RIMS- Mission Description

The Dynamics Explorer program was an important part of the broad Solar-Terrestrial Physics program of the National Aeronautics and Space Administration (NASA) dedicated to achieving an understanding of the processes which control man's near space environment. Two Dynamics Explorer spacecraft were launched in August 1981 for the purpose of studying coupling between the magnetosphere, ionosphere, and the atmosphere. Forms of this coupling include:

• electric current systems primarily along high latitude magnetic field lines with closure in the ionosphere
• a convection electric field system imposed upon the ionosphere and originating from the interaction of the solar wind with the Earth's magnetosphere
• energy transfer, including the acceleration of charged particles and thermal heat conduction
• thermal charged particle interchange between the ionosphere and magnetosphere
• wave, particle and plasma interactions in the ionospheric and magnetospheric plasmas

The program developed out of informal discussion in the late 1960s on the best practical way to study the strong and complex interactive processes within the atmosphere-ionosphere-magnetosphere system. The basic concept that a multi-satellite mission was necessary for a comprehensive study of these coupling problems emerged in the early 1970s, and evolved further through a number of formal and informal meetings. The strong case that the scientific community prepared and maintained for a program in atmosphere-ionosphere-magnetosphere coupling led to the recognition that these scientific problems represented many of the highest priority problems in space plasma physics.

The instruments selected for the mission were capable of measuring the following parameters:

• thermal electrons: temperature and density
• thermal ions: temperature, density, composition and bulk motion
• suprathermal particles: electron and ion distribution functions and composition
• vector magnetic field
• vector electric field
• plasma waves
• auroral optical images

A unique and necessary feature of the program involved the launch of two spacecraft into coplanar orbits, which occurred on August 3, 1981 from the Western Test Range. This orbit configuration provided for data acquisition at two altitudes within common magnetic flux tubes, fulfilling the requirement for simultaneous data sets in the magnetosphere and in the ionosphere/atmosphere. The 90 degree inclination of the orbits also maintained coplanarity, since the orbit planes did not precess. DE-1 was launched into a highly elliptical orbit with apogee near 4.7 Earth radii and perigee altitude below 1000 km. DE-2 was launched into a slightly elliptical orbit with apogee and perigee altitudes of 1012 km and 309 km, respectively.


B. Description of Spacecraft
On August 3, 1981, a Delta rocket launched from Vandenberg Air Force Base successfully placed the two Dynamics Explorer (DE) spacecraft into coplanar polar orbits. The purpose of the mission was to acquire data relevant to processes coupling the magnetosphere, ionosphere, and upper atmosphere. The two spacecraft were manufactured by the Government System Division of RCA, and the instruments were provided by various principal investigators. A complete description of the mission, spacecraft, instruments, and ground data systems appears in an issue of Space Science Instruments (Volume 5, pp. 345-573, 1981), edited by R. A. Hoffman. The high-altitude satellite, DE-1, carried an auroral imager and detectors designed primarily for field and particle measurements. The low-altitude satellite carried more aeronomic-type instruments. The orbits of the two satellites remained very nearly coplanar during their joint lifetimes, providing the unique opportunity to acquire data at two altitudes along common magnetic flux tubes. DE-2, operating at ionospheric altitudes, re-entered the atmosphere in February 1983. DE-1 ceased data collection operations in March 1991.

Both spacecraft were power limited and considerable effort was expended in selecting portions of orbits for maximum science return. DE-1 had a duty cycle of nearly 90% during the early portion of the mission, but in general varied between 16-55%. DE-2 had a duty cycle between 16 and 36%. Times of magnetic conjunction between the spacecraft, or between one spacecraft and a collaborating ground observatory, were given high priority. Also, data acquisition during passages through geophysical regions like the dayside cusp and plasmapause was emphasized.


C. Orbit Information
The DE-1 satellite, carrying the RIMS experiment, was launched into an elliptical polar orbit on August 3, 1981. The ~7.5 hour orbit has perigee of 675 km altitude and apogee of 24,875 km altitude. The orbit plane drifts westward at a rate of ~1 hour MLT every 15.4 days (24 hours in 12 months); the orbit line of apsides drifts about 0.328 degrees geographic latitude each day (moving from one pole to the other in 18 months). Thus, the orbit which started in the dawn-dusk plane with apogee over the north pole precessed to noon-midnight, with apogee over the midnight equator by the spring of 1982. The apogee continued to sweep around in latitude, bringing its position over the south pole during the summer of 1983, the noon sector equator during the spring of 1984, and returned to approximately its original position during the winter of 1984.

Note that while the DE-1 orbit provides coverage for most all local times and latitudes out to the apogee at 4.65 RE, the 3 to 1 ratio between the drift of the line of apsides and the westward drift of the orbital plane results in uneven coverage in local time for a given altitude. In particular, in the 12-18 hour magnetic local time sector, the higher altitudes in the auroral latitude region were not fully sampled.

The spacecraft spins in a reverse cartwheel mode, at 10 rpm with its spin axis perpendicular to the orbit plane. The spacecraft orbital-plane is roughly coincident with the magnetic field meridian plane so that the radial head (mounted perpendicular to the spin axis) responds to ion fluxes for nearly the full range of magnetic pitch angles (-180 degrees to +180 degrees).

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