SA33C-01 INVITED
Overview of TIMED CEDAR observations showing the MLTI system response to changing drivers from solar maximum to solar minimum
TIMED CEDAR observations have spanned a large range of solar activity conditions from just past solar maximum during which active region eruptions dominate, through the descending phase during which corotating interaction regions reach a maximum, and now fast approaching a 22-year minimum in magnetic activity during which inputs from below become increasingly prominent against a magnetically quieting state. During this interval, TIMED observed the response of the MLTI to a large number of powerful X-class flares, severe solar proton events, 7 superstorms, newly identified sawtooth events, high dynamic pressure during both southward and northward IMF, and to the strongest CIR-driven magnetic activity of the last 4 solar cycles. We will summarize new knowledge about the response of the MLTI to the very different characteristic inputs from this range of drivers and explore resulting insights into the behavior of the geospace system. As the ascending phase of solar cycle 24 begins, TIMED will observe the response of the MLTI to a new regime in the balance between solar irradiance and magnetic activity, specifically to rising F10.7 in the presence of low magnetic activity. An developing list of unanswered questions related to the MLTI state under these driving conditions will be presented in preparation for continuing community interactions in support of TIMED CEDAR MLTI science.
SA33C-02
Solar Cycle and Seasonal Variation of Thermospheric Density and Composition
Simulations with the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) have been conducted from solar maximum to solar minimum using measurements made by the TIMED Solar Extreme-ultraviolet Experiment (SEE) as input. These simulations have demonstrated good correspondence with satellite drag measurements of thermospheric density. However, in order to obtain this agreement, the semi- annual and annual terms of seasonal variation in the thermosphere must be accounted for. Large-scale thermospheric dynamics have been proposed as a mechanism for the semiannual variation, but cannot explain the annual variation observed as global differences between December and June densities shown in satellite drag measurements. In the MSIS model, seasonal changes are represented by empirical variations of thermospheric composition and temperature. For these TIEGCM simulations, seasonal variation of the eddy diffusivity parameter in the lower thermosphere is imposed. This alters density and temperature throughout the thermosphere by influencing the atomic/molecular balance. In this presentation, we explore the question of whether this description of seasonal variation in density is consistent with observations of thermospheric composition. Measurements from the TIMED Global Ultraviolet Imager (GUVI) of the atomic oxygen to molecular nitrogen ratio (O/N2) are analyzed as a function of solar cycle and season, and compared to calculations using the TIEGCM with TIMED/SEE data. Through these comparisons, we examine the effects of solar forcing, eddy diffusion, and thermospheric large-scale dynamics on thermospheric O/N2 and their roles in producing global seasonal variation in the thermosphere.
SA33C-03
Energy Balance in the Thermosphere from TIMED and SORCE data
Using a combination of data from the TIMED and SORCE satellites we have examined the macroscopic radiative energy balance of the atmosphere above 100 km. Time series of absorbed solar radiation from 0 to 120 nm and 120 to 175 nm have been developed. In addition, we compute the rates at which infrared energy is radiated by the atmosphere at 5.3 microns (by nitric oxide), 15 microns (by carbon dioxide), and 63 microns (atomic oxygen) using both measured emission rates and model computations. The effects of the declining phase of the current solar cycle are evident in the time series of both the absorbed solar radiation and the infrared emission at all wavelengths. In addition, spectral analyses yields several interesting periodic effects present in the infrared time series. Most notable is the occurrence of a statistically significant 9-day periodicity in the infrared data that is absent in the solar ultraviolet data, but is present in the time series of the Ap and Kp indexes. These results imply a coupling of the geomagnetic environment to the infrared energy budget of the thermosphere. We will also examine the energy balance above 100 km with these data.
SA33C-04
Characterization of Storm-Time E-region Electron Densities from TIMED/SABER 4.3 um Emission and Comparisons with Incoherent Scatter Radar Measurements
Observations of thermospheric infrared 4.3 um limb emission from the TIMED/SABER instrument have fostered the development of new data products, models, and analysis tools for the study of upper atmospheric and ionospheric response to solar-geomagnetic disturbances. Enhancements in nighttime 4.3 um emission during storm periods are due to vibrational excitation of NO+ (i.e., NO+(v)), caused by auroral dosing and subsequent ion-neutral chemical reactions, followed by radiative emission at 4.3 um. The fundamental observation-based quantity used to study the E-region from SABER 4.3 um emission measurements is the NO+(v) volume emission rate (VER). The NO+(v) VER is a new SABER data product and analysis tool which is derived by (1) removing the background CO2 infrared emission using SABER-based non-LTE radiation transfer models, and (2) by performing a standard Abel inversion on the residual radiance. We have found the NO+(v) VER to be a highly versatile data product, useful for characterizing the morphology of the E-region during solar-geomagnetic storms, studying E-region chemistry and energetics during auroral dosing, and for quantifying the enhancement in the E- region electron density as a response to magnetic disturbances. The end-goal of the latter study is to develop an empirical E-region storm-time correction to the International Reference Ionosphere (IRI) model. The IRI model is a widely used empirical model for the specification of ionospheric parameters and is recommended for international use by the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI). However, the specification of the ionospheric response to solar-geomagnetic disturbances in IRI remains largely incomplete, and there is currently no storm-time correction to IRI parameters in the E-region. We report on the initial steps toward developing a correction to IRI E-region electron density, in which we study correlations between storm-time enhancements of NO+(v) VER and various solar-geomagnetic indices. In addition, we compare the SABER-derived NO+(v) VER peak enhancements and vertical structure during storm periods with measurements from high-latitude incoherent scatter radar, e.g., from the European Incoherent SCATer (EISCAT) radar facility.
SA33C-05
Earth's Thermosphere under extreme solar EUV radiation environment
It has been suggested that the exobase temperature of early terrestrial planetary atmosphere could have reached over 10,000 K (Kulikov et al. 2006) due to the extreme (up to 100 times that of today) solar EUV energy flux from the young Sun during the early stage of planetary evolution. Such high exobase temperature should have caused the dominant species at the exobase to escape at significant rate. Extremely fast escape of major gases in planetary atmospheres will lead to deviation from hydrostatic equilibrium. A newly developed 1-D, multi-component, hydrodynamic model has been used to investigate the response of Earth's thermosphere/ionosphere to extreme solar EUV conditions (Tian et al. 2007). We found that Earth's thermosphere/ionosphere could experience the transition from a hydrostatic equilibrium regime into a hydrodynamic regime when exposed to solar EUV fluxes exceeding certain critical level. In this regime, adiabatic cooling related to the hydrodynamic flow must be taken into the energy consideration. Due to extreme solar EUV fluxes, atomic nitrogen may have been the dominant species in upper thermosphere instead of atomic oxygen. In this work, we couple the hydrodynamic thermosphere model with an expanded GLOW model (including the electron impact ionization and excitation of nitrogen atoms) to investigate the contributions of photoelectrons and secondary electrons to thermospheric energetics under extreme conditions. The combined model provides self- consistent heating efficiency estimates for the Earth's atmosphere under extreme conditions. Implications of the simulation results to other early planetary atmospheres and their evolutions will be discussed.
SA33C-06
Monitoring the Dayside Equatorial Anomalies With 135.6 nm Limb Emission
The TIMED/GUVI instrument is able to resolve 135.6nm emission from atomic oxygen ion recombination on the dayside limb (above the lower altitude day-glow emission). Signatures of the equatorial anomalies are clearly visible in this data. These signatures can be used to study the magnitude, asymmetry, separation and, to a large extent, the height of the anomalies. These quantities, in turn, contain information about electric fields in the F region of the ionosphere. We present here an analysis of this data taken during selected strong storm periods and contrast the storm-time behavior with quiet time variations.
SA33C-07
Comparison of Column O/N2 From GUVI Limb Profiles and Disk Images
We present the first statistical, direct comparison among column O/N2 values obtained from GUVI limb profiles and disk-image measurements of the OI 135.6 nm and N2 LBH emissions. In particular, we focus on geomagnetically quiet periods when the compositional structure of the thermosphere is uncomplicated and the measurement and analysis methods for both types of data should produce highly accurate values. Our comparisons of data from 2002-2005 show that limb-derived values are higher than disk-derived values by 5-10%. One possible cause for this difference is the value of the cross section for electron impact on N2 that is the source of the LBH emission but has proved difficult to resolve between various measurements. Thermospheric O and N2 profiles retrieved from limb airglow profiles have been found to be largely independent of this cross section, while the analysis of disk images to obtain column O/N2 relies on knowledge of the absolute magnitude of the intensities and thus is dependent on the cross sections for electron impact on O and N2. Our results from these years suggest the need to increase the cross section for N2 LBH an additional 10-20% from the factor of 1.4 that is currently applied to the Ajello and Shemansky (1985) cross section in the analysis of the GUVI disk images that are available in the online data products. However, limb O/N2 measurements during the solar minimum period of 2006 are lower than the disk values, suggesting a correlation may exist between the solar EUV and the determination of the LBH excitation cross section.