Browsing by Author "Pi, X."
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Item Restricted CEDAR Electrodynamics Thermosphere Ionosphere (ETI) Challenge for systematic assessment of ionosphere/thermosphere models: NmF2, hmF2, and vertical drift using ground‐based observations(American Geophysical Union, 2011-12-31) Shim, J. S.; Kuznetsova, M.; Rastätter, L.; Hesse, M.; Bilitza, D.; Butala, M.; Codrescu, M.; Emery, B.; Foster, B.; Fuller-Rowell, T.; Huba, J.; Mannucci, A. J.; Pi, X.; Ridley, A.; Scherliess, L.; Schunk, R. W.; Stephens, P.; Thompson, D. C.; Zhu, L.; Anderson, D.; Chau Chong Shing, Jorge Luis; Sojka, J. J.; Rideout, B.Objective quantification of model performance based on metrics helps us evaluate the current state of space physics modeling capability, address differences among various modeling approaches, and track model improvements over time. The Coupling, Energetics, and Dynamics of Atmospheric Regions (CEDAR) Electrodynamics Thermosphere Ionosphere (ETI) Challenge was initiated in 2009 to assess accuracy of various ionosphere/thermosphere models in reproducing ionosphere and thermosphere parameters. A total of nine events and five physical parameters were selected to compare between model outputs and observations. The nine events included two strong and one moderate geomagnetic storm events from GEM Challenge events and three moderate storms and three quiet periods from the first half of the International Polar Year (IPY) campaign, which lasted for 2 years, from March 2007 to March 2009. The five physical parameters selected were NmF2 and hmF2 from ISRs and LEO satellites such as CHAMP and COSMIC, vertical drifts at Jicamarca, and electron and neutral densities along the track of the CHAMP satellite. For this study, four different metrics and up to 10 models were used. In this paper, we focus on preliminary results of the study using ground‐based measurements, which include NmF2 and hmF2 from Incoherent Scatter Radars (ISRs), and vertical drifts at Jicamarca. The results show that the model performance strongly depends on the type of metrics used, and thus no model is ranked top for all used metrics. The analysis further indicates that performance of the model also varies with latitude and geomagnetic activity level.Item Restricted Ensemble Modeling with Data Assimilation Models: A New Strategy for Space Weather Specifications, Forecasts, and Science(American Geophysical Union, 2014-02-23) Schunk, R. W.; Scherliess, L.; Eccles, V.; Gardner, L. C.; Sojka, J. J.; Zhu, L.; Pi, X.; Mannucci, A. J.; Wilson, B. D.; Komjathy, A.; Wang, C.; Rosen, G.The Earth’s Ionosphere-Thermosphere-Electrodynamics (I-T-E) system varies markedly on a range of spatial and temporal scales and these variations have adverse effects on human operations and systems, including high-frequency communications, over-the-horizon radars, and survey and navigation systems that use Global Positioning System (GPS) satellites. Consequently, there is a need to elucidate the underlying physical processes that lead to space weather disturbances and to both mitigate and forecast near-Earth space weather. The meteorologists and oceanographers have shown that data assimilation models are superior to global physics-based models for specifications and forecasts, but only during the last 15 years have they been used for near-Earth investigations as more global (space and ground-based) measurements became available. Although data assimilation models produce better specifications and forecasts than global physicsbased models, there is still a spread in results for a given simulation scenario when different data assimilation models are used. This spread occurs because the different data assimilation models use different data types, data amounts, assimilation techniques, and background physics-based models. This data assimilation issue is being addressed with the launching of the “NASA/NSF Space Weather Modeling Collaboration” program. Currently, our team has seven physics-based data assimilation models for the ionosphere, plasmasphere, thermosphere, and electrodynamics. These models assimilate a myriad of different ground- and space-based observations, and there is more than one data assimilation model for each near-Earth space domain. These data assimilation models are being used to create a Multimodel Ensemble Prediction System (MEPS), which will allow ensemble modeling of the I-T-E system with different data assimilation models that are based on different physical assumptions, assimilation techniques, and initial conditions. The application of ensemble modeling with several different data assimilation models will lead to a paradigm shift in how basic physical processes are studied in near-Earth space, and it is expected to lead to a significant advance in space weather specifications and forecasts.Item Restricted Flux tube analysis of L‐band ionospheric scintillation(American Geophysical Union, 2013-04-23) Shume, E. B.; Mannucci, A. J.; Butala, M. D.; Pi, X.; Valladares, C. E.This manuscript presents magnetic flux tube analysis of L‐band signal scintillation in the nighttime equatorial and low‐latitude ionosphere. Residues of the scintillation index urn:x wiley:jgra:media:jgra50285:jgra50285-math-0001 estimated from the L‐band signals received from Geostationary Earth Orbit (GEO) satellites are employed in the analysis. The urn:x wiley:jgra:media:jgra50285:jgra50285-math-0002 estimates have been shown to be associated with simultaneous GPS VTEC variations derived from JPL's GIPSY‐GIM package. We have applied the wavelet decomposition technique simultaneously on the urn:x-wiley:jgra:media:jgra50285:jgra50285-math-0003 time series in a flux tube over the equatorial and low‐latitude regions. The technique decomposes the urn:x-wiley:jgra:media:jgra50285:jgra50285-math-0004 signal to identify the dominant mode of variabilities and the temporal variations of scintillation‐producing irregularities in the context of a flux tube. Statistically significant regions of the wavelet power spectra considered in our study have mainly shown that (a) dominant plasma irregularities associated with urn:x-wiley:jgra:media:jgra50285:jgra50285-math-0005 variabilities in a flux tube have periods of about 4 to 15 minutes (horizontal irregularity scales of about 24 to 90 km). These periods match short period gravity waves, (b) scintillation‐producing irregularities are anisotropic along the flux tube and in the east‐west direction, and (c) the occurrences of scintillation‐producing irregularities along the flux tube indicate that the entire flux tube became unstable. However, plasma instability occurrences were not simultaneous in most cases along the flux tube, there were time delays of various orders. Understanding the attributes of L‐band scintillation‐producing irregularities could be important for developing measures to mitigate L‐band signal degradation.Item Restricted Systematic evaluation of ionosphere/thermosphere (UT) models: CEDAR Electrodynamics Thermosphere Ionosphere (ETI) Challenge (2009–2010)(American Geophysical Union, 2014-03) Shim, J. S.; Kuznetsova, M.; Rastätter, L.; Bilitza, D.; Butala, M.; Codrescu, M.; Emery, B. A.; Foster, B.; Fuller‐Rowell, T. J.; Huba, J.; Mannucci, A. J.; Pi, X.; Ridley, A.; Scherliess, L.; Schunk, R. W; Sojka, J. J.; Stephens, P.; Thompson, D. C.; Weimer, D.; Zhu, L.; Anderson, D.; Chau Chong Shing, Jorge Luis; Sutton, E.In order to model and predict the weather of the near‐Earth space environment, it is necessary to understand the important coupling mechanisms from the surface of the Sun to the Earth's ionosphere, including its coupling with the atmosphere below. This chapter reports the simulations of the mid‐latitude to low‐latitude ionosphere. Multiday simulations during the Whole Heliosphere Interval (WHI) 2008 are performed using two versions of SAMI3 model: (1) SAMI3 with externally specified E X B drifts and (2) SAMI3 with a potential solver to self‐consistently specify electric fields. The results are compared with GPS‐derived global total electron content (TEC) maps. The chapter details the E X B drifts calculated by the self‐consistent SAMI3 and compares these results with an empirical model. It provides initial results for a multi‐year run of the descending phase of Solar Cycle 23 to illustrate the broader range of Integrated Sun‐Earth System (ISES) activity underway