The sun was first observed to emit intense radio radiation in 1942. At the end of World War Two the advances in radio receivers were turned to peaceful studies of extra-terrestrial signals. Astronomical radio telescopes were set up to observe solar radio waves at fixed frequencies, recording changes in signal intensity.
It was soon found that at some frequencies the solar signal remained quite steady, while in other areas of the radio spectrum the intensity might suddenly become a million times more powerful. These giant outbursts appeared to be associated with sunspots which could be seen on the sun's surface with optical telescopes. In 1949, J. P. Wild and L. L. McCready made the first sweep-frequency observations of the sun at Dapto, Australia, 20 miles south of Sydney. Instead of recording radiation at a single of fixed frequency, their receivers were able to record intensity at all frequencies from 70 to 130 megacycles per second (later the band was extended from 20 to 240 megacycles). By this means it became possible to observe fluctuations of solar outbursts not only as a function of time, but also as a funtion of frequency. Astronomers hoped these records would help explain the mechanism which created the intense signals.
In 1955 the United State Air Force and the Harvard College Observatory agreed on a joint project for a solar sweep-frequency observatory in the U.S. The Air Force supplied the funds and Harvard was in charge of the research. Dr. Alan Maxwell, a young New Zealand physicist who had been trained in radio astronomy at Britain's Jodrell Bank station, became the director of the project.
200 Miles from El Paso
The site of the proposed station was Cook Flat, a bowl-shaped valley in the Davis Mountains of west Texas. This location is roughly 200 miles from the nearest large city, El Paso, and thus relatively free from television and radio broadcasts which might blot out solar signals. Although some man-made radiation does reach Cook Flat, the hills which rise 1500 feet above the valley's floor reduce this interference by a factor of over 100. Eight miles from Cook Flat is the town of Fort Davis (population 600), where the Harvard station has set up offices and dark-rooms for developing film records.
The Fort Davis station began its observations in August, 1956. Since that time the observatory has been recording radiation from the sun 10 hours a day, every day of the year, except for the infrequent interruptions due to failures of the equipment. At 7 a.m. the station automatically turns itself on. A wire parabolic dish, 28 feet in diameter and mounted above the ground on steel legs, begins to follow the sun in its path through the sky. At the focus of this dish a complex antenna system receives the signals from the sun as they are reflected off the wire screen. In the nearby dust-proof laboratory building, electronic equipment analyzes the solar data.
The heart of the project are the sweep-frequency receivers mounted on steel racks in the lab. Each receiver is simply an elaborate radio with a radio telescope for an antenna and lacks any sort of speaker for the sound to come out. Like an ordinary radio, these receivers can be tuned in on different frequencies; this is done by a small electric motor which mechanically tunes or sweeps the receiver through its entire range three times each second. In place of a speaker which would make the solar outbursts audible, each receiver has a cathode ray tube. The spot moves up and down the fluorescent cathode ray screen in a straight line, synchronized so that as the receiver sweeps from higher to lower frequencies the spot seep from bottom to top of the screen. Thus the vertical displacement of the spot corresponds to frequency, and the intensity of the spot is made to vary with intensity of the solar signal at that frequency.
When the station began in 1956 it had three spectral receivers covering the frequency ranges 100-800, 180-320, and 320-580 megacycles. Later three more receivers were added: 25-50, 50-100, and 2,000-4,000 megacycles. The six cathode ray screens are mounted one over another, with the highest frequencies at the bottom, and photographed with 70mm film which slowly winds past the display screens at the rate of 2.5 feet per hour. When the 100 foot rolls of film are later developed in Fort Davis, they give a permanent visual record of solar radio activity.
Analysis of radio records from Texas and Australia has answered many questions about the sun. By combining these records with the motion pictures taken with optical solar observatories all over the world, astronomers have pieced together a theory for the general mechanism which causes giant explosions on the face of the sun--solar flares. The most important concept for analyzing the records is the fact that the frequency of a radio outburst corresponds to the height above the sun's surface emit high frequency signals; bases higher in the solar corona radiate at lower frequencies. Thus, a change in the frequency of a solar signal is caused by a change in the height of the emitting region. With this in mind four general types of solar outbursts have been distinguished.
I. Noise Storm. A noise storm consists of a series of short individual bursts of radiation, each lasting a minute or less. The source of these bursts is a turbulent area in the corona above an active sunspot. An entire noise storm may last for hours of days. The bursts are generally confined to a narrow range of frequencies, not greater than 30 megacycles, because the emitting region is at a fairly constant height in the corona.
II. Slow-Drift Burst. As the name suggests, this is a narrow band of radiation which gradually drifts from higher to lower frequencies at roughly one megacycle per second. The drift-rate of these Type II bursts suggests that the emitting gases are moving away from the sun at roughly 1,000 kilometers per second (1/300th the speed of light). Such bursts are often associated with large flares and are often followed in one or two days by auroral displays and geomagnetic storms on earth. Thus the slow-drift burst probably heralds the ejection of solar protons. The theoretical velocity of the protons--1,000 km/sec, producing a sun-earth travel time of 40 hours correlates well with the observed delay between solar and terrestrial events.
III. Fast-Drift Bursts. These bursts also drift from high to low frequencies, but with great rapidity. Each burst lasts roughly one second, and the drift rate corresponds to an outward velocity of 100,000 km/sec. Such radiation may be due either to fast traveling nuclear particles of to shock waves in the corona. Although Type III events have been associated with numerous small flares, they do not appear to have any effect on the earth.
IV. Continuum Radiation. Type IV bursts consist of intense radiation that persists over a broad-band of frequencies for an hour or more. On the Harvard film records, a continuum burst looks like a white streak, perhaps an inch high, running horizontally across the film. This type of radiation accompanies the largest flares. It may be followed almost immediately by a burst of high-energy particles which travel toward the earth at almost the speed of light. These powerful particles can cause terrestrial radio blackouts. These days later a shower cloud of particles may reach the earth and cause geomagnetic storms and aurorae.
The continuum bursts themselves are caused by a process known as synchrotron radiation: when electrons traveling at speeds near that of light are accelerated they emit a broad-band radiation. Any modern theory concerning solar flares must somehow explain where these high-speed electrons come from and how they are accelerated in order to explain Type IV radio emission.
Next year is the low-point in the 11 year cycle of solar activity; it will be known to the world's scientific community as the International Year of the Quiet Sun (IYQS). For the Fort Davis station this means that there is little reason to operate the solar equipment, for the exciting radio activity that led to so many discoveries at solar maximum (1959) will be virtually nonexistent. The observations will continue only to preserve continuity of the records. The team out in Texas will have plenty to do, however, working with a new antenna which was built last year.
The Harvard Radio Astronomy Station was originally conceived strictly as a solar research project, but it is now branching out to study some of the other radio sources. A new $1,000,000 instrument, financed by the Air Force, now rises over 100 feet above the rocky soil of Cook Flat, dwarfing the solar equipment.
The antenna has a dish 85 feet in diameter made of giant metal plate rather that simple wire mesh. The massive structure, aptly described as a "moveable bridge" can be pointed at any spot in the sky by throwing a switch in the laboratory which has been constructed nearby. Unlike the sweep-frequency receivers, this instrument can only do observations at 950-megacycles (L-band) and 5,000 megacycles (C-band). No other comparable antenna in the world (and there are few of them) is operating at C-band and so any work which Harvard does at this frequency will be original.
The first serious project with the new antenna was begun early this past summer. Dennis N. Downes '65 and Michael P. Hughes, Dr. Maxwell's research associate at the station, observed the passage of the solar corona in front of the Crab Nebula. In late June the Crab Nebula, the gaseous remnants of a star which exploded in 1054, was in the daytime sky near the sun.
A team of workers at the Air Force Cambridge Research Laboratories, using an 85 foot antenna similar to that in Fort Davis, had written a paper announcing that the radio waves from the Cran were sharply distored as they passed through the outer extensions of the sun's atmosphere. The Harvard group repeated the observations and found no distortion.
Later in the summer the new antenna was used to measure the effective temperatures of someof the planets and to map the center of our galaxy at 5,000 megacycles. Operations ended in September, however, because rain water leaked into the equipment at the focus of the giant dish, and caused the receiver to mal-function. Once this equipment is made watertight, work at the station should proceed rapidly. Dr. Maxwells hopes to make extensive studies of individual sources of celestial radiation. With over 100 sources to be examined, the Harvard Radio Astronomy Station will not be idle during the Year of the Quiet Sun.