SA21B-1536
Comparison of TIDI Wind Observations and the HWM07 Model
The latest version of the Horizontal Wind Model (HWM07) is an empirical model of the Earth's winds from the surface to 500 km. Data sources include measurements from satellite, rocket, and ground-based instrumentation that have been collected over the last 50 years. One notable source of wind information not included in the model is from the TIMED Doppler Interferometer (TIDI) on the TIMED satellite. TIDI provides an independent set with which to evaluate to the model in the mesosphere and lower thermosphere. This paper provides a comparison of the two data sets.
SA21B-1537
Thermospheric Neutral Density Responses to Changes in IMF Sector Polarity
The thermospheric density is important not only for satellite orbital tracking, but also in understanding the thermosphere-ionosphere coupling process as well. Thermospheric density variations are controlled by various sources such as Joule/particle heating, Lorentz force, thermal expansion, upwelling and horizontal wind circulation. These sources are directly or indirectly associated with the direction and/or strength of the interplanetary magnetic field (IMF). That is, there is an intimate relationship between IMF variation and thermospheric density variation. In order to examine how thermospheric density variations are influenced on the orientation and/or strength of the IMF, we used total mass density around 400 km, derived from the high- accuracy accelerometer on board the Challenging Minisatellite Payload (CHAMP) spacecraft, in 2003 when the IMF exhibited a well-defined sector polarity change with a ~27-day periodicity; directed toward the Sun (i.e., +Bx and –By) and away the Sun (-Bx and +By). It has been known that the IMF By in GSE coordinates makes a positive or negative IMF Bz offset in GSM coordinate. We discuss whether the thermospheric total mass density from CHAMP changes with the IMF sector polarity.
SA21B-1538
New Applications for the Jacchia 77 Model
We examine the Jacchia 77 model and compare model densities to the 2001-2005 densities derived from the CHAMP and GRACE accelerometer data. Of particular interest is the model's unique formulation of exospheric temperature directly from the solar flux (F10) as opposed to a nighttime minimum temperature in the earlier Jacchia 70 and Jacchia 71 models. We compare this calculation directly to average global exospheric temperatures derived from the CHAMP and GRACE accelerometer neutral density data using the hydrostatic equation. The average global exospheric temperature is important because the model density profiles are all derived from this quantity. The Jacchia 77 model includes a special 81-day weight averaged F10 as a model proxy. This approach uses the F10 from the last three solar rotations instead of a centered F81 index, which means the model can be used in real time by using an 81-day weighted "boxcar" index. With new solar proxies for EUV and Mg recently introduced, we discuss the possibility of incorporating these indices in a similar manner. Because the drivers of thermospheric density-- the semiannual variation, solar EUV and solar wind are treated as separate modules in the model-- we examine the strengths and weaknesses of each one as consideration for future model upgrades.
SA21B-1539
UV emissions observed by the SCIFER2 sounding rocket
The SCIFER2 sounding rocket was launched into a Poleward Moving Auroral Form (PMAF) event on January 18, 2008. As is typical, the event was characterized by the presence of soft electron precipitation, driving discrete auroral arcs at 630 nm. The rocket payload, which included a UV photometer as well as several other instruments, flew in the vicinity of the dayside cusp and reached an apogee of approximately 1500 km. The photometer was included on the payload in order to explore the possibility that sunlight might scatter from upwelling neutral gases as a result of the electron precipitation (and associated microphysical processes). Near these altitudes, the photometer was pointed approximately perpendicular to local zenith. In this configuration, highly structured UV emissions were measured emanating from sources located several hundred km, perhaps 1000 km or more, above the ground. In this study, we examine rocket and ground data to determine the source of these emissions, including the possibilities that the structures are associated with auroral phenomena, flourescence or, as suggested above, from sunlight scattered from high-altitude (upwelling) neutral gases.
SA21B-1540
An Empirical Model of the Earth's Horizontal Wind Fields: HWM07
The new Horizontal Wind Model (HWM07) provides a statistical representation of the horizontal wind fields of the Earth's atmosphere from the ground to the exosphere (0 to 500 km). It represents over fifty years of satellite, rocket, and ground-based wind measurements via a compact Fortran-90 subroutine. The computer model is a function of geographic location, altitude, day of the year, solar local time, and geomagnetic activity. It includes representations of the zonal mean circulation, stationary planetary waves, migrating tides, and the seasonal modulation thereof. The objective of the work presented is to address known HWM93 discrepancies by improving the mathematical formulation and assimilating new data sets. In this paper we present an overview of the new observational database, mathematical formulation, model parameter estimation procedure, and overall results with comparisons to data.
SA21B-1541
Properties of Traveling Atmospheric Disturbances (TADs) Inferred From CHAMP and GRACE Accelerometer Observations
The accelerometers on the CHAMP and GRACE satellites have made it possible to accumulate near- continuous records of thermosphere density between about 370 and 490 km since May 2001, and July 2002, respectively. They have recorded the response to virtually every significant geomagnetic storm during this period. CHAMP and GRACE are in (near) polar and quasi-circular orbits, sampling 24 hr local time approximately every 4 and 5 months, respectively. These capabilities offer unique opportunities to study the temporal and latitudinal responses of the thermosphere to geomagnetic disturbances. Data from initially 34 geomagnetic storms were explored, but significant and unambiguous TAD activity in the observed response of the thermosphere was detected for about half the events. The atmospheric variability is evaluated by de-trending the data, allowing the extraction of specific ranges in horizontal scale, and analyzing density "residuals". The scale of the perturbation is decisive for its lifetime and relative amplitude. Sometimes the disturbances represent wave-like structures propagating far from the source, and these so- called 'TADs' were detected and described for the May 2003 storm for the first time. Some TADs traveled over the pole into the opposite hemisphere; this was found in both CHAMP and GRACE data. Most TADs propagate equatorward, but poleward propagating TADs have on occasion been detected too. The estimated speeds of the observed TADs are of the order of 400-900 m/s, and their mean scale is approximately 2000 km. The TADs observed with GRACE are significantly slower than those seen in the CHAMP data. The opposite is expected from theory: speed increasing with altitude.
SA21B-1542
Observation and Modeling of Ion Upwelling Above Aurora
Auroral electron precipition heats the ionospheric plasma. Especially at F-region altitudes, this leads to increased plasma pressure and a pressure gradient force that accelerates plasma away from the heated region. The resulting upward ion velocities have been observed by the incoherent scatter radar at Poker Flat (PFISR). The upward moving ions cause an increased ion density well above typical auroral ionization altitudes. N2+ ions that are lifted to altitudes above the shadowheight will resonantly scatter sunlight. This is observed by coincident overflights of the Solar Mass Ejection Imager (SMEI) on the Coriolis satellite, looking up from 840 km altitude. We will present a study that combines modeling and observations by PFISR and SMEI to illustrate and explain this process.
SA21B-1543
Thermospheric Mixing by Intense Small-Scale Joule Heating
Intense small-scale Joule heating structures can induce mixing of thermospheric constituents, which may significantly supplement the mixing caused by the global-scale meridional circulation driven by the total Joule heating. Associated with the net upward flux of molecular nitrogen and oxygen due to the small-scale mixing is an additional upward heat flux. The relative importance of small-scale mixing, as compared with the importance of mixing by the meridional circulation, likely increases with magnetic activity. A parametrization is presented in terms of an effective eddy diffusivity. For a moderately active level of Joule heating, the effective eddy diffusivity may be a sizable fraction of molecular diffusivity between 120~km and 250~km altitude at auroral latitudes.
SA21B-1544
Principal Modes of Thermospheric Density Variability: Empirical Orthogonal Function Analysis of CHAMP 2001-2007 Data
In this paper we characterize dominant modes of global density variability as empirical orthogonal functions (EOFs) using densities obtained from the accelerometer experiment on board the CHAMP satellite during 2001-2007. From a sequential non-linear regression analysis of sparse observations of the density along satellite trajectories, we obtain EOFs in latitude/local-time coordinates and its orbit-time dependent amplitude. The analysis unravels not only the importance of each mode to the overall density variability but also underlying drivers of the primary modes of the density variability. The primary mode of the variability takes form of the diurnal variation. It correlates highly with daily F10.7 index and moderately with geomagnetic indices (Kp and Dst). It underscores that solar EUV is far the strongest driver of the overall thermospheric density variability. The secondary mode has a latitudinally asymmetric structure, and represents the summer-to-winter annual density variation. In contrast to this annual variation of the secondary mode, the primary mode indicates semiannual variation with peaks at around equinoxes. The quaternary mode reflects effects of the high-latitude forcing.
SA21B-1545
Nitric Oxide in the Lower Thermosphere
Nitric Oxide (NO) is a crucial minor species in the lower thermosphere. It plays a strong role in the thermospheric energy balance as it emits efficiently in the infrared, it is the terminal ion in the lower ionosphere, and if transported to lower altitudes will catalytically destroy ozone. NO is primarily produced through the reaction of excited atomic nitrogen with molecular oxygen. One of the primary loss mechanisms of NO is photodissociation by solar ultraviolet irradiance. In order to produce the excited atomic nitrogen atom, the strong N2 molecular bond must be broken. At low latitudes, solar soft X-ray irradiance is the energy source that leads to NO. At high latitudes, auroral electrons and the energetic secondary electrons provide the source of energy that leads to the large amounts of NO observed there. While the governing processes controlling production and loss of NO have been settled, our understanding of these processes is still evolving. In recent years there has been new progress in relevant chemical reaction rates and their temperature dependencies, diffusion rates, and the magnitude and variability of solar energy deposition. In this talk we discuss the new results and assess their impact with a 1D photochemical model. Model results will be compared to observations by the Student Nitric Oxide Explorer (SNOE). SNOE observed fluorescently scattered sunlight by NO at 215 and 237 nm to obtain global concentrations of NO in the lower thermosphere daily from February 1998 through December 2003.
SA21B-1546
On the geomagnetic storm response and recovery timescales of the thermosphere
The temperature of the Earth's thermosphere can be substantially increased during geomagnetic storms mainly due to high-latitude Joule heating induced by magnetospheric convection and auroral particle precipitation. The main cooling mechanism controlling the recovery of neutral temperature and density to geomagnetic activity is the infrared emission from nitric oxide (NO) emission at 5.3 micrometers. NO is produced by both solar and auroral activity, the first due to solar EUV and X-rays the second due to particle dissociation of N2, and has a typical lifetime of 12 to 24 hours in the mid and lower thermosphere. NO cooling in the thermosphere peaks between 150 and 200 km altitude. In this paper, a global, three-dimensional, time-dependent, non-linear coupled model of the thermosphere, ionosphere, plasmasphere, and electrodynamics (CTIPe) has been used to determine the response and recovery timescale of the upper atmosphere to geomagnetic activity. In these simulations, realistic NO storm increases are defined by the three-dimensional nitric oxide empirical model (NOEM) based on measurements from the Student Nitric Oxide Explorer (SNOE) scientific satellite. The F10.7 index is used to define solar EUV heating. The magnetospheric energy input into the system is characterized by the time variation of the solar wind velocity, the interplanetary magnetic field (IMF) magnitude and direction, and the auroral precipitation index derived from the TIROS/NOAA satellite observations. The solar wind parameters and auroral indices are used to define the magnetospheric convection electric field and auroral ionization/heating rates. The energy is subsequently lost from the system primarily by infrared radiation, particularly by NO cooling. The source is therefore the time integral of the electromagnetic energy input and the loss is radiative cooling. Together they combine to provide the characteristic response and recovery of the system to geomagnetic activity. Comparisons of the neutral density observed by the CHAMP satellite with predictions of CTIPe are presented for selected geomagnetic storm events.
SA21B-1547
Simulations of Tsunami Effects in the F-Region Ionosphere
Recent observations of F-region electron densities and of total electron content (TEC) have revealed disturbances that appear to be correlated with tsunamis. These observations show large electron density perturbations (~ 100%) and large TEC fluctuations (~ 30%) propagating at speeds of ~ 200 m/s and with a characteristic horizontal wavelength of ~ 300 km to 500 km. Published simulations of tsunami propagation through the atmosphere and subsequent interaction with the ionosphere have revealed striking similarities with the observations. However, important physical processes known to affect gravity wave propagation were not included in these prior analyses, while the ionospheric perturbation models they included were oversimplified. Here we describe numerical simulations of the upward propagation of a spectrum of gravity waves forced by a traveling deformation of the lower boundary and the interaction of these waves with the F-region ionosphere. The tsunami is assumed to travel as a steady-state disturbance at the lower boundary (z=0) with a vertical displacement described by a modified Airy function in the horizontal direction. It travels at the shallow water wave speed of 200 m/s. The horizontal wavenumber spectrum of the tsunami is first calculated and from this the vertical velocity spectrum at the surface is calculated. This spectrum is used to provide the forcing at the lower boundary of a spectral full-wave model. This model describes the propagation of linear, steady-state acoustic-gravity waves in a non-isothermal atmosphere with the inclusion of eddy and molecular diffusion of heat and momentum, ion drag, Coriolis force, and height-dependent mean winds. A steady-state 1-D ionospheric perturbation model including O+, NO+, O2+ and N2+ and electrons is used to calculate the electron density response to the tsunami. Electron density perturbations as a function of height and the total electron content (TEC) are calculated as a function of horizontal position. We perform simulations for an assumed maximum tsunami amplitude of 5 cm. Our simulations show that the effect of molecular diffusion is to strongly damp the waves in the topside (> 300 km altitude) ionosphere. In spite of this, the F-region response is large, with vertical displacements of ~ 2 to 5 km and electron density perturbations of ~ 100%. Mean winds have a profound effect on the ability of the waves to propagate into the F-region ionosphere. The higher frequency gravity waves in the spectrum are Doppler shifted to even higher frequencies for propagation into a headwind, which inhibits the propagation of the disturbance to F-region altitudes. We summarize our simulations by comparison with some ionospheric observations.
SA21B-1548
Numerical Estimates of Polar Cusp Neutral Upwelling Using Satellite Conjunction Data
Recent observations have confirmed neutral particle upwelling at high latitudes which are localized to the polar cusp region and seem to be correlated to high auroral activity. For decades, thermospheric upwelling has been recognized as an important topic and has been studied observationally and theoretically, with efforts largely focused on Joule heating being the basic driver. As data and models have improved over the years, Joule heating has indeed proven to be fundamental to upwelling, at least at lower altitudes. At higher latitudes, however, the situation appears to be more complex and recent results indicate that Joule heating alone is not adequate. We investigate this issue with numerical models using data acquired by FAST and CHAMP satellites. Field and particle data from FAST and accelerometer data CHAMP are used from a single favorable conjunction alignment event. We compare results from two different models: a Joule heating model ("thermodynamic") and an auroral precipitation model "electrodynamic").
SA21B-1549
Poynting Flux Response to Strong Solar Wind Pressure Enhancements
Strong solar wind dynamic pressure changes cause dramatic variations in the ionospheric transpolar potential and coupling efficiency between the solar wind and the terrestrial magnetosphere. Magnetospheric particle precipitation into the high latitude ionosphere also shows strong responses to solar wind pressure pulses. Neutral density data from low Earth orbiting satellites suggest that these pulses are a source of unmodeled error in satellite drag computations. Improved data processing for measurements from DMSP satellites allows us to investigate the Poynting flux response to strong solar wind pulses that precede some magnetic storms. Including these and other data in the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure provides perspective on the temporal and spatial variations of energy available to fuel thermospheric upheavals associated with enhanced satellite drag. We compare the Poynting flux and corresponding ionospheric and thermospheric perturbations from storms that were preceded by large solar wind dynamic pressure increases during May 15-17, 2005 and Aug 24-26, 2005 with those of a storm without such pre-conditioning.
SA21B-1550
Periodic Modulations in Thermospheric Composition by Solar Wind High Speed Streams
We report on periodicities observed in the column density ratio, ΣO/N2, correlated with periodicities in solar wind speed and Kp index. The recently discovered solar-terrestrial connection between rotating solar coronal holes and mass density variations in the Earth's thermosphere by Lei et al. (2008) and Thayer et al. (2008) prompted this study and has led to further insight regarding the thermosphere response to periodic high-speed solar wind streams and recurrent geomagnetic activity. In particular, ΣO/ N2 ratios, measured by the Global Ultraviolet Imager (GUVI) instrument flown on the TIMED satellite, demonstrate strong 9 and 7 day oscillations in 2005 and 2006, respectively, that are well correlated with the solar wind speed and Kp index. More importantly, the ΣO/ N2 ratio response peaks at high latitudes, as opposed to the mass density response being global, indicating that vertical winds are active at the higher latitudes while lower latitudes experience primarily thermal expansion.
SA21B-1551
Thermospheric Density Fluctuations Derived from the Atmospheric Neutral Density Experiment Risk Reduction Mission
The Atmospheric Neutral Density Experiment (ANDE) Risk Reduction flight was launched on Dec 9, 2006 and deployed into orbit by the Space Shuttle Discovery on December 21, 2006. The primary mission objective is to test the deployment mechanism from the Shuttle for the ANDE flight in mid 2009. Scientific objectives of the ANDE risk reduction flight include: monitor total neutral density along the orbit for improved orbit determination of resident space objects, monitor the spin rate and orientation of the spacecraft, and provide a test object for polarimetry studies. The two ANDERR spacecraft decayed on December 25, 2007 and May 21, 2008, atmospheric densities derived from observations of the ANDERR spacecraft will be presented and compared to atmospheric models and drivers.
SA21B-1552
Geomagnetic Storm Associated Enhancements of High-Latitude Meridional Winds
During geomagnetically active times, thermospheric winds are strongly enhanced by large pressure gradients
and interaction with ionized particles. In many physical and empirical models, these winds are underspecified
due to the sparsity of wind measurements in the thermosphere. We use cross-track wind speeds measured
by the accelerometer aboard the CHAMP satellite to characterize the response of the high-latitude
thermosphere to magnetic activity. With an orbital inclination of 87.25°, the cross-track axis of CHAMP
is nominally oriented in the zonal direction; however, near the poles CHAMP is able to produce ~16
measurements per day of meridional winds at both the north and south poles. This dataset represents one of
the few available satellite-based thermospheric wind datasets that spans solar cycle 23 from maximum
through solar minimum (2001--2008). High-latitude meridional wind dependencies on magnetic activity, local
time and latitude, phase of the solar cycle, and solar wind configuration become apparent when employing a
statistical analysis of the entire dataset.
http://sisko.colorado.edu/sutton/
SA21B-1553
Signatures of the universal time effect in the thermosphere and ionosphere
Thermospheric neutral and ionospheric electron densities measured in situ by the CHAMP satellite at nearly constant local time show significant longitudinal/universal time variations under geomagnetically quiet conditions. An NCAR-Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) run under the same geophysical conditions as the CHAMP measurements reproduced these observed longitudinal variations. Further analyses of the TIEGCM results demonstrate that the displacement of the geomagnetic equator from the geographic equator is the cause of this longitudinal variability. A discussion of the physical processes involved, including neutral wind circulation and electrodynamics, indicate that the dynamo is critical in producing these variations.
SA21B-1554
Lower Atmospheric Gravity Wave Impacts on the Thermosphere
Gravity waves are a significant source of mass, momentum and energy in the mesosphere/lower- thermosphere regions. But little direct evidence is available for waves in the middle- and upper- thermosphere. A new 2-D non-hydrostatic, non-isothermal, gravity wave model has been developed, which allows lower atmospheric gravity waves produced via thunderstorms, flow over local topography, geostrophic adjustment, or wave-wave interactions, to be coupled into a global thermosphere model. The current study- uses the 2-D model to determine the characteristics of the gravity waves at thermospheric altitudes using realistic temperature profiles determined from atmospheric heating and cooling, along with self-consistently calculated thermal diffusion and viscosity. The temperature, velocity, and densities computed are total quantities, rather than just the perturbation quantities.
SA21B-1555
Possible equatorial detection of the nighttime thermal effect of the evening solar terminator wave
Fabry-Perot temperature measurements obtained at Arequipa, Peru (-16.5 S, 74 W) during the period of solar maximum activity in 1999-2001 during the fall-winter-spring period of April to October occasionally exhibited a secondary peak enhancement of 10 to 40 K amplitude in addition to the more normal midnight temperature maximum enhancement of 50 to 150 K. Temperature measurements obtained for each cycle of directions (N,E,S,W, and Z) were averaged to reduce the standard deviation of the error from 40-65 K for each point to 20-35 K. A total of about 20 nights exhibits such peaks as well as the MTM feature. These peaks were seen in the period prior to local midnight generally within one or two hours after evening twilight. The results of observations from 1999 to 2001 will be reviewed to obtained an estimate of the statistical rate of appearance of these secondary peaks. Data obtained during the recent solar minimum period will be discussed in regard to possible secondary peak detection. It is proposed that the these temperature enhancements may be attributed to the density wave associated with the motion of the solar terminator wave through the thermosphere as described by the recent work of Forbes and collaborators reporting on the analysis of CHAMP data.
SA21B-1556
Thermospheric Density in the Earth's Magnetic Cusp as Observed by the Streak Mission
Measurements of neutral gas density in the thermosphere at the base of the Earth's south magnetic cusp from the Streak mission are reported and discussed. In contrast to recent reports of enhanced density in the cusp, these measurements show the density to be depleted relative to the surrounding region. The difference is interpreted as an altitude effect. This observation and interpretation lead to new constraints on the physical mechanisms that could explain cusp upwelling and are inconsistent with a Joule heating mechanism. A mechanism based on heating by soft cusp particle precipitation is put forth, and model calculations are used to show how it explains the relative depletions observed by Streak as well as the enhancements at higher altitudes reported previously.
SA21B-1557
Paired Ionosphere-Thermosphere Orbiters (PITO): Science and Implementation
The Paired Ionosphere-Thermosphere Orbiters mission is described and discussed. The mission utilizes a pair of orbiting vehicles in eccentric, high-inclination, coplanar orbits. The orbits have arguments of perigee that differ by 180 degrees and are phased such that one vehicle is at perigee (200 km) while the second is at apogee (2000 km). Half an orbit later, the vehicles switch positions. Three types of science instruments are envisioned to take advantage of this scenario: in-situ instruments that measure, parameters locally, downlooking imagers that provide areal coverage below the satellite, and profilers that measure parameters long a long vertical line-of-sight. The main idea is that in addition to the two point measurements provided by the in-situ instrumentation, context information for the low-altitude measurements is obtained by the high- altitude imagers. In addition, profiling instruments such as sounders and spaceborne lidar can be added to create vertical profiles. Such an observation system is capable of providing elements of global coverage, regional coverage, and coverage in three dimensions. Science goals are presented, as are the results of a detailed implementation plan, including several trade studies on key elements of the mission. The conclusion is that the mission would enable significant new understanding of the ionosphere-thermosphere system within a resource envelope that is consistent with that of NASA's Medium Explorer (MIDEX) line of science missions.
SA21B-1558
Impact of non-hydrostatic processes on the thermospheric density and winds
One common assumption used in many theoretical thermosphere/ionosphere models is hydrostatic equilibrium, under which the pressure gradient force is balanced with the gravity force in the vertical direction. This assumption represents the large-scale atmosphere behavior very well, but on small spatial scales and during short time periods, the system can be non-hydrostatic. Non-hydrostatic processes can cause large vertical winds and strong disturbances of neutral density in the upper atmosphere. It is currently unknown what are the global ramifications of the non-hydrostatic processes. Comparison between the non- hydrostatic Global Ionosphere Thermosphere Model (GITM) and the hydrostatic Coupled Thermosphere Ionosphere Plasmasphere model (CTIP) helps us to quantify the non-hydrostatic coupled response of the system to strong driving. Specifically, the investigation begins with turning off all non-hydrostatic terms in GITM and forcing hydrostatic equilibrium at each time-step. The results are compared with a CTIP simulation to show the effects of model differences. The non-hydrostatic terms then are added back in GITM and the results are compared with CTIP again to determine their relative contributions. The simulation results are also compared with ground-based and satellite measurements of the neutral density and winds.
SA21B-1559
Investigation of longitudinal distribution of thermospheric density by an empirical model derived from the CHAMP data
The CHAMP satellite observes thermospheric mass density around 400km. By applying the least-squares approximate polynomial method to the CHAMP observation data, we constructed an empirical model to reproduce the thermospheric mass density at about 400km altitude. Geographical longitude, latitude, latitude, local time on the CHAMP orbit and so on are indepedendent variable of our model. For example, the model can reproduce a mass density distribution in [longitude,latitude] at the spring equinox and on fixed local time sector. In the present paper, we will show results of an empirical model that reproduces longitudinal dependence of thermospheric density on LT and season. Furthermore, we will refer to physical relations between the ionospheric structure like plasma density and thermospheric density.
SA21B-1560
The difference in thermospheric response to solar and magnetospheric inputs
In this talk we present CHAMP accelerometer data interpreted in terms of thermospheric density. In a multi- years statistical study we investigate how the air density in a latitude band of ±30° about the equator on the day and on the night side responds to external forcing. The prime driver of thermospheric density is the solar flux. For magnetically quiet days a high degree of correlation (cc>0.9) is obtained when comparing the mass density with the solar flux index. The slope of the regression line on the day side, however, is steeper by a factor of 2 than on the night side. This ratio is independent of season and solar flux level. Magnetic activity is another driver. In this case the energy input takes place at auroral latitudes. Density bulges created at high latitudes in both hemispheres propagate subsequently towards the equator. On the night side we observe a delayed response compared to the day side, which implies a later energy input in that time sector. Furthermore, we investigate which magnetic activity index is suited best to characterize the air density increase. As an example for magnetospheric input we present the thermospheric response to substorms. The presented results can be used as constraints for the improvement of atmospheric models.
SA21B-1561
New Thermospheric Infrared Radiative Flux and Power Results From the SABER Experiment
The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite measures the vertical distribution of infrared radiation emitted by various atmospheric gases (ozone, water vapor, nitric oxide, and carbon dioxide), providing important information about the radiation budget in the upper atmosphere. From these measurements, the infrared power and energy radiated by nitric oxide (NO) and carbon dioxide (CO2) have been computed. It has been demonstrated that NO, in particular, acts as a natural thermostat, providing a mechanism for solar storm energy to be lost from the atmosphere via infrared emission. A new version (Version 1.07) of the SABER data set has been released and new computations of flux, power and energy have been made with these data over the full mission timeline (2002-2008). A pubic database of the computed daily zonal power values is being developed. Cooling by the atomic oxygen fine structure line at 63 μm based on atmospheric model profiles will also be computed and included in the database. Results for the new data version and comparison with the previous version will be presented. The seven- year span of this data set provides information on long-term solar variability, and the inclusion of more recent data aids the search for evidence of the start of solar cycle 24. These data provide fundamental information on the climate of the thermosphere and enable detailed investigation of short and long-term variability as a function of latitude.
SA21B-1562
Thermospheric Neutral Density Cooling Trends: 1962-2008
Satellite drag measurements have permitted detection of long-term changes in thermospheric density. The effect is most pronounced during solar minimum conditions. We have extended our previous database of thermospheric densities deduced from satellite drag data back to 1962 to capture the solar minimum period in 1964-1965 and through the solar minimum into 2008 to cover five solar minima for the first time. All data are generated from actual radar tracking observations to form precise orbit and drag/density data with improved accuracy and one-day resolution. Satellites with relatively high eccentricities were used to achieve long lifetimes and relatively localized latitude and local time resolution. Trends are presented as ratios to the empirical neutral density models. The model representations have also been enhanced by replacing their climatological average semiannual variations with realistic values derived for each satellite. This analysis permits accurate representation of thermospheric cooling as a function of solar flux. In qualitative agreement with theoretical models, the cooling is sensitive to solar cycle with the largest changes occurring at solar minimum. Trends are also deduced for the first time in terms of thermospheric temperature changes. The derived thermospheric cooling is further analyzed as a function of altitude (250-550 km), latitude, season, local time, solar flux and geomagnetic activity. Results are compared to both empirical and theoretical model predictions.
SA21B-1563
Fall 2008 Observations of neutral oxygen emissions at 844.6 nm using the Millstone Hill Spatial Heterodyne Spectrometer
In Fall 2008, after extensive realignment and insertion of new optical components, the Millstone Hill Spatial Heterodyne Spectrometer made twilight (pre-sunrise and post-sunset) observations of 844.6 nm emissions from atmospheric neutral oxygen from the F-region. Analysis of the resulting spectrum for absolute brightness and temperature will be used to feed a new forward model of the mesopshere and lower thermospheric (MLT) region, extracting new results for the density of neutral oxygen in that region, following the techniques described by Drs. Redgie Lancaster and Lara Waldrop. Improvements in the Spatial Heterodyne Spectrometer for these measurements include the insertion of new, nearly pristine reflection diffraction gratings; a reset of the "zero path" of the instrument to try to improve fringe contrast; and the elimination of noise in the detected images equal to, and even at times slightly greater than, the expected signal. All of these issues will be discussed, along with the challenge of seeing the 8446 line emission over the twilight continuum during observations.