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Pardini C.,CNR Institute of Information Science and Technologies Alessandro Faedo | Moe K.,Space Environment Technologies, LLC | Anselmo L.,CNR Institute of Information Science and Technologies Alessandro Faedo
Planetary and Space Science | Year: 2012

Uncertainties in the neutral density estimation are the major source of aerodynamic drag errors and one of the main limiting factors in the accuracy of the orbit prediction and determination process at low altitudes. Massive efforts have been made over the years to constantly improve the existing operational density models, or to create even more precise and sophisticated tools. Special attention has also been paid to research more appropriate solar and geomagnetic indices. However, the operational models still suffer from weakness. Even if a number of studies have been carried out in the last few years to define the performance improvements, further critical assessments are necessary to evaluate and compare the models at different altitudes and solar activity conditions. Taking advantage of the results of a previous study, an investigation of thermospheric density model biases during the last sunspot maximum (October 1999 - December 2002) was carried out by analyzing the semi-major axis decay of four satellites: Cosmos 2265, Cosmos 2332, SNOE and Clementine. Six thermospheric density models, widely used in spacecraft operations, were analyzed: JR-71, MSISE-90, NRLMSISE-00, GOST-2004, JB2006 and JB2008. During the time span considered, for each satellite and atmospheric density model, a fitted drag coefficient was solved for and then compared with the calculated physical drag coefficient. It was therefore possible to derive the average density biases of the thermospheric models during the maximum of the 23 rd solar cycle. Below 500 km, all the models overestimated the average atmospheric density by amounts varying between 7% and 20%. This was an inevitable consequence of constructing thermospheric models from density data obtained by assuming a fixed drag coefficient, independent of altitude. Because the uncertainty affecting the drag coefficient measurements was about 3% at both 200 km and 480 km of altitude, the calculated air density biases below 500 km were statistically significant. The minimum average biases were obtained with JB2008, NRLMSISE-00 and GOST-2004. Above 500 km, where only one satellite was analyzed (at 630 km), and errors tend to increase with altitude, it cannot be asserted that the calculated biases are significant. Nevertheless, they are presented to show how the various models diverge at higher altitudes. Around 630 km, NRLMSISE-00 had a negligible average bias, while the other models underestimated (GOST-2004) or overestimated the average density, by amounts varying between 6% and 16%. However, in terms of semi-major axis root mean square residuals, JB2006 and JB2008 were the best in any case. Below 500 km, the short-term behavior of the models was also investigated by fitting the semi-major axis decay over 30-day arcs. The resulting fitted drag coefficients displayed a significant variability, probably associated with mismodeled density variations, but JB2008, followed by JB2006, provided the smallest semi-major axis residuals and a reduced short-term variability of the density bias at just a few frequencies, having been probably successful in removing a significant fraction of the mismodeling sources. © 2012 Elsevier Ltd. All rights reserved.

Moe K.,Space Environment Technologies, LLC | Wu Q.,High Altitude Observatory
Journal of Geophysical Research: Space Physics | Year: 2014

The recent HIWIND (High-Altitude Interferometer Wind Observations) balloon measurements have revealed persistent equatorward winds in the dayside thermosphere during geomagnetically quiet times. Although this result does not agree with some current thermospheric density models, it is consistent with an earlier thermospheric density model (M1975) which includes the energy input through the magnetospheric dayside cusps during geomagnetically quiet times. We show the thermospheric density distribution with and without the magnetospheric input to illustrate the effect of the density gradient on the winds at high latitudes. We review the early history of the development of our understanding of the energy input to the high-latitude thermosphere. Future HIWIND measurements can add to our understanding and lead to improved models of thermospheric densities and winds. Key Points Recent HIWIND measurements disagree with most thermospheric density models They do agree with a model which includes energy from the magnetosphere More HIWIND measurements are needed ©2014. American Geophysical Union. All Rights Reserved.

Weimer D.R.,Virginia Polytechnic Institute and State University | Bowman B.R.,U.S. Air force | Sutton E.K.,Air Force Research Lab | Tobiska W.K.,Space Environment Technologies, LLC
Journal of Geophysical Research: Space Physics | Year: 2011

The total Poynting flux flowing into both polar hemispheres as a function of time, computed with an empirical model, is compared with measurements of neutral densities in the thermosphere at two altitudes obtained from accelerometers on the CHAMP and GRACE satellites. The Jacchia-Bowman 2008 empirical thermospheric density model (JB2008) is used to facilitate the comparison. This model calculates a background level for the "global nighttime minimum exospheric temperature," ΔTc, from solar indices. Corrections to this background level due to auroral heating, ΔTc, are presently computed from the Dst index. A proxy measurement of this temperature difference, ΔTc, is obtained by matching the CHAMP and GRACE density measurements with the JB2008 model. Through the use of a differential equation, the Tc correction can be predicted from IMF values. The resulting calculations correlate very well with the orbit-averaged measurements of ΔTc, and correlate better than the values derived from Dst. Results indicate that the thermosphere cools faster following time periods with greater ionospheric heating. The enhanced cooling is likely due to nitric oxide (NO) that is produced at a higher rate in proportion to the ionospheric heating, and this effect is simulated in the differential equations. As the ΔTc temperature correction from this model can be used as a direct substitute for the Dst-derived correction that is now used in JB200, it could be possible to predict ΔTc with greater accuracy and lead time. Copyright 2011 by the American Geophysical Union.

Bruinsma S.L.,French National Center for Space Studies | Doornbos E.,Technical University of Delft | Bowman B.R.,Space Environment Technologies, LLC
Advances in Space Research | Year: 2014

Atmospheric densities from ESA's GOCE satellite at a mean altitude of 270 km are validated by comparison with predictions from the near real time model HASDM along the GOCE orbit in the time frame 1 November 2009 through 31 May 2012. Except for a scale factor of 1.29, which is due to different aerodynamic models being used in HASDM and GOCE, the agreement is at the 3% (standard deviation) level when comparing daily averages. The models NRLMSISE-00, JB2008 and DTM2012 are compared with the GOCE data. They match at the 10% level, but significant latitude-dependent errors as well as errors with semiannual periodicity are detected. Using the 0.1 Hz sampled data leads to much larger differences locally, and this dataset can be used presently to analyze variations down to scales as small as 150 km. © 2014 COSPAR. Published by Elsevier Ltd. All rights reserved.

Agency: National Aeronautics and Space Administration | Branch: | Program: SBIR | Phase: Phase I | Award Amount: 100.00K | Year: 2011

Commercial aircrew members and frequent flyers face radiation hazards from the effects of cosmic rays and solar energetic particles. During significant solar events, dose rates can exceed safety thresholds. To mitigate the radiation dose rate and total dose hazards, a unique, state-of-the-art system of physics-based models and real-time data characterizing the aviation radiation environment called Nowcast of Atmospheric Ionizing Radiation for Aviation Safety (NAIRAS) is undergoing development. However, validation of the NAIRAS system must occur to provide confidence that accurate nowcasts, and eventually forecasts, can be made for the aviation radiation environment. The Automated Radiation Measurements for Aviation Safety (ARMAS) project will provide that validation in a cost-effective manner. The Tissue Equivalent Proportional Counter (TEPC) radiation detector measures the rate and total quantities of absorbed dose and dose equivalent during aircraft flights. These measurements help estimate the biological risk associated with radiation exposure to humans. Up to three flights of TEPC will be flown during the first half of the performance period. The flight regimes are designed to test a range of representative radiation environments. TEPC results will be analyzed in the second half of the performance period and compared with NAIRAS to validate modeled flight profile results.

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