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.