[1] The availability of unprecedented amounts of real-time data from Global Navigation Satellite Systems and ionosondes coupled with new and more stringent requirements for specification and forecast of the neutral and electron densities in the thermosphere-ionosphere system are driving a new wave of development in data assimilation schemes for the thermosphere and ionosphere. However, such schemes require accurate knowledge of any biases affecting the state-propagating models, and characterizing such biases involves significant effort. A first step in the estimation of the model biases, a steady state neutral temperature comparison with the empirical Mass Spectrometer Incoherent Scatter model, was published in Space Weather in 2008. Here we present another step in the validation of the Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics (CTIPe) general circulation model in preparation for its future inclusion in a data assimilation scheme. We describe an implementation of the model at the Space Weather Prediction Center (SWPC) and present real-time comparisons between CTIPe and GPS total electron content and F 2 layer ionosonde measurements. The CTIPe results are generated automatically about 20 min ahead of real time. The model inputs are based on NASA's Advanced Composition Explorer and F10.7 data available in the SWPC database. The results and the comparison with measurements for the current 2-week period are available at http://helios.swpc.noaa.gov/ctipe/. The results are quite encouraging and offer hope that physics-based models can compete with empirical models during quiet times and have tremendous potential to provide more reliable forecasts during periods of geomagnetic disturbance.

To address challenges of assessing space weather modeling capabilities, the Community Coordinated Modeling Center is leading a newly established International Forum for Space Weather Modeling Capabilities Assessment. This paper presents preliminary results of validation of modeled foF2 (F 2 layer critical frequency) and TEC (total electron content) during the first selected 2013 March storm event (17 March 2013). In this study, we used eight ionospheric models ranging from empirical to physics-based, coupled ionosphere-thermosphere and data assimilation models. The quantities we considered are TEC and foF2 changes and percentage changes compared to quiet time background, and the maximum and minimum percentage changes. In addition, we considered normalized percentage changes of TEC. We compared the modeled quantities with ground-based observations of vertical Global Navigation Satellite System TEC (provided by Massachusetts Institute of Technology Haystack Observatory) and foF2 data (provided by Global Ionospheric Radio Observatory) at the 12 locations selected in middle latitudes of the American and European-African longitude sectors. To quantitatively evaluate the models' performance, we calculated skill scores including correlation coefficient, root-mean square error (RMSE), ratio of the modeled to observed maximum percentage changes (yield), and timing error. Our study indicates that average RMSEs of foF2 range from about 1 MHz to 1.5 MHz. The average RMSEs of TEC are between~5 and~10 TECU (1 TEC Unit = 10 16 el/m 2 ). dfoF2[%] RMSEs are between 15% and 25%, which is smaller than RMSE of dTEC[%] ranging from 30% to 60%. The performance of the models varies with the location and metrics considered.To address the needs and challenge of assessment of our current knowledge about space weather effects on IT system and current state of IT modeling capabilities, the Community Coordinated Modeling Center (CCMC) has been supporting community-wide model validation projects, including Coupling, Energetics and Dynamics of Atmospheric Regions (CEDAR) and Geospace Environment Modeling (GEM)-CEDAR modeling challenges. The CCMC initiated the CEDAR electrodynamics thermosphere ionosphere challenge in 2009 focusing on the evaluation of basic IT system parameters modeled, such as electron and neutral densities, the ionospheric F 2 layer peak electron density (NmF2) and peak height (hmF2), and vertical drift (Shim Key Points: • foF2/TEC and foF2/TEC changes during a storm predicted by eight ionosphere models were compared with GIRO foF2 and GPS TEC measurements • Skill scores (e.g., correlation coefficient, RMSE, yield, and timing error) were calculated • Model performance strongly depends on the quantities considered, the type of metrics used, and the location considered Supporting Information: • Supporting Information S1 Correspondence to: J. S. Shim, jasoon.shim@nasa.gov Citation: Shim, J. S., Tsagouri, I., Goncharenko, L., Rastaetter, L., Kuznetsova, M., Bilitza, D., et al. (2018). Validation of ionospheric specifications du...

The primary operational impact of upper atmospheric neutral density variability is on satellite drag. Drag is the most difficult force to model mainly because of the complexity of neutral atmosphere variations driven by solar UV and EUV radiation power, magnetospheric energy input, and the propagation from below of lower atmosphere waves. Taking into account the self‐consistent interactions between neutral winds, composition, ion drifts, and ionization densities, first‐principles models are able to provide a more realistic representation of neutral density than empirical models in the upper atmosphere. Their largest sources of uncertainty, however, are the semiannual variations in neutral density and the magnitude, spatial distribution, and temporal evolution of the magnetospheric energy input. In this study, results from the physics‐based coupled thermosphere‐ionosphere‐plasmasphere electrodynamics (CTIPe) model and measurements from the CHAMP satellite are compared and used to improve the modeled thermospheric neutral density estimates. The good agreement between modeled and observed densities over an uninterrupted yearlong period of variable conditions gives confidence that the thermosphere‐ionosphere system energy influx from solar radiation and magnetospheric sources is reasonable and that Joule heating, the dominant source during geomagnetically disturbed conditions, is appropriately estimated. On the basis of the correlation between neutral density and energy injection, a global time‐dependent Joule heating index (JHI) is derived from the relationship between Joule heating computed by the CTIPe model and neutral density measured by the CHAMP satellite. Preliminary results show an improvement in density estimates using CTIPe JHI, demonstrating its potential for neutral density modeling applied to atmospheric drag determination.

The magnetosphere is a major source of energy for the Earth's ionosphere and thermosphere (IT) system. Current IT models drive the upper atmosphere using empirically calculated magnetospheric energy input. Thus, they do not sufficiently capture the stormtime dynamics, particularly at high latitudes. To improve the prediction capability of IT models, a physics-based magnetospheric input is necessary. Here, we use the Open Global General Circulation Model (OpenGGCM) coupled with the Coupled Thermosphere Ionosphere Model (CTIM). OpenGGCM calculates a three-dimensional global magnetosphere and a two-dimensional high-latitude ionosphere by solving resistive magnetohydrodynamic (MHD) equations with solar wind input. CTIM calculates a global thermosphere and a high-latitude ionosphere in three dimensions using realistic magnetospheric inputs from the OpenGGCM. We investigate whether the coupled model improves the storm-time IT responses by simulating a geomagnetic storm that is preceded by a strong solar wind pressure front on August 24, 2005. We compare the OpenGGCM-CTIM results with low-earth-orbit satellite observations and with the model results of Coupled Thermosphere-Ionosphere-Plasmasphere electrodynamics (CTIPe). CTIPe is an up-to-date version of CTIM that incorporates more IT dynamics such as a low-latitude ionosphere and a plasmasphere, but uses empirical magnetospheric input. OpenGGCM-CTIM reproduces localized neutral density peaks at~400 km altitude in the high-latitude dayside regions in agreement with in situ observations during the pressure shock and the early phase of the storm. Although CTIPe is in some sense a much superior model than CTIM, it misses these localized enhancements. Unlike the CTIPe empirical input models, OpenGGCM-CTIM more faithfully produces localized increases of both auroral precipitation and ionospheric electric fields near the high-latitude dayside region after the pressure shock and after the storm onset, which in turn effectively heats the thermosphere and causes the neutral density increase at 400 km altitude.

The Solar Cycle 23 -24 minimum has been considered unusually deep and complex. In this article we study the ionospheric behavior during this minimum, and we have found that, although observable, the ionosphere response is minor and marginally exceeds the range of normal geophysical variability of the system. Two main ionospheric parameters have been studied: vertical TEC (vTEC, total electron content) and NmF2 (peak concentration of the F region). While vTEC showed a consistent modest decrease of the mean value, NmF2 behavior was less clear, with instances where the mean value for the minimum 23 -24 was even higher that for the minimum 22 -23. More extensive work is required to gain a better understanding of the ionospheric behavior under conditions similar to those presented in the last minimum.

A principal limitation of physics‐based modeling in the Thermosphere‐Ionosphere system is the large uncertainty associated with the implementation of the external forcing of the system, including high‐latitude convection and particle precipitation, solar UV/EUV fluxes, and waves propagating from below. As measuring these quantities with sufficient spatial and temporal resolution is prohibitively costly, a more practical approach to improve model results is to assimilate more readily available measurements of the system. We discuss considerations in implementing an Ensemble Kalman Filter (EnKF) for a strongly forced system versus a chaotic system, overview an EnKF implementation for the strongly forced Coupled Thermosphere, Ionosphere, Plasmasphere, and Electrodynamics model, and present encouraging improvements to neutral density specification obtained by assimilating Challenging Minisatellite Payload (CHAMP) neutral density measurements. The model results show improvement in comparisons with both CHAMP and Gravity Recovery and Climate Experiment (GRACE) measurements during a geomagnetically quiet period at solar minimum.

It has recently been suggested that observations of neutral density from satellite accelerometer data indicate a strong cooling occurred in the upper thermosphere during the January 2009 sudden stratospheric warming (SSW). The 2009 warming was a major event with winter polar stratospheric temperatures increasing by 70 K. This January period has been re‐examined with three independent models: the NRLMSISE‐00 empirical model; the physics‐based coupled thermosphere, ionosphere, plasmasphere, electrodynamics model (CTIPe); and the whole atmosphere model (WAM). The analysis of this period and comparison with the neutral density observations reveals that there is, in fact, no evidence at any latitude for a large‐scale or global decrease in upper thermosphere density or temperature in response to the SSW. The observed decrease in density and temperature can be amply accounted for by small changes in geomagnetic activity during this period. On the contrary, the WAM numerical simulations of the period suggest a possible small globally averaged upper thermosphere warming and neutral density increase by 5% during the SSW. This warming would have been difficult to discern in the local‐time sampling of the CHAMP observations due to likely change in the diurnal density variation during the SSW, and due to a much larger contribution to the variability from geomagnetic sources. At this stage, therefore, it is not possible to ascertain if a cooling or warming occurred in the upper thermosphere in response to the stratospheric warming.

The specification and prediction of density changes in the thermosphere is a key challenge for space weather observations and modeling, because it is one result of complex interactions between the Sun and the terrestrial atmosphere and also because it is of operational importance for tracking objects orbiting in near-Earth space. For low Earth orbit, neutral density variation is the most important uncertainty for propagation and prediction of orbital elements. A recent international conference conducted under the auspices of the National Aeronautics and Space Administration Community Coordinated Modeling Center included a workshop on neutral density modeling, using both empirical and numerical methods, and resulted in organization of an initial effort in model comparison and evaluation. Here we report on the exploitable density data sets available, the selected years and storm events, and the metrics for complete model assessment. Comparisons between five models (three empirical and two numerical) and neutral density data sets that include measurements by the CHAMP, GRACE, and GOCE satellites are presented as examples of the assessment procedure that will be implemented at Community Coordinated Modeling Center. The models in general performed reasonably well, although seasonal errors sometimes are present, and impulsive geomagnetic storm events remain challenging. Numerical models are still catching up to empirical methods on a statistical basis, but hold great potential for describing these short-term variations.