Largest Flares
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.
Quiet Sun
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 Crab
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.