SA14A-01 16:00h
Theoretical and Empirical Descriptions of Thermospheric Density
The longest-term and most accurate overall description the density of the upper thermosphere is provided by analysis of change in the ephemeris of Earth-orbiting satellites. Empirical models of the thermosphere developed in part from these measurements can do a reasonable job of describing thermospheric properties on a climatological basis, but the promise of first-principles global general circulation models of the coupled thermosphere/ionosphere system is that a true high-resolution, predictive capability may ultimately be developed for thermospheric density. However, several issues are encountered when attempting to tune such models so that they accurately represent absolute densities as a function of altitude, and their changes on solar-rotational and solar-cycle time scales. Among these are the crucial ones of getting the heating rates (from both solar and auroral sources) right, getting the cooling rates right, and establishing the appropriate boundary conditions. However, there are several ancillary issues as well, such as the problem of registering a pressure-coordinate model onto an altitude scale, and dealing with possible departures from hydrostatic equilibrium in empirical models. Thus, tuning a theoretical model to match empirical climatology may be difficult, even in the absence of high temporal or spatial variation of the energy sources. We will discuss some of the challenges involved, and show comparisons of simulations using the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) to empirical model estimates of neutral thermosphere density and temperature. We will also show some recent simulations using measured solar irradiance from the TIMED/SEE instrument as input to the TIE-GCM.
SA14A-02 16:15h
Estimating the Upper Atmospheric Forcing through Observing and Modelling the Thermospheric Response
A data assimilation system for specifying the thermospheric composition and denity has been developed over the last several years. Although the drivers of the upper atmosphere are well understood, quantifying the thermospheric forcing still remains a primary error source in the data assimilation system during geomagnetic storm periods. Correctly modelling the high latitude forcing requires knowledge of the spatial and temporal variations of the convection electric field and the auroral precipitation. At best, the globally averaged Joule heating is known only within a factor or two. At a given location, this uncertainty can rise to a factor of ten. However, since the solar heating at low latitudes and the magnetospheric sources at high latitudes control the magnitude and spatial distribution of the global circulation, these strongly affect the neutral composition and density structure. Observations of the change in the neutral density and composition structure consequently provide an additional information source in specifying the upper atmospheric driver. The enhanced driver specification may, in turn, improve the thermospheric specification in the data assimilation scheme.
SA14A-03 16:30h
Global Observations of Exospheric Temperature During the Bastille Day Event and Comparisons With the TIMEGCM Model and NRLMSIS-2000
We present observations of the global exospheric temperature and its variation during the Bastille Day geomagnetic storm that occurred on July 14-18, 2000. The Bastiile Day event was initiated by an X-class solar flare that was followed by a coronal mass ejection. The CME eventually produced a major geomagnetic storm. Global exospheric temperatures were derived by fitting the topside intensity distribution of limb scan data observed by the Low Resolution Airglow and Aurora Spectrograph (LORAAS) on the {\it Advanced Research and Global Observation Satellite (ARGOS)}. The {\it ARGOS} was launched into a sun-synchronous orbit on 23 February 1999 at 2:29:55 AM Pacific Standard Time. The LORAAS obtained limb scans every 90 seconds providing soundings spaced by approximately 5.4$\deg$ of latitude. We compare the LORAAS-derived exospheric temperatures with exospheric temperatures predicted by the Mass Spectrometer and Incoherent Scatter (MSIS) with Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIMEGCM). We find that the measured temperatures are systematically higher, by $\sim$200 K, than predicted by the NRLMSIS-2000 model and the temporal evolution of the temperature changes is also not well reproduced by the model. The TIMEGCM model, with its more accurate picture of the geomagnetic and solar effects, is shown to better capture both the time evolution of the storm-time temperature changes and to also more accurately predict the quiet time temperatures.
SA14A-04 16:45h
The Semiannual Thermospheric Density Variation From 1970 to 2002 Between 200-1100 km
This study characterizes the semiannual thermospheric density variation over the last three solar cycles, covering the height range of 200 to 1100 km. Historical radar observational data have been processed with special orbit perturbations on 13 satellites with perigee heights ranging from 200 to 1100 km. The semiannual variation has been found to be extremely variable from year to year. The magnitude of the maximum yearly difference, from the July minimum to the October maximum, is used to characterize the yearly semiannual variability. It has been found that this maximum difference can vary by as much as 100 percent from one year to the next. A high correlation has been found between this maximum difference and solar EUV data. The semiannual variation for each year has been characterized based on analyses of annual and semiannual cycles.
SA14A-05 17:00h
Atmospheric Neutral Density Observed With Accelerometers
The German satellite CHAMP was launched in a circular, nearly-polar orbit at 460 km altitude in July 2000. The two main mission objectives are the mapping of the magnetic and gravity fields of the Earth. The US/German GRACE satellites were launched in March 2002, also in a circular, nearly-polar orbit, but at 500 km altitude. The GRACE satellites are separated from each other by approximately 220 km, so the satellites pass the same geographical location within about 30 seconds of each other. The mission objective is to track changes in the Earth's gravity field. The CHAMP and GRACE satellites carry STAR accelerometers, positioned at their centers of mass, and GPS receivers and attitude sensors in order to determine accurate satellite positions as well as accurate accelerometer calibration parameters. Total atmospheric density has been derived from the CHAMP/STAR data, and presently about 3 years of observations are available. The GRACE/SuperSTAR accelerometer data are available to the scientific community since August 2004, and atmospheric density can be derived using a similar procedure as the one succesfully applied to CHAMP/STAR data. The CHAMP and GRACE satellites do not orbit at the same altitude (CHAMP is in an approximately 100 km lower orbit), and their orbital planes generally do not coincide. Therefore, the local time vs. latitude sampling is much better using data from both missions. On the other hand, if one wants to study the propagation of waves or disturbances, the orbital planes must be coincident or nearly so. In this study, the densities observed by CHAMP and GRACE are intercompared, both for (almost) coincident orbital planes as well as (almost) perpendicular ones. Secondly, the densities of GRACE-A and GRACE-B are compared in order to investigate if short-scale variations of the order of 100-200 km can be observed. The density observations will also be compared to thermosphere models, and certain modelling errors will be discussed.
SA14A-06 INVITED 17:15h
Mesosphere-Thermosphere Coupling
The mesosphere supplies energy, momentum and heat to the thermosphere via upward-propagating tides, gravity waves and planetary waves that originate at various levels in the atmosphere. These waves redistribute neutral and ionized constituents, and generate electric fields. Their relevance to thermosphere-ionosphere variability becomes particular evident during solar minimum, when effects due to solar particle and EUV radiation are subdued. In this paper, three aspects of the problem are elucidated: (1) The use of general circulation models to provide information on wave coupling into the thermosphere; (2) the use of Hough Mode Extensions to estimate total mass density variability consistent with wind or temperature observations measured near the mesopause; and (3) the combined use of measurements and theory to estimate the zonal mean winds and temperatures generated due to dissipation of upward-propagating tides.
SA14A-07 17:30h
The midnight temperature maximum: seasonal and solar cycle climatology
Examination of the series of low-latitude thermospheric temperatures observed in the early evening between solar minimum and solar maximum with the Fabry-Perot interferometer located at Arequipa, Peru (16.5 S, 71.5 W) have shown an increase from typical values of 650-700 K to 1100-1150 K between early 1997 and mid-2000. These results represent 396 nights of temperature measurements between 0 and 9 UT. Comparison with the MSIS 00 NRL model showed generally good agreement but with frequent large deviations of 50 to 100 K illustrating the large variability of the thermospheric temperature at low latitudes. The Arequipa temperature observations were used to study the morphology of the midnight temperature maximum (MTM) which is a heating of the low-latitude thermosphere that takes place during the midnight hours and is not reproduced in the MSIS 00 model. The mean MTM amplitude was found to be typically 75 K during solar minimum and about 20 K lower at solar maximum. The peak amplitude of the MTM occurs between 06 and 08 UT during the winter solstice and between 05 to 07 near the equinoxes. The nighttime thermal variation of the MTM is well represented by a Gaussian distribution with a full width extent of 3 to 5 hours. These results display considerable variability from night to night in the amplitude and timing of the MTM appearance, which probably implies a strong degree of variability in the tidal forcing believed to be the cause of the MTM phenomenon.
SA14A-08 17:45h
Day-to-Day Variability of the F-layer at Sunrise
Using a 30-day continuous incoherent scatter radar (ISR) data base from Millstone Hill and the Utah State University Time Dependent Ionospheric Model (TDIM), we investigate the day-to-day variability in the rate of increase of the electron densities as the ionosphere enters sunrise. The data shows this variability to be quite large. Rishbeth et al [1995] studied the F-layer at sunrise and showed that there is a seasonal dependence, which was explained as a consequence of seasonal variation in the atomic to molecular ratio of the neutral atmosphere in the F2-layer. The month-long data base used in this study extends from 4 October to 4 November 2002. The observations provide full electron density, electron temperature, ion temperature, and vertical ion drift profiles throughout the F-layer. Using the TDIM ionospheric model, simulations are carried out to find the source of the marked day-to-day differences in the sunrise response of the F-layer at the Millstone Hill location. Possible drivers of this variability include the temperature and drift variation as well as neutral density and ratio effects, the latter being associated with geomagnetic forcing of the thermosphere. The role of pre-dawn conditioning of the ionosphere is also considered.