Carl A. Reber

The Upper Atmosphere Research Satellite (UARS) mission

The Upper Atmosphere Research Satellite (UARS) is a NASA program aimed at improving our knowledge of the physical and chemical processes controlling the stratosphere, mesosphere, and lower thermosphere, emphasizing those levels that are known to be particularly susceptible to change by human activities. The spacecraft was launched by the Space Shuttle Discovery on September 12, 1991 into a near-circular orbit at 585 km altitude and 57 deg inclination. Measurements include vertical profiles of temperature, many trace gases, and horizontal wind velocities, as well as solar energy inputs. Many of the limb-scanning instruments can measure to as high as 80 deg latitude, providing near-global coverage. The mission is supported by a large international correlative measurement program, yielding data both for validation of the UARS measurements and for complementary scientific studies. A dedicated data system provides rapid processing to geophysical quantities and makes these data available to UARS scientists.

The Upper Atmosphere Research Satellite (UARS)

The Upper Atmosphere Research Satellite (UARS) was launched by the Space Shuttle on September 12, 1991 into a near circular orbit at 585 km altitude inclined 57 degrees to the Equator. Measurements were initiated a few days later, including solar energy inputs to the atmosphere and vertical profiles of temperature, important minor gas species, and wind fields. The orbital parameters, combined with the sensor measurements characteristics, yield a measurement pattern that produces near global coverage with a duty cycle that periodically favors the Northern or the Southern hemispheres. A few spacecraft and instrument anomalies have impacted the total amount of data obtained to date, but the overall performance of the mission has been very good.

The upper atmosphere research satellite

The Upper Atmosphere Research Satellite (UARS) [Burr and Reber, 1985; Reber, 1985] is an important new National Aeronautics and Space Administration program aimed at improving our knowledge of the stratosphere, mesosphere, and lower thermosphere, emphasizing those levels that are known to be particularly susceptible to change by external agents. Using a combination of measurements and theoretical studies, UARS will provide a focus for resolving scientific questions about the chemistry, dynamics, and overall energy balance of the upper atmosphere.

Investigation of the major constituents of the April–May 1963 heterosphere by the explorer XVII satellite

Abstract The mass spectrometer experiment on the Explorer XVII satellite has yielded new data on the concentrations of the major components of the neutral upper atmosphere over a two month period in the spring of 1963. These data compare favourably with results from the total density experiments on the same spacecraft as well as satellite drag data and direct measurements made from rockets. The night measurements are in general consistent with an isothermal atmosphere at a temperature of 650–700°K while the daytime scale heights indicate a temperature of 825 ± 75°K. There was a large variability in the number densities at a given altitude, indicating a strong sensitivity to changes in energy inputs to the atmosphere, particularly to changes in magnetic activity. Average quiet daytime conditions near 260 km altitude were observed to be: N2,1.5 ± 0.5 × 108 cm−3; O, 2.4 ± 0.4 × 108 cm−3; He, 1.4 ± 0.3 × 106 cm−3. Average quiet nighttime conditions at 400km were: N2, 2.0 ± 1.0 × 106 cm−3; O, 1.0 ± 0.4 × 107cm−3; He, 6.0 ± 2.0 × W cm−3. The large horizontal change in the location of the satellite during a measurement precludes the interpretation of the data in terms of a simple vertical profile, but does oner the possibility of investigating local time or other horizontal gradient effects on the atmospheric constituents.

Global exospheric temperatures and densities under active solar conditions

Temperatures measured by the OGO-6 satellite using the 6300 A airglow spectrum are compared with temperatures derived from total densities and N2 densities. It is shown that while the variation of the total densities with latitude and magnetic activity agree well with values used for CIRA (1972), the temperature behavior is very different. While the temperatures derived from the N2 density were in much better agreement there were several important differences which radically affect the pressure gradients. The variation of temperature with magnetic activity showed seasonal and local time variations. Neutral temperature, density, pressure and boundary oxygen variations for the storm of 8 March 1970 are presented.

Ozone diurnal variations observed by UARS and their model simulation

Several years of ozone measurements from the Microwave Limb Sounder onboard the Upper Atmosphere Research Satellite are analyzed using a two-dimensional Fourier series in day of year and time of day. Because of limited temporal coverage near local noon, only the diurnal and semidiurnal components are included. Data are investigated in detail at 28°N in the middle stratosphere to lower mesosphere, where the data are considered most reliable. The observations show that ozone is a maximum in the afternoon at 3 mbar and a minimum in the afternoon at 1 mbar and above with a narrow transition zone of reduced diurnal variation in between. This strong dependency on altitude in the transition from a maximum in the afternoon to a minimum in the afternoon, coupled with the small percentage changes in ozone, imposes strict requirements on the data and on the analysis of the data. Comparisons are made with results from a photochemical box model run at 11 levels between 0.46 mbar and 21.5 mbar for 28°N at spring equinox and near the solstices. This is the first time that a data analysis and model comparison of this kind has been made, leading to the identification of relatively small diurnal variations, especially in the transition zone. In the middle stratosphere the model results are in poor agreement with the observations because of the influence of stratospheric dynamics which are neglected in the model runs. In the upper stratosphere the model shows the expected underprediction of absolute ozone amounts, although the percentage change from the midnight value is in excellent agreement with the observations and in particular correctly simulates the diurnal variation in the transition zone between 3 and 1 mbar. Model sensitivity studies are performed to determine the effects of major reaction rate changes and simplified tidal effects.

Equatorial phenomena in neutral thermospheric composition

Abstract Several interesting phenomena relating to the equatorial ionosphere have been observed in the data from the OGO-6 mass spectrometer, chief among which are: 1. (1) The diurnal variations during equinox at an altitude of 450km show the N 2 and O densities peaking near 1500 hr while He peaks near 1000 hr. 2. (2) The latitudinal variation in N 2 during the day is very similar to the F -region electron density exhibiting the well known features of the ionospheric anomaly. 3. (3) During periods of intense geomagnetic disturbance (e.g. the large storm of 8 March 1970), the low latitude thermospheric temperature increases on the order of 50°–150°K, while at mid latitudes, increases of more than 1000°K are observed. These results point to the limitations of the static diffusion models and indicate the need for the inclusion of dynamics in the interpretation of thermospheric phenomena.

Large-Scale Waves in the Thermosphere Observed by the AE-C Satellite

Atmosphere Explorer C (AE-C) data are analyzed to study wavelike perturbations in the thermosphere at an altitude of about 260 km. The data were measured during one orbit on January 20, 1975. The examples shown are typical of many other orbits of both satellites AE-C and AE-E. Four geophysical parameters are analyzed: nitrogen and oxygen densities, electron density, and ion temperature, as measured by three different instruments. The data are processed by normalizing them to their average values and extracting their trends. Their fluctuations are obtained by passing the normalized detrended data through a high-pass filter. Strong periodicities are apparent that persist throughout the records, regardless of filter cutoff frequency. Fluctuations are compared by computing cross correlation functions. Spectra are obtained using the MEM and FFT procedures. It is demonstrated, for the first time, that relationships between ion variations and neutral variations are coherent over a wide range of scale sizes over global distances. It is also inferred from plane wave modeling studies and from the persistent periodicities over global distances that a quasi-stationary wave structure is present in the thermosphere that may slowly dissipate and be regenerated by auroral region sources. Large-scale structures are found in all four parameters with horizontal scale sizes ranging from about 400 to about 4000 km. The spectra for all parameters contain peaks at wavelength that are confirmed by the periods of the fluctuations, and decrease with decreasing wavelength with a power law type of variation.

Dynamical effects in the distribution of Helium in the thermosphere

Abstract Helium, because it is non-reactive, relatively light, and a minor constituent in the lower thermosphere, provides an excellent measuring tool for the effects of transport processes in the thermosphere and exosphere. It has long been recognized that the abundance of helium in the high atmosphere is closely connected with the turbulent diffusion coefficient and the ‘height of the turbopause’, and more recently it has been shown that dynamics, or wind systems, play perhaps an even greater role in redistributing this gas. The best known phenomenon in this category is probably the winter enhancement of helium, which implies prevailing winds blowing across the equator of 40–50 m/s in June and somewhat higher velocities near the December solstice. In addition, an annual variation in the total helium content in the heterosphere indicates a change in the effective global eddy diffusion of about a factor of two, with a maximum near June. Magnetic storms and high latitude heating of the thermosphere during magnetically quiet times also cause significant depletions in the local helium content, and here helium provides a tracer for the wind cells generated by these mechanisms. These phenomena, along with others somewhat less dramatic, have provided some of the most convincing evidence that the behavior of the upper atmosphere must be explained by including dynamic effects, and cannot be explained merely by changes in exospheric temperature.

Geomagnetic storm effects on the thermosphere and the ionosphere revealed by in situ measurements from OGO 6

Abstract The temporal response of ion and neutral densities to a geomagnetic storm has been investigated on a global scale with data from consecutive orbits of OGO-6 (>400km) for 4 days covering both magnetically quiet and disturbed conditions. The first response of the neutral atmosphere to the storm takes place in the H and He densities which start to decrease near the time of the storm sudden commencement. The maximum decreases in H and He were more than 40% of the normal density at high latitudes. A subsequent increase in O and N 2 densities occurs about 8 hours later than the change in H and He densities, while the relative O and N 2 density changes indicate a depletion of atomic oxygen in the lower thermosphere by more than a factor of two. The overall features of the change in the neutral atmosphere, especially the patterns of change for individual species, strongly support the physical picture that energy is deposited primarily at high latitudes during the storm, and the thermosphere structure changes through (1) heating of the lower thermosphere and (2) generation of large scale circulation in the atmosphere with upwelling at high latitudes and subsidence at the equator. The storm-time response of H + occurs in two distinct regions separated by the low latitude boundary of the light ion trough. While on the poleward side of the boundary the H + density decreases in a similar manner to the decrease in H density, on the equatorward side of the boundary the H + decrease occurs about half a day later. It is shown that the decrease of H + density is principally caused by the decrease in H density for both regions. The difference in H + response between the two regions is interpreted as the difference in H + dynamics outside and inside the plasmasphere. The O + density shows an increase, the pattern of which is rather similar to that for O. Two possibilities for explaining the observed change in O + density are suggested. One attributes the observed increase in O + density to an increase in the plasma temperature during the storm. The other possibility is that the increase in the production rate of O + due to an increase in O density exceeds the increase in the loss rate of O + due to an increase in N 2 density, especially around the time of sunrise. Hence the change in O + density in the F -region may actually be controlled by the change in O density.