Data obtained from the Stratospheric Aerosol Measurement (SAM) II instrument, which flew on board the Nimbus-7 satellite, are used to determine the vertical distribution of stratospheric aerosols in the polar regions of both hemispheres.
Scientific Objectives
The Stratospheric Aerosol Measurement (SAM) II instrument, aboard the Earth-orbiting Nimbus 7 spacecraft, was designed to measure solar irradiance attenuated by aerosol particles in the Arctic and Antarctic stratosphere. The scientific objective of the SAM II experiment was to develop a stratospheric aerosol database for the polar regions by measuring and mapping vertical profiles of the atmospheric extinction due to aerosols.
This database allows for studies of aerosol changes due to seasonal and short-term meteorological variations, atmospheric chemistry, cloud microphysics, volcanic activity and other perturbations. The results obtained are useful in a number of applications, particularly the evaluation of any potential climatic effect caused by stratospheric aerosols.
Project Description
The SAM II instrument consists of a single-channel Sun photometer with a 0.04 micron passband centered at a wavelength of 1.0 micron. This is a region of the spectrum where absorption by atmospheric gases is negligible; consequently, any attenuation of sunlight is due to scattering by aerosol particles and air molecules.
Instrument Operation
In operation, the instrument is activated shortly before each sunrise or sunset encountered by the satellite. A sensor with a wide field-of-view is used to indicate the Sun's presence. Two similar sensors then point the SAM II to within ±0.03 degrees in azimuth (left and right). A mirror begins a rapid vertical scan until the Sun's image is acquired by the SAM II telescope. The mirror then slowly scans vertically across the Sun at a rate of 0.25 degree per second reversing itself each time a Sun-limb crossing occurs.
Technical Specifications
The entrance window to the SAM II telescope only passes sunlight of wavelengths greater than 0.9 micron. A circular aperture placed at the image plane serves to define the instantaneous field of view of the instrument to be 0.5 minute of arc. This corresponds to a vertical resolution in the atmosphere of approximately 0.5 km altitude. From the telescope, the light is directed through an interference filter, which rejects all but the 1.0 micron wavelength (±0.02 micron) passband, to a photodiode detector. The solar intensity as a function of time is digitized, recorded, and periodically telemetered back to Earth.
Orbital Characteristics and Coverage
The SAM II instrument, along with a number of other sensors, is mounted on the Nimbus 7 Earth-orbiting spacecraft. The orbital characteristics of this spacecraft determine the frequency and geographic locations of the SAM II measurements. The mode of operation of the SAM II instrument is such that it takes data during each sunrise and sunset encountered. The Nimbus 7 spacecraft has an orbital period of 104 minutes, which means that it circles the Earth nearly 14 times per day.
Measurement Patterns
The Nimbus 7 spacecraft was placed in a high-noon, Sun-synchronous orbit; that is, the spacecraft crossed the Equator during each orbit at local noon. In general terms, this means that the orbital plane of the spacecraft was fixed with respect to the Sun, and thus all sunsets occur in the Arctic region and all sunrises occur in the Antarctic region.
In the course of a single day, measurements of the stratospheric aerosol are obtained at 14 points spaced 26 degrees apart in longitude in the Arctic region and similarly for the Antarctic region. All the points obtained during 1 day in a given region are at very nearly the same latitude, but as time progresses, the latitudes of the measurements slowly change with the season by 1 to 2 degrees each week, gradually sweeping out the area from approximately 64.0 to 83.0 degrees.
Orbital Degradation and Data Collection Changes
However, due to an orbit degradation associated with the Nimbus 7 spacecraft, there has been a change and disruption in the collection of SAM II data beginning in 1987. During the period of time from 1987 through 1993, orbital precession caused the Nimbus 7 spacecraft to cross the equator earlier than the planned high-noon crossing.
Coverage Changes Over Time
- Initial Antarctic coverage: 64.5 degrees (solstices) to 81.0 degrees (equinoxes)
- 1992 Antarctic coverage: 53.1 degrees (solstices) to 69.2 degrees (equinoxes)
- Initial Arctic coverage: 64.1 degrees (solstices) to 83.0 degrees (equinoxes)
- 1991 Arctic coverage: Extended to 86.2 degrees at equinoxes
Data Collection Interruptions
The orbital precession also affected the spacecraft orientation and prevented the SAM II instrument from acquiring the Sun for certain periods:
Arctic Region Sunset Events Lost:
- Mid-June through mid-August 1988
- Mid-March through mid-September 1989
- Mid-January through September 1990
- January 7, 1991, through the present
- Antarctic Region: SAM II was unable to acquire the Sun from mid-January through October 1993. The final 2 months of SAM II data for the Antarctic region were collected during November and December 1993, with no further data expected beyond 1993.
Data Processing
The SAM II satellite data are processed after being telemetered to the ground, with the data on solar intensity versus time being mathematically inverted to yield extinction coefficient versus altitude (extinction profile) for each sunrise or sunset event. The mathematical inversion used is described by Chu and McCormick (1979).