UARS Science Highlights

A Summary of the Upper Atmosphere Research Satellite (UARS)

Facts | Significant Scientific Achievements | Top Eight Science Highlights | Other Notable Achievements
(Chemical Constituents, Dynamics, Particles, and External Influences)

• Charles Jackman, PS
• Anne Douglass, Deputy PS

UARS was deployed from the shuttle Discovery into a 585 km, 57 degree inclination orbit on September 15, 1991. After deployment, all UARS instruments began checkout and operation. There were no initial instrument failures or major problems. The solar and MLS instruments began to take data very soon after deployment, the infrared limb sounding instruments were turned on later after to allow for instrument outgassing.

The UARS solar array extends off to one side of the spacecraft, and rotates to track the sun. During one of the monthly yaw maneuvers, the rotation of the solar array is stopped. Then the observatory is yawed around and the array is restarted, rotating in the opposite direction.

On June 2, 1992, the rotating UARS solar array began to exhibit motion anomalies. All instruments were turned off while the problem was diagnosed. The array was later restarted on June 17 using a work around procedure. The motion anomalies arose from the drive clutch system. Engineering model tests on the clutch suggested that metal debris is generated during the periods when the array is started and stopped. The accumulation of this debris prevents the clutch spring from engaging the array drive shaft. If the clutch does not fully grip the drive shaft, slippage occurs which wears the shaft and generates more debris. After the restarting on June 17, the array functioned nominally until September 1993 when the clutch began to slip again on the drive shaft. After some effort, the backup drive was engaged. The clutch system operated nominally until and Earth sensor failure in March 1995. Complications resulting from this failure led to additional solar array drive problems, and a decision was made in May 1995 stop the solar array and operate the instruments on a reduced power plan. UARS has been operating in this mode for over a year now and all working instruments have gotten enough time to continue producing data.

UARS experienced a further reduction in power in June 1997 with the loss of one of three batteries. In October 1999, the UARS experienced difficulties with its remaining tape recorder. Use of only 25% of a tape recorder is now possible and the UARS is in a "real-time" operation through communication using two Tracking and Data Relay Satellites (TDRS's). The two TDRS's allow capture of about two-thirds of measurements in a given orbit.

The priority instruments in the reduced power and power-sharing mode are the HALOE, SOLSTICE, and SUSIM. Other instruments are operated as the available power allows. Power is plentiful during the "day" part of the UARS orbit and two instruments (HRDI and PEM) take advantage of this situation by also operating in a "daytime-only" mode. UARS has been operating in this mode for awhile now and all working instruments have gotten enough time to continue producing data.

After functioning from turn on October 28, 1991 the ISAMS chopper wheel ceased to rotate in mid-January, 1991. Attempts to restart the wheel were unsuccessful until it suddenly began to rotate again two months later. The mechanism failed again in July 1992. The ISAMS instrument contains the first spaceborn Sterling cycle cooler. This cooler continues to operate as part of the lifecycle test even though the instrument does not take data.

In April 1993, the CLAES cryogen expired on schedule and the instrument ceased taking data. The MLS 183 GHz radiometer failed about the same time. This channel measured stratospheric water vapor and mesospheric ozone. The 63 and 205 GHz radiometers continue to operate measuring T, ClO, stratospheric ozone and lower stratospheric HNO3. The Particle Environment Monitor (PEM) instrument has experienced failures (October 1991 and May 1999) and degradations (June 1994 and November 1998) in all of its low-energy proton sensors (<35 keV); however, its proton sensors detecting energies greater than 35 keV remain operational as well as its electron and X-ray channels. All other instruments are fully operational.

UARS data is processed at the Central Data Handling Facility (CDHF), at Goddard Space Flight Center. UARS data is transferred to Goddards Distributed Active Archive Center (DAAC) for public distribution electronically.

It is impossible to rank the major scientific discoveries from UARS in order of importance. Each of the 10 instruments has produced high quality data and important unique or complementary results. 

Here are ten significant scientific achievements by UARS instruments.

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1. Seasonal mapping of chlorine radicals and reservoirs in the lower stratosphere.

   A few months after launch, MLS was able to map ClO (chlorine monoxide - an ozone destroying radical) within the Arctic vortex showing the extent of ClO formation and its close association with polar stratospheric cloud formation temperatures. Not only was this was an important confirmation of earlier aircraft results, but it also showed the extent of the zone of elevated ClO. Since these initial observations UARS has continued to monitor both the Arctic and Antarctic late winter-spring ozone depletions. The northern hemisphere depletion in January-March 1996 was the largest ever.

2. Containment of polar vortex chemistry within the vortex region.

   At the time of the launch of UARS some scientists speculated that the ozone destroying chemicals within the polar vortex would leak to mid-latitudes. Other scientists argued from a dynamical perspective that containment of the chemicals must occur. It was not until the launch of UARS and the mapping of trace species by UARS instruments that containment could be demonstrated.

3. Descent in the center of the polar vortex.

   HALOE scientists were the first to notice very low concentrations of the long lived trace gas methane (CH4) in the center of the spring Antarctic polar vortex. Analysis showed that very low values of methane exist within the mid and upper stratosphere in late fall and these values descend to the lower stratosphere by late spring, a net change 12-15 km over six months. This amount of descent is remarkable in any part of the atmosphere, and was later confirmed by measurements by CLAES (N2O, and CH4) and ISAMS (N2O and CO).

4. Infrared mapping of aerosols and PSCs.

   Mt. Pinatubo erupted on June 15, 1991 injecting up to 20 megatons of sulfur dioxide directly into the stratosphere. Reaction of SO2 with stratospheric OH produces sulfuric acid which condenses into aerosols at stratospheric temperatures and pressures. UARS observations by the instruments CLAES, HALOE and ISAMS were used to track the aerosol cloud from its IR emissions. This is the first near synoptic mapping of volcanic aerosol layers.

5. First direct measurement of winds from space.

   Both WINDII and HRDI on UARS were built to measure winds from space. Although the techniques are different, both rely on the Doppler shift of an oxygen emission line in the mesosphere. HRDI, additionally detects daytime stratospheric winds using the Doppler shift of an oxygen absorption line in the stratosphere. These are the first remote space born wind sounders. HRDI and WINDII both have been able to give the first complete global picture of the atmospheric tide. HRDI has also been able to measure the tropical quasibiennial oscillation winds in the stratosphere.

6. First global maps of chlorofluorocarbons and their products from space.

   Some people outside the scientific community believe that chlorofluorocarbons are not responsible for ozone loss at the poles. They argue that chlorofluorocarbons are heavy molecules and will never rise into the stratosphere. High levels of observed stratospheric chlorine are due to volcanic activity, they argue.
   CLAES has detected both CFCl3 (F11) and CF2Cl2 (F12) in the stratosphere. Both are found to decrease strongly with altitude above the tropopause. As the chlorofluorocarbons are breakdown in the stratosphere, they release Cl and F which form HF and HCl. HF is a long lived trace gas with no important natural sources. HALOE has detected HF and Hcl in the stratosphere and found that both increases with altitude as the CFCs decrease and are increasing with time.

7. Tropical transport in the stratosphere.

   The long lifetime of the UARS mission has lead to some remarkable trace gas trend information. One of the more dramatic observations has been the vertical transport of water vapor upward in the tropical stratosphere. The amount of water vapor entering the stratosphere changes throughout the year as the tropical tropopause gets colder and warmer. These variations in water vapor ascend slowly into the stratosphere and appear coherent to about 30 km (from 16 km). The water vapor observations tell us that the tropical region must be quite isolated from the rest of the stratosphere or these variations would be diluted.

8. Measurement of the UV and Visible component of solar variability.

   ACRIM records the total solar irradiance while SOLSTICE and SUSIM measure the UV flux from Lyman alpha (~121.6 nm) up to around 400 nm. SOLSTICE uses stars, while SUSIM uses onboard calibration lamps to correct for instrument changes over time. UARS was launched near the end of the maximum of solar cycle 22 and now the sun has reached the minimum between solar cycles 22 and 23. A comparison of energy change over this period by these instruments shows that the UV variation accounts for a significant 40% of the change in the total solar irradiancee.

9. The role of energetic particles in stratospheric chemistry.

   Energetic particle observations by PEM have shown that most of the relativistic electrons observed at geosynchronous altitudes (by GOES, for example) are trapped. Only about 1- 10% of the relativistic electron precipitations (REPs) measured at geosynchronous altitudes actually precipitate into the earths atmosphere. These measurements thus show that REPs have a relatively small global impact on stratospheric odd nitrogen which was a controversy before the UARS launch.

10. Upper tropospheric water vapor in the presence of clouds.

    The MLS team noticed after launch that they were getting some spectral interference from water vapors. Further analysis showed that they could use the MLS instrument to actually measure upper tropospheric water. Since MLS is a microwave emission instruments, the measurements could be made even if ice clouds are present. This new water vapor measurement is currently being used to study how cirrus clouds impact the climate.

In January 2000, UARS investigators re-examined science highlights during the UARS mission. Here is a list of those highlights, grouped by the Project Science Office. They are given in categories of the "Top Eight Science Highlights from UARS" and "Other Notable Science Achievements from UARS," which are provided in the four categories of Chemical Constituents, Dynamics, Particles, and External Influences.

1. The direct correlation between three-dimensional distributions of observed ozone depletion and reactive chlorine was established.

2. The dominance of human-made chlorofluorocarbons in the chlorine and fluorine amounts in the stratosphere was clearly demonstrated.

3. Using several years of observations the movement and mixing of air in the stratosphere and mesosphere were derived globally and seasonally.

4. The longest time series (in excess of 8 years) of absolutely calibrated solar ultraviolet spectral irradiances over both maximum and minimum levels of solar activity were measured.

5. Methods were developed for identification of stratospheric particle formation and composition through analysis of spectral signatures.

6. The evolution of reactive chlorine over the winter and its role in the springtime polar ozone destruction has been confirmed and quantified for both southern and northern hemispheres over several years.

7. The seasonal variation in water vapor transport from the tropical troposphere to the stratosphere indicated the link between the amount of water entering the stratosphere and the seasonal cycle in the upper tropospheric tropical temperature.

8. Observations of water vapor and methane in the tropics show the coherent upward propagation of the seasonal cycle for time scales longer than one year, indicating that horizontal mixing between the tropics and middle latitudes is limited.

1. Mesospheric chlorine and fluorine amounts are generally consistent with ground-based measurements of chlorine and fluorine source gases when factoring in a five year lag for transport of these constituents from the ground to the mesosphere.

2. Global and seasonal mapping of chlorine radicals and reservoirs in the stratosphere has provided much insight into the formation and partitioning of the chlorine family constituents.

3. The first global maps of chlorofluorocarbons and their products were provided showing that chlorofluorocarbons do reach the stratosphere and their altitude and latitude distributions are consistent with the production of more active forms of chlorine.

4. The first demonstration of the dramatic difference between hemispheres of the seasonal evolution of the major nitrogen and chlorine reservoirs was provided which results from the hemispheric meteorological differences.

5. The variations in stratospheric water vapor and methane have been quantified over the eight years of the mission.

6. Upper tropospheric water vapor in the presence of clouds was measured and is being used to study how cirrus clouds impact the climate.

7. Observations in the troposphere are serving as a very valuable test data set for applications of the retrieval codes being developed for future satellite missions.

8. Ozone measurements quantified the effect of aerosols on SAGE II satellite ozone retrievals especially during the recovery phase from the Mt. Pinatubo eruption.

1. Chemical constituents appear to be contained in the polar vortex during the wintertime in both poles.

2. The descent of chemical constituents in the center of the polar vortex has been quantified over several years at both poles.

3. Atmospheric tides, the filtering of gravity waves as they propogate upwards, and the effects of gravity wave breaking upon the establishment of upper atmospheric temperature fields were measured globally and seasonally over several years.

4. The barriers between the tropics and middle latitudes and middle and polar latitudes were clearly demonstrated.

5. Direct observations of the mesospheric and thermospheric winds were used to characterize the elusive diurnal tide.

6. Planetary scale disturbances were found to be very regular and extremely large in the mesosphere and lower thermosphere.

7. The diurnal tide and planetary scale disturbances were shown to lead to major perturbations in the global scale distribution of atomic oxygen which can be greater than that from the photochemistry.

1. A quantification of the decline in stratospheric sulfate aerosol abundance after the 1991 Mt. Pinatubo volcanic eruption to the near background levels in 1999 was provided.

2. A quantification of the variation in chemical constituents caused by stratospheric sulfate aerosol changes was provided.

3. Daily or near daily maps of water and nitric acid have been used to identify processes which affect polar stratospheric cloud formation.

4. Global maps of aerosols and polar stratospheric clouds in both vertical and areal extent were derived.

5. The retrievals of aerosol extinction at multiple wavelengths in the infrared has shown that the wavelength dependence of the extinction differs for particles of different composition and size.

1. A time series of the solar Mg II core-to-wing ratio index, an important proxy for solar spectral irradiance originating in the chromosphere, transition region, and upper photosphere, is being produced which can be used to bridge the gap in the time series of past and future measurements of a Mg II index by the SBUV and SBUV/2 instruments aboard other satellites. When combined, the composite time series will span more than two solar cycles.

2. The effects of electron precipitation on various chemical constituents minor species appear to be organized in geomagnetic coordinates rather than geographic coordinates.

3. The influence of relativistic electrons on the middle atmosphere has been more firmly established -
   a) Although relativistic electrons deposit most of their energy in the mesosphere, a study of the largest relativistic electron precipitation event during the UARS time period failed to find any related mesospheric ozone changes;
   and
   b) Most of the energy deposited in the atmosphere by electron precipitation (and corresponding bremsstrahlung radiation) was shown to come from the steady long duration diffuse auroral precipitation rather than from relativistic electron precipitation events.

As is evident from the discussions in the previous sections, UARS has provided an enormous amount of new information on the middle and upper atmosphere. As of this writing, over seven hundred refereed scientific papers have been written on the results from UARS. In every sense, the mission has been a success and continues to be successful.

Some useful facts:

• Original UARS mission lifetime was 18 months.
• UARS cost about $750 million to build.
• It is the largest Mission to Planet Earth spacecraft ever launched or now planned to be launched.
• Dr. Paul Crutzen - co-winner of the 1995 Noble Prize in Chemistry - is a member of the UARS Science Team.

• Charles H. Jackman, PS

• Anne R. Douglass, Deputy PS

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