Orbital atmospheric physics and dynamics

There are two ways of modeling the upper atmosphere. One is the empirical model that makes use of experimental data on means and excursions from the mean and fits the data in a self-consistent manner. The other approach is to deal directly with the physical processes. This is difficult since what is happening is extremely complex. Data measured using an interferometer to give Doppler shifts of airglow lines showed 300 to 800 m/sec winds with a complex structure in the upper region of the thermosphere at high latitudes. Ionospheric electric fields, strongly influenced by interaction with the solar wind, drive the ionized component and large neutral winds result due to momentum transfer between the charged particles and the neutrals. Frictional heating results from movement of ions through the neutrals, which also influences the compositional structure. These are examples of the complex interactions involved. The NCAR General Circulation Model (tropospheric) was adapted for use at thermospheric altitudes: the Thermospheric General Circulation Model (TGCM). The model makes use partly of primitive equations and partly of empirical data for some quantities such as electron density, magnetic field, and ion drift

Mesospheric Dynamical Changes Induced by the Solar Proton Events in October-November 2003

The very large solar storms in October-November 2003 caused solar proton events (SPEs) at the Earth that impacted the upper atmospheric polar cap regions. The Thermosphere Ionosphere Mesosphere Electrodynamic General Circulation Model (TIME-GCM) was used to study the atmospheric dynamical influence of the solar protons that occurred in Oct-Nov 2003, the fourth largest period of SPEs measured in the past 40 years. The highly energetic solar protons caused ionization, as well as dissociation processes, and ultimately produced odd hydrogen (HOx) and odd nitrogen (NOy). Significant short-lived ozone decreases (10-70%) followed these enhancements of HOx and NOy and led to a cooling of most of the lower mesosphere. This cooling caused an atmospheric circulation change that led to adiabatic heating of the upper mesosphere. Temperature changes up to plus or minus 2.6 K were computed as well as wind (zonal, meridional, vertical) perturbations up to 20-25% of the background winds as a result of 22 the solar protons. The solar proton-induced mesospheric temperature and wind perturbations diminished over a period of 4-6 weeks after the SPEs. The Joule heating in the mesosphere, induced by the solar protons, was computed to be relatively insignificant for these solar storms with most of the temperature and circulation perturbations caused by ozone depletion in the sunlit hemisphere

A technique for recovering the vertical number density profile of atmospheric gases from planetary occultation data

The occultation technique of determining the properties of the atmosphere using absorption spectroscopy is examined. The intensity of a star, in certain atmospheric absorption bands, is monitored by a satellite tracking the star during occultation by the Earth's atmosphere. The intensity data in certain wavelength intervals, where absorption is attributed to a single species, are related to the tangential column number density of the absorbing species through Beer's law. The equation for the tangential column number density is the Abel integral equation which is inverted to obtain the number density profile of the absorbing species at the occultation tangent ray point. Two numerical schemes for inverting the Abel integral equation for signals of low intensity with statistical noise superimposed are presented; one for determining the number density profile of atmospheric species that decrease exponentially with height, and the second for determining the profile of constituents having a more complex vertical structure, such as ozone. The accuracy of retrieving the number density distribution from planetary occultation data is examined. A theoretical analysis of the errors in determining the number density from occultation data of very low signal intensity is also presented. The errors in retrieving the number density profile are related to the intensity of the source, the number of data points per scan, and the degree of data smoothing required before inversion. As a specific example, calculations are made of the errors in retrieving the molecular oxygen and ozone number density profiles from occultation intensity data in the Schumann-Runge continuum of molecular oxygen at 1450 A and the Hartley continuum of ozone at 2450 A.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/34033/1/0000310.pd

Lidar Studies of Interannual, Seasonal, and Diurnal Variations of Polar Mesospheric Clouds at the South Pole

Polar mesospheric clouds (PMC) were observed by an Fe Boltzmann temperature lidar at the South Pole in the 1999–2000 and 2000–2001 austral summer seasons. We report the study of interannual, seasonal, and diurnal variations of PMC using more than 430 h of PMC data. The most significant differences between the two seasons are that in the 2000– 2001 season, the PMC mean total backscatter coefficient is 82% larger and the mean centroid altitude is 0.83 km lower than PMC in the 1999–2000 season. Clear seasonal trends in PMC altitudes were observed at the South Pole where maximum altitudes occurred around 10–20 days after summer solstice. Seasonal variations of PMC backscatter coefficient and occurrence probability show maxima around 25–40 days after summer solstice. Strong diurnal and semidiurnal variations in PMC backscatter coefficient and centroid altitude were observed at the South Pole with both in-phase and out-of-phase correlations during different years. A significant hemispheric difference in PMC altitudes was found, that the mean PMC altitude of 85.03 km at the South Pole is about 2–3 km higher than PMC in the northern hemisphere. Through comparisons with the NCAR Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM), the hemispheric difference in PMC altitude is attributed to the hemispheric differences in the altitudes of supersaturation region and in the upwelling vertical wind, which are mainly caused by different solar forcing in two hemispheres that the solar flux in January is 6% greater than the solar flux in July due to the Earth’s orbital eccentricity. Gravity wave forcing also contributes to these differences.Ope

Unstable Layers in the Mesopause Region Observed with Na Lidar During the Turbulent Oxygen Mixing Experiment (TOMEX) Campaign

The Na wind/temperature lidar located at Starfire Optical Range near Albuquerque, New Mexico, provided real time measurements of wind, temperature, and Na density in the mesopause region during the TOMEX rocket campaign in October 2000. The state of the atmosphere in which the rocket was launched into was examined using the lidar measurements. Both convectively and dynamically unstable layers were observed at various times and altitudes during the night. The low convective stability region below 90 km was found to be associated with the diurnal tide. The unstable layers are the combined results of wave and tidal perturbations. Comparison with the thermosphere/ionosphere/mesopshere/electrodynamics general circulation model (TIMEGCM) simulation showed that the model can produce the general feature of the observed atmospheric structure (but with a much smaller diurnal amplitude in temperature), which likely leads to underestimate of instability and gravity wave effects

Unstable Layers in the Mesopause Region Observed with Na Lidar During the Turbulent Oxygen Mixing Experiment (TOMEX) Campaign

The Na wind/temperature lidar located at Starfire Optical Range near Albuquerque, New Mexico, provided real time measurements of wind, temperature, and Na density in the mesopause region during the TOMEX rocket campaign in October 2000. The state of the atmosphere in which the rocket was launched into was examined using the lidar measurements. Both convectively and dynamically unstable layers were observed at various times and altitudes during the night. The low convective stability region below 90 km was found to be associated with the diurnal tide. The unstable layers are the combined results of wave and tidal perturbations. Comparison with the thermosphere/ionosphere/mesopshere/electrodynamics general circulation model (TIMEGCM) simulation showed that the model can produce the general feature of the observed atmospheric structure (but with a much smaller diurnal amplitude in temperature), which likely leads to underestimate of instability and gravity wave effects

A computer model of global thermospheric winds and temperatures

A computer model of the global, time-dependent, thermospheric horizontal vector neutral wind and neutral temperature fields has been constructed based on output from the NCAR thermospheric general circulation model (NCAR-TGCM). The wind field is represented by a vector spherical harmonic (VSH) expansion in the horizontal, a fourier expansion in Universal Time, and a polynomial expansion in altitude. The global temperature field representation differs in that a scalar spherical harmonic expansion is used in the horizontal and a Bates model temperature profile is used in altitude. A set of suitably-truncated spectral coefficients contains the wind and temperature description for a diurnally-reproducible run of the NCAR-TGCM. The VSH model is coded in a FORTRAN subroutine that returns vector wind and temperature values for a given UT, geographic location, and altitude. The model has applicability for studies of thermospheric and/or ionospheric physics were reasonable time-dependent neutral wind and temperature values are of interest. The routine is novel since portable computer models of thermospheric wind fields have not previously been available to researchers. The current version of the model is valid for solar maximum, December solstice only, although the model can be extended to any season and specific set of geophysical conditions for which TGCM results are available. Results from the VSH computer model are presented to compare with global-scale wind measurements from the Dynamics Explorer (DE-2) satellite. The agreement between the computer model results and data from individual orbits of DE-2 is good, indicating that the model provides reasonable wind values, having the appropriate characteristic latitudinal, diurnal, and Universal-Time-dependent signatures observed from the satellite at upper thermospheric altitudes. The VSH thermospheric temperature values are in general agreement with MSIS-83 temperatures but illustrate smaller-scale horizontal temperature structures than are resolved by MSIS-83, owing to the larger number of spectral harmonics retained.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26911/1/0000477.pd

Thermosphere and ionosphere dynamics during 20-30 March 1979 time period: Comparison of TIGCM calculated densities with observations

The Thermosphere Ionosphere General Circulation Model developed at the National Center for Atmospheric Research, (NCAR TIGCM), has been used to simulate the time-dependent variations of global thermosphere and ionosphere structure and dynamics during the 20-30 March 1979 time period. Thermospheric density variations predicted by the TIGCM during this period are statistically compared to satellite electrostatic triaxial accelerometer neutral density measurements obtained between 170 and 240 km altitude and to predictions made by the Mass Spectrometer Incoherent Scatter empirical model (MSIS-86). In its present state of development, the TIGCM has attained about the same accuracy (standard deviation), as MSIS-86. Incorporation of improved representation of ion drag, resulting from the downward flow of magnetospheric plasma on the nightside, has contributed to the TIGCM model accuracy.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30290/1/0000692.pd

Thermospheric composition changes seen during a geomagnetic storm

The largest magnitude winds observed using the instruments on board the Dynamics Explorer 2 (DE-2) satellite were measured during the large geomagnetic storm that occured on the 24th of November 1982. Neutral temperatures exceeded 2000 K during this strom, these high temperatures, combined with the very large observed winds and the very full instrumental coverage available in both hemispheres, make it a unique event to study. In this paper we present results obtained using these DE-2 data and a time dependent simulation of the event made using the National Center for Atmospheric Research Thermosphere/Ionosphere General Circulation Model (NCAR-TIGCM). In general, the agreement between model calcuations and the data is very good, implying that most of the important physical processes controlling the energetics and dynamics of the thermosphere are reasonably well represented in the model. The modelled summer hemisphere changes in the mass mixing ratio of N2([Psi]N2) are in very good agreement with the DE-2 data, and the overall global pattern of [Psi]N2 in the model is also in good agreement with the averaged data in both hemispheres. This agreement allows us to study the physical processes occurring in the model with confidence that they are the same as those occuring in the "real" thermosphere. This short paper describes model-experiment comparisons for the November 24, 1982 geomagnetic storm, but does not include the processes responsible for these changes. A full description of them is available in the set of papers/1,2,3,4/.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29808/1/0000154.pd