SA24A-01
Density Variations Observable by Precision Satellite Orbits
This research uses precision satellite orbits from the Challenging Minisatellite Payload (CHAMP) satellite to produce a new data source for studying density changes that occur on time scales less than a day. Precision orbit derived density is compared to accelerometer derived density. In addition, the precision orbit derived densities are used to examine density variations that have been observed with accelerometer data to see if they are observable. In particular, the research will examine the observability of geomagnetic storm time changes and polar cusp features that have been observed in accelerometer data. Currently highly accurate density data is available from three satellites with accelerometers and much lower accuracy data is available from hundreds of satellites for which two-line element sets are available from the Air Force. This paper explores a new data source that is more accurate and has better temporal resolution than the two-line element sets, and provides better spatial coverage than satellites with accelerometers. This data source will be valuable for studying atmospheric phenomena over short periods, for long term studies of the atmosphere, and for validating and improving complex coupled models that include neutral density. The precision orbit derived densities are very similar to the accelerometer derived densities, but the accelerometer can observe features with shorter temporal variations. This research will quantify the time scales observable by precision orbit derived density. The technique for estimating density is optimal orbit determination. The estimates are optimal in the least squares or minimum variance sense. Precision orbit data from CHAMP is used as measurements in a sequential measurement processing and filtering scheme. The atmospheric density is estimated as a correction to an atmospheric model.
SA24A-02
Understanding and forecasting solar EUV and UV irradiance variations
We describe the application and current status of the Solar Radiation Physical Modeling (SRPM) project for understanding the nature of the solar EUV and UV irradiance variations and building tools to forecast short and medium term variations of the solar irradiance spectrum at any location in the Heliosphere. These methods can be used for very detailed estimates of the EUV solar irradiance changes on the Earth and planetary atmospheres. Reliable EUV estimates are an important input for the modeling of the physical parameters of the thermosphere. In the presentation we show the current status, the solar atmospheric models, the synthetic EUV and UV spectra and the performance of the forecasting tool.
SA24A-03 INVITED
Thermosphere Density Response to High Speed Solar Wind Streams
Thermosphere densities at 400 km altitude from accelerometer measurements on the CHAMP satellite are used to investigate oscillations at periods of less than 13 days during the declining phase of solar cycle 23 (2002-2007). Density spectra reveal the presence of prominent periods around 5 days, 7 days and 9 days. Specifically, a pronounced 9-day periodicity is obtained in the years from 2004 to 2007, and a strong periodicity of about 7 days occurred in 2006 and 2007. The 5-day periodic oscillations were also seen during 2004-2007, but their powers are smaller than those of 7-day or 9-day oscillations. These periodic oscillations in neutral density tend to occur during the latter part of the declining solar cycle when periodic fast streams in the solar wind modulate the level of geomagnetic activity in the geospace environment. In fact, the periodic oscillations seen in neutral densities are simultaneously present in the solar wind and geomagnetic activity index Kp, whereas not in the SOHO solar EUV irradiance data. The periods of 5, 7 and 9 days apparently reflect subharmonics of the 27-day rotation, and may be related to the longitudinal distribution of coronal holes. The periodic oscillations in neutral density are felt globally, and are proportional to the periodic Kp perturbations at the same frequency. The neutral density data thus suggest that these oscillations are related to the solar wind, and not to the influences of planetary waves at similar periods originating in the lower atmosphere.
SA24A-04
The annual variation of globally averaged thermospheric total mass density
Solar extreme ultraviolet (EUV) irradiance is the primary source of energy in the thermosphere, and EUV irradiance variations are the dominant source of thermospheric density variability. Irradiance variations are driven by solar oscillations (e.g., due to the 11-year solar cycle and 27-day rotational modulation) and by the Earth's rotation and revolution (e.g., diurnal and annual cycles). The annual irradiance variation consists of a local component arising from the inclination of Earth's axis to its orbital plane (i.e., seasonal effects), and a global component arising from the eccentricity of Earth's orbit, which causes the Earth-sun distance to vary by +/-1.7 percent. Empirical density models typically use the EUV flux incident at the Earth (rather than at 1 AU) as an input argument, thereby combining the effects of solar-driven EUV irradiance variations with those of Earth-orbit-driven global annual variations. However, there is a significant global annual variation in thermospheric density that is not accounted for with this approach. In this presentation, we examine the annual variation in global average thermospheric density (derived from the orbits of many near-Earth objects) and its dependence on the phase of the solar cycle. We consider the relative merits of empirically representing this oscillation via the EUV irradiance or via harmonic terms. We conclude that it is more advantageous to use the EUV irradiance at 1 AU to represent purely solar-driven EUV variations, while employing separate annual Fourier terms to represent the global annual density oscillation.