improved post-storm overcooling capabilities relative to NRLMSIS 2.0 alone, enhancing the phenomena that it can capture.
Surface-to-space atmospheric waves from Hunga Tonga-Hunga Ha'apai eruption
Wright, Corwin J.; Hindley, Neil P.; Alexander, M. Joan; Barlow, Mathew; Hoffmann, Lars; Mitchell, Cathryn N.; Prata, Fred; Bouillon, Marie; Carstens, Justin; Clerbaux, Cathy; Osprey, Scott M.; Powell, Nick; Randall, Cora E.; Yue, Jia (Nature Portfolio, 2022-09-22)
The January 2022 Hunga Tonga-Hunga Ha'apai eruption was one of the most explosive volcanic events of the modern era(1,2), producing a vertical plume that peaked more than 50 km above the Earth(3). The initial explosion and subsequent plume triggered atmospheric waves that propagated around the world multiple times(4). A global-scale wave response of this magnitude from a single source has not previously been observed. Here we show the details of this response, using a comprehensive set of satellite and ground-based observations to quantify it from surface to ionosphere. A broad spectrum of waves was triggered by the initial explosion, including Lamb waves(5,6) propagating at phase speeds of 318.2 +/- 6 m s(-1) at surface level and between 308 +/- 5 to 319 +/- 4 m s(-1) in the stratosphere, and gravity waves(7) propagating at 238 +/- 3 to 269 +/- 3 m s(-1) in the stratosphere. Gravity waves at sub-ionospheric heights have not previously been observed propagating at this speed or over the whole Earth from a single source(8,9). Latent heat release from the plume remained the most significant individual gravity wave source worldwide for more than 12 h, producing circular wavefronts visible across the Pacific basin in satellite observations. A single source dominating such a large region is also unique in the observational record. The Hunga Tonga eruption represents a key natural experiment in how the atmosphere responds to a sudden point-source-driven state change, which will be of use for improving weather and climate models.
Can Column Formaldehyde Observations Inform Air Quality Monitoring Strategies for Ozone and Related Photochemical Oxidants?
Travis, K. R.; Judd, L. M.; Crawford, J. H.; Chen, Gao; Szykman, James; Whitehill, Andrew; Valin, Lukas C.; Spinei, Elena; Janz, Scott; Nowlan, Caroline R.; Kwon, Hyeong-Ahn; Fried, Alan; Walega, James (American Geophysical Union, 2022-07-16)
Formaldehyde column density (omega HCHO) showed a potentially useful correlation with surface ozone during the LISTOS campaign on Long Island Sound and the KORUS-AQ campaign in Seoul, South Korea. This builds on previous work that identified this relationship from in situ aircraft observations with similar findings for ground-based and airborne remote sensing of omega HCHO. In the Long Island Sound region, omega HCHO and surface ozone exhibited strong temporal (r(2) = 0.66) and spatial (r(2) = 0.73) correlation. The temporal variability in omega HCHO (similar to 1 Dobson units [DU]) was larger than the range in the spatial average (similar to 0.1 DU). The spatial average is most useful for informing ozone monitoring strategies, demonstrating the challenge in using omega HCHO satellite data sets for this purpose. In Seoul, high levels of NO2 resulted in O-x better correlating with omega HCHO than surface ozone due to titration effects. The omega HCHO-O-x relationship may therefore reflect the sum of surface ozone and related photochemical oxidants, relevant to air quality standards set to regulate this quantity such as the U.S. EPA National Ambient Air Quality Standard (NAAQS). The relationship of omega HCHO to O-x shifted in Seoul during the campaign demonstrating the need to evaluate this relationship over longer time periods. With sufficient precision in future satellite retrievals, omega HCHO observations could be useful for evaluating the adequacy of surface air quality monitoring strategies.
Comparison of a Neutral Density Model With the SET HASDM Density Database
Weimer, Daniel R.; Tobiska, W. Kent; Mehta, Piyush M.; Licata, R. J.; Drob, Douglas P.; Yoshii, Jean (American Geophysical Union, 2021-12)
The EXospheric TEMperatures on a PoLyhedrAl gRid (EXTEMPLAR) method predicts the neutral densities in the thermosphere. The performance of this model has been evaluated through a comparison with the Air Force High Accuracy Satellite Drag Model (HASDM). The Space Environment Technologies (SET) HASDM database that was used for this test spans the 20 years 2000 through 2019, containing densities at 3 hr time intervals at 25 km altitude steps, and a spatial resolution of 10 degrees latitude by 15 degrees longitude. The upgraded EXTEMPLAR that was tested uses the newer Naval Research Laboratory MSIS 2.0 model to convert global exospheric temperature values to neutral density as a function of altitude. The revision also incorporated time delays that varied as a function of location, between the total Poynting flux in the polar regions and the exospheric temperature response. The density values from both models were integrated on spherical shells at altitudes ranging from 200 to 800 km. These sums were compared as a function of time. The results show an excellent agreement at temporal scales ranging from hours to years. The EXTEMPLAR model performs best at altitudes of 400 km and above, where geomagnetic storms produce the largest relative changes in neutral density. In addition to providing an effective method to compare models that have very different spatial resolutions, the use of density totals at various altitudes presents a useful illustration of how the thermosphere behaves at different altitudes, on time scales ranging from hours to complete solar cycles.
Improved Neutral Density Predictions Through Machine Learning Enabled Exospheric Temperature Model
Licata, Richard J.; Mehta, Piyush M.; Weimer, Daniel R.; Tobiska, W. Kent (American Geophysical Union, 2021-12)
The community has leveraged satellite accelerometer data sets in previous years to estimate neutral mass density and exospheric temperatures. We utilize derived temperature data and optimize a nonlinear machine-learned (ML) regression model to improve upon the performance of the linear EXospheric TEMPeratures on a PoLyhedrAl gRid (EXTEMPLAR) model. The newly developed EXTEMPLAR-ML model allows for exospheric temperature predictions at any location with one model and provides performance improvements over its predecessor. We achieve reductions in mean absolute error of 2 K on an independent test set while providing similar error standard deviation values. Comparing the performance of both EXTEMPLAR models and the Naval Research Laboratory Mass Spectrometer and Incoherent Scatter radar Extended model (NRLMSISE-00) across different solar and geomagnetic activity levels shows that EXTEMPLAR-ML has the lowest mean absolute error across 80% of conditions tested. A study for spatial errors demonstrated that at all grid locations, EXTEMPLAR-ML has the lowest mean absolute error for over 60% of the polyhedral grid cells on the test set. Like EXTEMPLAR, our model's outputs can be utilized by NRLMSISE-00 (exclusively) to more closely match satellite accelerometer-derived densities. We conducted 10 case studies where we compare the accelerometer-derived temperature and density estimates from four satellites to NRLMSISE-00, EXTEMPLAR, and EXTEMPALR-ML during major storm periods. These comparisons show that EXTEMPLAR-ML generally has the best performance of the three models during storms. We use principal component analysis on EXTEMPLAR-ML outputs to verify the physical response of the model to its drivers.
Impact Angle Control of Local Intense dB/dt Variations During Shock-Induced Substorms
Oliveira, Denny M.; Weygand, James M.; Zesta, Eftyhia; Ngwira, Chigomezyo M.; Hartinger, Michael D.; Xu, Zhonghua; Giles, Barbara L.; Gershman, Daniel J.; Silveira, Marcos V. D.; Souza, Vítor M. (American Geophysical Union, 2021-12-01)
The impact of interplanetary shocks on the magnetosphere can trigger magnetic substorms that intensify auroral electrojet currents. These currents enhance ground magnetic field perturbations (dB/dt), which in turn generate geomagnetically induced currents (GICs) that can be detrimental to power transmission infrastructure. We perform a comparative study of dB/dt variations in response to two similarly strong shocks, but with one being nearly frontal and the other highly inclined. Multi-instrument analyses by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Los Alamos National Laboratory spacecraft show that nightside substorm-time energetic particle injections are more intense and occur faster in the case of the nearly head-on impact. The same trend is observed in dB/dt variations recorded by THEMIS ground magnetometers. THEMIS all-sky imager data show a fast and clear poleward auroral expansion in the first case, which does not clearly occur in the second case. Strong field-aligned currents computed with the spherical elementary current system (SECS) technique occur in both cases, but the current variations resulting from the inclined shock impact are weaker and slower compared to the nearly frontal case. SECS analyses also reveal that geographic areas with dB/dt surpassing the thresholds 1.5 and 5 nT/s, usually linked to high-risk GICs, are larger and occur earlier due to the symmetric compression caused by the nearly head-on impact. These results, with profound space weather implications, suggest that shock impact angles affect the geospace driving conditions and the location and intensity of the subsequent dB/dt variations during substorm activity.
Characterization of multi-scale ionospheric irregularities using ground-based and space-based GNSS observations
Peng, YuXiang; Scales, Wayne A.; Hartinger, Michael D.; Xu, Zhonghua; Coyle, Shane (2021-07-12)
Ionospheric irregularities can adversely affect the performance of Global Navigation Satellite System (GNSS). However, this opens the possibility of using GNSS as an effective ionospheric remote sensing tool. Despite ionospheric monitoring has been undertaken for decades, these irregularities in multiple spatial and temporal scales are still not fully understood. This paper reviews Virginia Tech’s recent studies on multi-scale ionospheric irregularities using ground-based and space-based GNSS observations. First, the relevant background of ionospheric irregularities and their impact on GNSS signals is reviewed. Next, three topics of ground-based observations of ionospheric irregularities for which GNSS and other ground-based techniques are used simultaneously are reviewed. Both passive and active measurements in high-latitude regions are covered. Modelling and observations in mid-latitude regions are considered as well. Emphasis is placed on the increased capability of assessing the multi-scale nature of ionospheric irregularities using other traditional techniques (e.g., radar, magnetometer, high frequency receivers) as well as GNSS observations (e.g., Total-Electron-Content or TEC, scintillation). Besides ground-based observations, recent advances in GNSS space-based ionospheric measurements are briefly reviewed. Finally, a new space-based ionospheric observation technique using GNSS-based spacecraft formation flying and a differential TEC method is demonstrated using the newly developed Virginia Tech Formation Flying Testbed (VTFFTB). Based on multi-constellation multi-band GNSS, the VTFFTB has been developed into a hardware-in-the-loop simulation testbed with external high-fidelity global ionospheric model(s) for 3-satellite formation flying, which can potentially be used for new multi-scale ionospheric measurement mission design.
Ionospheric Remote Sensing with GNSS
Peng, YuXiang; Scales, Wayne A. (MDPI, 2021-11-22)
The Global Navigation Satellite System (GNSS) plays a pivotal role in our modern positioning, navigation and timing (PNT) technologies. GNSS satellites fly at altitudes of approximately 20,000 km or higher. This altitude is above an ionized layer of the Earth’s upper atmosphere, the so called “ionosphere”. Before reaching a typical GNSS receiver on the ground, GNSS satellite signals penetrate through the Earth’s ionosphere. The ionosphere is a plasma medium consisting of free charged particles that can slow down, attenuate, refract, or scatter the GNSS signals. Ionospheric density structures (also known as irregularities) can cause GNSS signal scintillations (phase and intensity fluctuations). These ionospheric impacts on GNSS signals can be utilized to observe and study physical processes in the ionosphere and is referred to ionospheric remote sensing. This entry introduces some fundamentals of ionospheric remote sensing using GNSS.
Trends in the polar summer mesosphere temperature and pressure altitude from satellite observations
Bailey, Scott M.; Thurairajah, Brentha; Hervig, Mark E.; Siskind, David E.; Russell, James M. III; Gordley, Larry L. (2021-09-01)
Time series of mesospheric temperature and pressure altitude are produced through combining observations by the Halogen Occultation Experiment (HALOE), Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER), and Solar Occultation for Ice Experiment (SOFIE) instruments. Time series of both temperature and pressure altitude are produced through the combination of HALOE/SABER providing 29 years in length and HALOE/SOFIE providing 22 years in length. The different sampling of the three instruments constrains the time series to June in the northern hemisphere and December in the southern hemisphere and 6470 degrees in both hemispheres. We interpret the time series by fitting them to simple descriptions of the variations including solar, intra-hemispheric, inter-hemispheric, and linear trend terms. The inferred intra- and inter-hemispheric terms show that dynamical influences rival solar variability in the mesosphere. We find a robust result that the mesosphere is in general cooling at most altitudes at approximately 1-2 K per decade in response to greenhouse gas increases. That cooling leads to a shrinking of the atmosphere on the order of 100-200 m per decade. The shrinking leads to a reduction in cooling and eventually a warming near 0.005 hPa due to hydrostatic contraction.
Testing the electrodynamic method to derive height-integrated ionospheric conductances
Weimer, Daniel R.; Edwards, Thom (2021-01-14)
We have used empirical models for electric potentials and the magnetic fields both in space and on the ground to obtain maps of the height-integrated Pedersen and Hall ionospheric conductivities at high latitudes. This calculation required use of both "curl-free" and "divergencefree" components of the ionospheric currents, with the former obtained from magnetic fields that are used in a model of the field-aligned currents. The second component is from the equivalent current, usually associated with Hall currents, derived from the ground-level magnetic field. Conductances were calculated for varying combinations of the interplanetary magnetic field (IMF) magnitude and orientation angle, as well as the dipole tilt angle. The results show that reversing the sign of the Y component of the IMF produces substantially different conductivity patterns. The Hall conductivities are largest on the dawn side in the upward, Region 2 field-aligned currents. Low electric field strengths in the Harang discontinuity lead to inconclusive results near midnight. Calculations of the Joule heating, obtained from the electric field and both components of the ionospheric current, are compared with the Poynting flux in space. The maps show some differences, while their integrated totals match to within 1 %. Some of the Poynting flux that enters the polar cap is dissipated as Joule heating within the auroral ovals, where the conductivity is greater.
Effect of polyoxymethylene (POM-H Delrin) off-gassing within the Pandora head sensor on direct-sun and multi-axis formaldehyde column measurements in 2016-2019
Spinei, Elena; Tiefengraber, Martin; Mueller, Moritz; Gebetsberger, Manuel; Cede, Alexander; Valin, Luke; Szykman, James; Whitehill, Andrew; Kotsakis, Alexander; Santos, Fernando; Abbuhasan, Nader; Zhao, Xiaoyi; Fioletov, Vitali; Lee, Sum Chi; Swap, Robert (2021-01-28)
Analysis of formaldehyde measurements by the Pandora spectrometer systems between 2016 and 2019 suggested that there was a temperature-dependent process inside the Pandora head sensor that emitted formaldehyde. Some parts in the head sensor were manufactured from the thermal plastic polyoxymethylene homopolymer (E.I. Du Pont de Nemour & Co., USA; POM-H Delrin (R)) and were responsible for formaldehyde production. Laboratory analysis of the four Pandora head sensors showed that internal formaldehyde production had exponential temperature dependence with a damping coefficient of 0.0911 +/- 0.0024 degrees C-1 and the exponential function amplitude ranging from 0.0041 to 0.049 DU. No apparent dependency on the head sensor age and heating and cooling rates was detected. The total amount of formaldehyde internally generated by the POM-H Delrin components and contributing to the direct-sun measurements were estimated based on the head sensor temperature and solar zenith angle of the measurements. Measurements in winter, during colder (< 10 degrees C) days in general, and at high solar zenith angles (> 75 degrees) were minimally impacted. Measurements during hot days (> 28 degrees C) and small solar zenith angles had up to 1 DU (2.69 x 10(16 )molec. cm(-2)) contribution from POM-H Delrin parts. Multi-axis differential slant column densities were minimally impacted (< 0.01 DU) due to the reference spectrum being collected within a short time period with a small difference in head sensor temperature. Three new POM-H Delrin free Pandora head sensors (manufactured in summer 2019) were evaluated for temperature-dependent attenuation across the entire spectral range (300 to 530 nm). No formaldehyde absorption or any other absorption above the instrumental noise was observed across the entire spectral range.
Improving Neutral Density Predictions Using Exospheric Temperatures Calculated on a Geodesic, Polyhedral Grid
Weimer, Daniel R.; Mehta, P. M.; Tobiska, W. K.; Doornbos, E.; Mlynczak, M. G.; Drob, Douglas P.; Emmert, J. T. (2019-12-10)
A new model of exospheric temperatures has been developed, with the objective of predicting global values with greater spatial and temporal accuracy. From these temperatures, the neutral densities in the thermosphere can be calculated, through use of the Naval Research Laboratory Mass Spectrometer and Incoherent Scatter radar Extended (NRLMSISE-00) model. The exospheric temperature model is derived from measurements of the neutral densities on several satellites. These data were sorted into triangular cells on a geodesic grid, based on location. Prediction equations are derived for each grid cell using least error fits. Several versions of the model equations have been tested, using parameters such as the date, time, solar radiation, and nitric oxide emissions, as measured with the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. Accuracy is improved with the addition of the total Poynting flux flowing into the polar regions, from an empirical model that uses the solar wind velocity and interplanetary magnetic field. Given such inputs, the model can produce global maps of the exospheric temperature. These maps show variations in the polar regions that are strongly modulated by the time of day, due to the daily rotation of the magnetic poles. For convenience the new model is referred to with the acronym EXTEMPLAR (EXospheric TEMperatures on a PoLyhedrAl gRid). Neutral densities computed from the EXTEMPLAR-NRLMSISE-00 models combined are found to produce very good results when compared with measured values.
First Observation of Ionospheric Convection From the Jiamusi HF Radar During a Strong Geomagnetic Storm
Zhang, J. J.; Wang, W.; Wang, C.; Lan, A. L.; Yan, J. Y.; Xiang, D.; Zhang, Q. H.; Ruohoniemi, J. Michael; Kunduri, B. S. R.; Nishitani, Nozomu; Shi, X.; Qiu, H. B. (2019-12-11)
The Super Dual Auroral Radar Network (SuperDARN) is an international low-power high-frequency (HF) radar network, which provides continuous observations of the motion of plasma in the ionosphere. Over the past 15 years, the network has expanded dramatically in the middle latitudes of the Northern Hemisphere to improve the observation capabilities of the network during periods of strong geomagnetic disturbance. However, a large coverage gap still exists in the middle latitudes. A newly deployed middle-latitude HF radar in China (the Jiamusi radar) is about to join the network. This paper presents the first observation of the ionospheric convection from the Jiamusi radar during the strong geomagnetic storm on 26 August 2018. The Jiamusi measurements are compared with the simultaneous measurements from the SuperDARN Hokkaido East radar. The features of the high-velocity westward flows including the equatorward expansion and variation tendency of the line-of-sight velocities observed by the two radars are consistent with each other. According to joint analysis with auroral images, we can confirm that the westward flows observed by the two radars are sunward return flows of the duskside convection cell in the auroral region. The impact the Jiamusi data had on the calculation of SuperDARN convection patterns is also examined. The results show that the inclusion of the Jiamusi data can increase the number of gridded line-of-sight velocity measurements by up to 24.42%, the cross-polar cap potential can be increased by up to 13.90% during the investigated period.
Conjugate observations of electromagnetic ion cyclotron waves associated with traveling convection vortex events
Kim, Hyomin; Clauer, C. Robert; Gerrard, Andrew J.; Engebretson, Mark J.; Hartinger, Michael D.; Lessard, Marc R.; Matzka, Juergen; Sibeck, David G.; Singer, Howard J.; Stolle, Claudia; Weimer, Daniel R.; Xu, Zhonghua (2017-07)
We report on simultaneous observations of electromagnetic ion cyclotron (EMIC) waves associated with traveling convection vortex (TCV) events caused by transient solar wind dynamic pressure (P-d) impulse events. The Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft located near the magnetopause observed radial fluctuations of the magnetopause, and the GOES spacecraft measured sudden compressions of the magnetosphere in response to sudden increases in Pd. During the transient events, EMIC waves were observed by interhemispheric conjugate ground-based magnetometer arrays as well as the GOES spacecraft. The spectral structures of the waves appear to be well correlated with the fluctuating motion of the magnetopause, showing compression-associated wave generation. In addition, the wave features are remarkably similar in conjugate hemispheres in terms of bandwidth, quasiperiodic wave power modulation, and polarization. Proton precipitation was also observed by the DMSP spacecraft during the wave events, from which the wave source region is estimated to be 72 degrees-74 degrees in magnetic latitude, consistent with the TCV center. The confluence of space-borne and ground instruments including the interhemispheric, high-latitude, fluxgate/induction coil magnetometer array allows us to constrain the EMIC source region while also confirming the relationship between EMIC waves and the TCV current system.
Correlations Between the Thermosphere's Semiannual Density Variations and Infrared Emissions Measured With the SABER Instrument
Weimer, Daniel R.; Mlynczak, M. G.; Emmert, J. T.; Doornbos, E.; Sutton, E. K.; Hunt, L. A. (2018-10)
This paper presents measurements of the amplitudes and timings of the combined, annual, and semiannual variations of thermospheric neutral density, and a comparison of these density variations with measurements of the infrared emissions from carbon dioxide and nitric oxide in the thermosphere. The density values were obtained from measurements of the atmospheric drag experienced by the Challenging Minisatellite Payload, Gravity Recovery and Climate ExperimentA, Gravity field and Ocean Circulation Explorer, and three Swarm satellites, while the optical emissions were measured with the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. These data span a time period of 16years. A database containing global average densities that were derived from the orbits of about 5,000 objects (Emmert, 2009, https://doi.org/10.1029/2009JA014102, 2015b, https://doi.org/10.1002/2015JA021047) was employed for calibrating these density data. A comparison with the NRLMSISE-00 model was used to derive measurements of how much the density changes over time due to these seasonal variations. It is found that the seasonal density oscillations have significant variations in amplitude and timing. In order to test the practicality of using optical emissions as a monitoring tool, the SABER data were fit to the measured variations. Even the most simple fit that used only filtered carbon dioxide emissions had good correlations with the measured oscillations. However, the density oscillations were also well predicted by a simple Fourier series, contrary to original expectations. Nevertheless, measurements of the optical emissions from the thermosphere are expected to have a role in future understanding and prediction of the semiannual variations. Plain Language Summary The uppermost atmosphere, known as the thermosphere, undergoes oscillations in the density of the neutral atoms and molecules, producing two peaks and valleys in the density in each year. The timing of of these "semiannual" variations or oscillations, as well as their amplitudes, tends to vary. Their unpredictability makes it harder to accurately model the amount of drag experienced by orbiting satellites. It had been noticed that the infrared light emitted by carbon dioxide molecules in the thermosphere has a tendency to follow the semiannual oscillations. Such emissions have been measured by an instrument on a NASA satellite for the past 16years. We have compared these emissions with the variations in the semiannual oscillations that were derived from measurements of the drag seen by six different satellites flown by both NASA and the European Space Agency during the same time period, though not at the same time. The results of the comparison show how well the infrared emissions match the density oscillations, due to changes in both the composition and temperature of the thermosphere that influence both. Results show that further study will be needed to be able to accurately predict the density oscillations.

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