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The extremely high-precision pointing of the spacecraft was achieved through reaction wheels which were housed in an attitude control unit on the spacecraft. Unfortunately, during the first year of operation (1980), the reaction wheels failed one by one and by November 1980 the SMM spacecraft could only crudely point in the solar direction. This left only a few (non-imaging) instruments able to function. NASA conceived the repair of the spacecraft using the newly operating Space Shuttle, and in April 1984 the Space Shuttle Challenger went into orbit with a team of astronauts who were instructed to replace the entire attitude control unit and do some other instrument repairs. The highly eventful week-long mission was ultimately successful, the SMM spacecraft was captured, placed in the bay of the Shuttle, repairs were made, and finally SMM was placed back into orbit. It was the finest space spectacular since the Apollo lunar landings, and it had enormous world-wide press coverage.
Rutherford Appleton Laboratory was involved in the building, operating, and analysis of data from two instruments, the Flat Crystal Spectrometer (FCS) and Bent Crystal Spectrometer (BCS). Both these instruments were extremely high-resolution X-ray spectrometers designed to observe the soft X-ray line spectrum from solar flares and active regions which occurred in profusion at the start and end of the mission, and were still in evidence during the middle years of the 1980s when there was a minimum in solar activity.
The spectrometer was among the highest resolution X-ray instruments, space-based or ground-based, anywhere in the world, and the line spectra obtained and published in the scientific literature were unprecedented in their astonishing detail. Only the advent on the large tokamaks at Princeton and elsewhere as well as the relatively short-lived Japanese Hinotori spacecraft were the achievements of these spectrometers eventually surpassed. However, the insights into the detailed structure of solar flare and active region plasmas (i.e. extremely high-temperature gas where the component particles are ionized atoms or ions and free electrons) were extremely valuable and have not been superseded.
The other instruments on board SMM were as follows:
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Gamma Ray Spectrometer (GRS) |
measuring gamma-ray spectra; first to resolve clearly the electron-positron line at 551keV and deuteron-formation line at 2.2MeV, so proving the existence of nuclear reactions in flares; and also the first instrument to detect free neutrons from flares. |
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Hard X-ray Burst Spectrometer (HXRBS) |
observed thousands of flares in X-rays with energies up to 300keV. |
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Hard X-ray Imaging Spectrometer (HXIS) |
Joint UK/Dutch spectrometer that imaged flares at energies of up to 30keV. |
Ultraviolet Spectrometer and Polarimeter (UVSP) |
imaged flares in the far ultraviolet (1150 -- 3600 Angstroms) and detected many examples of plasma flow along magnetic loops making up active regions. |
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Coronagraph/Polarimeter (C/P) |
white-light coronagraph observing to distances of 6 solar radii from sun centre, made first high-resolution observations of coronal mass ejections. |
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Active Cavity Radiometer (ACRIM) |
high-precision instrument measuring total irradiance from sun, finding for the first time that there is a very small variation with solar cycle (a minimum at solar activity minimum) and with sunspots (which cause a temporary dip in solar output). |
The international team of experimenters were based at the NASA Goddard Space Flight Center, Greenbelt, Maryland, for most of the SMM operation period (1979-1989). Workshops were organised during this period and shortly after to discuss the many scientific results. It was one of the first times that flare theories, particularly involving MHD theory and those of particle acceleration, could be compared in detail with observations. A particular focus was the site and nature of the primary energy release, identified with a reconnection of magnetic field near the top of a flaring magnetic loop.
One of the startling finds of the FCS and BCS spectra was the identification of very large density regions at the start of the soft X-ray flare using ratios of line intensities. Normally particle densities in flares were thought to be around 1011 per cubic centimeter but observations of lines due to carbon-like iron (Fe XXI) showed there was a period just after the flare impulsive stage when densities were at least 10 times greater. This is of much importance as it implies that the cooling of a flare is by radiation rather than the more conventional idea of thermal conduction cooling.
It was recognized early on that there were substantial flows of plasma in the early stages of flares -- the X-ray lines of helium-like iron (Fe XXV), formed at 20 million degrees K, showed huge Doppler shifts to shorter wavelengths, revealing the presence of motions towards the observer. This gave rise to many theoretical models of flares where "evaporation" of material in the chromosphere (the region of the solar atmosphere just above the visible photosphere) occurred with the evaporated plasma moving up into flare loops. This is still controversial, since quite possibly the motions are due to individual loops which start to expand at the flare impulsive stage.
Element abundances became a hot topic during the SMM era, when it was claimed by several people that the abundance of Fe and Ca varied during flares. Even now, these claims have not been settled. Another controversial topic was the possibility that element abundances in the corona (hot outer part of the sun's atmosphere) varied according to their first ionization potential. This seemingly esoteric effect has been explained in various theoretical models, though there remain some who are sceptical of its existence.
As it is always sobering to bear in mind, the sun is merely a star so its properties are likely to be reproduced in stars that have similar surface temperature and magnetic field activity as the sun's. This has recently been borne out in studies of X-ray spectra of stars -- Capella, for instance, is a binary system consisting of two giant stars with similar surface temperatures to the sun. Capella has been observed as an X-ray source for many years, and recent Chandra spectra show that the corona(e) of Capella have an X-ray spectrum remarkable similar to small solar flares, i.e. have a temperature of about 6 million degrees K (the "quiet" solar corona has a temperature of only 1 or 2 million degrees K). The diagram below illustrates a Chandra spectrum of Capella and a spectrum of a solar flare from the FCS instrument on SMM.
Data from SMM have proved to be a highly valuable insight into the mechanisms of solar activity -- flares, coronal mass ejections, active regions, plasma flows etc. The scientists who have worked on the data analysis have an international background and have made important advances in solar physics. It is worth remembering the contribution of the Challenger crew in their successful bid to repair the ailing SMM spacecraft in 1984 which gave scientists the opportunity to continue observing with the instruments until 1989. Sadly, the pilot of the Challenger crew, Francis R. Scobee, was subsequently killed in the last flight of Challenger. The debt to him is enormous and will be remembered for many years to come.
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Author: Professor Ken Phillips Last updated: February 1, 2001 |
