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

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

Photometric and interferometric observations of a mid-latitude stable auroral red arc

Observational results of the mid-latitude stable auroral red arcs of October 30/31 and October 31/November 1, 1968 are presented. The measurements were made with a turret photometer and a 6 in. dia. high resolution, pressure scanning Fabry-Perot interferometer. The structure, intensity, and position of the red arc were determined from photometer scan measurements which show that the arc was stable, persisted for more than 12 hr, and moved southward during the night. The arc appeared to extend across the eastern part of the United States tilted to lines of constant magnetic L-shells. The results of the Doppler temperature measurements made with the Fabry-Perot interferometer show no measurable neutral gas temperature increase within the red arc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32793/1/0000166.pd

Stable mid-latitude red arcs: Observations and theory

The observational features of the arc are fairly well established. At present, the thermal conduction model appears to explain the red arc features most consistently, but it must be noted that a soft electron flux would give very similar results. Ion temperature measurements in the vicinity of an arc, which should be forthcoming in the very near future, can establish conclusively whether transverse electric fields play any important role in the formation of the arcs. Accepting the assumption that the arcs are the result of energy flowing down from the plasmasphere, the major remaining question is: where does the energy come from and how does it get into the plasmasphere? The various proposed mechanisms discussed in the previous chapter appear feasible, but much work needs to be done before this problem is completely resolved.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43762/1/11214_2004_Article_BF00177029.pd

The altitude region sampled by ground-based Doppler temperature measurements of the OI 15867 K emission line in aurorae

Measurements of atmospheric optical emissions with ground-based spectrometers give columnintegrated line profiles. Therefore, measurements from a single station are insufficient to infer the height of emission and, thus, the height of temperature and wind determinations. In aurorae the temperature measured by a ground-based spectrometer can be lower than similar measurements in the nightglow because the 15867 K (630.0 nm ; 1 K = 1 cm -1) emitting region may occur at lower altitudes. Temperature measurements obtained on an individual night from College, Alaska, illustrate this effect.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26183/1/0000262.pd

The calculated and observed ionospheric properties during Atmospheric Explorer-C satellite crossings over Millstone Hill

The Atmospheric Explorer-C (AE-C) satellite passed almost directly over the Millstone Hill incoherent scatter radar station on 14 February 1974 and passed within the near vicinity of the station on 15 February 1974. Measurements of ionospheric and atmospheric properties were made simultaneously by the incoherent scatter radar and the AE-C satellite instruments. The incoherent scatter radar measured vertical profiles of the electron and ion temperatures and electron density and these data were used to derive a neutral gas temperature profile. The AE-C satellite measured the electron and ion densities and electron and ion temperatures, neutral gas composition, solar EUV flux, photoelectron spectra, the 6300 A volume emission rate profile and the distribution of NO along the satellite path. These simultaneous measurements provide a consistent set of data to examine current F-region theory in the daytime ionosphere. We used a time-dependent coupled model of the ionospheric E- and F-region to calculate the ionospheric properties over Millstone Hill at the times of the AE-C crossings and then compared the calculated structure to the observed structure. The results show good agreement between the incoherent scatter radar measurements and the model calculations. There is also good agreement among satellite and incoherent scatter radar measurements and model calculations for the altitude of the satellite crossing, 161 km. The satellite measurements along the orbital path, however, reveal considerable horizontal gradients in the measured ionospheric properties.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22695/1/0000249.pd

Modelling of time-dependention outflows at high geomagnetic latitudes

In a recent paper, Gombosi and Killeen (1987) applied a highly parameterized thermospheric Joule heat source as a boundary condition in the time-dependent, ion outflow model of Gombosi et al. (1985) to show that episodic ion outflows at high geomagnetic latitudes could result from low altitude ion frictional heating. To delineate more realistically the time-dependent thermosphere/ionosphere environment, we extend this previous study by using output from the Thermospheric General Circulation Model (TGCM) of the National Center for Atmospheric Research (NCAR) as input to the same hydrodynamic polar wind code for a set of case studies which follow the thermal forcing history of individual, ionospheric, convecting flux tubes. Using derived, time-varying frictional heating rates such as those experienced by these flux tubes, we show that transverse ion heating below 500 km can provide sufficient energy to perturb the velocity distribution of the major ion species. The time-dependent flux tube heating results in localized regions of field-aligned O+ upflows. These results demonstrate that localized heating, generated from thermosphere/ionosphere interactions, may initiate heavy ion upwellings which, through further energization at higher altitudes, could evolve into the transient ion outflows as seen by the Dynamics Explorer 1 satellite.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27472/1/0000513.pd

Modelling of composition changes during F-region storms: a reassessment

A recalculation of the global changes of thermospheric gas composition, resulting from strong heat inputs in the auroral ovals, shows that (contrary to some previous suggestions) widespread increases of mean molecular mass are produced at mid-latitudes, in summer and at equinox. Decreases of mean molecular mass occur at mid-latitudes in winter. Similar results are given by both the `UCL' and `NCAR TIGCM' three-dimensional models. The computed composition changes now seem consistent with the local time and seasonal response observed by satellites, and can broadly account for `negative storm effects' in the ionospheric F2-layer at mid-latitudes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29311/1/0000375.pd