Solar Radio Monitoring Program

The Solar Radio Monitoring Program is a service operated jointly by National Research Council Canada and the Canadian Space Agency. Its function is to provide current and archival values of the 10.7cm Solar Flux solar activity index to any user to whom it might be of use.

The various manifestations of solar activity are driven by the total amount of magnetic flux emerging through the photosphere into the chromosphere and corona, and its temporal and spatial distribution. For reasons not clearly understood, solar activity ebbs and flows over a cycle of about 11 years. The 10.7cm Solar Flux is a measurement of the integrated emission at 10.7cm wavelength from all sources present on the disc. It is almost completely thermal in origin, and directly related to the total amount of plasma trapped in the magnetic fields overlying active regions. This in turn is related to the amount of magnetic flux. A comparison made over more than a solar activity cycle show that there is indeed a linear correlation between the 10.7cm Solar Flux and the total photospheric magnetic flux in active regions.

The 10.7cm Solar Flux, i.e., the solar flux density at 10.7cm wavelength is measured using two fully automated radio telescopes (called "Flux Monitors"), located at the Dominion Radio Astrophysical Observatory.

The left-hand flux monitor is the one that provided the "Ottawa" flux for many years; the one on the right, on a tower, was built in 1990. The two instruments record the strength of the solar radio emission at 10.7cm wavelength each day for as long as the Sun is above the horizon. In addition, the instruments interrupt the continuous monitoring each day to make three precise measurements of the solar flux density. These measurements constitute the 10.7cm Solar Flux index. These measurements are transmitted immediately by E-Mail and fax to a worldwide list of users. In addition, current and archival data are available from www.spaceweather.ca.

History of the Solar Radio Monitoring Program

The history of the 10.7cm Solar Flux is intimately tied in with the beginnings of Canadian radio astronomy. Immediately after the Second World War, Arthur Covington and his colleagues at the National Research Council in Ottawa used bits of military surplus radar and test equipment to make a radio telescope. The antenna was a 4ft (1.2m) paraboloid from a Type IIIC Gun Laying Radar, mounted on a prototype mount casting for a Model 268 radar. By leaning the mount so that the azimuth axis was pointed at the Pole Star, it was converted into a simple polar mount, which made tracking the Sun much easier. The receiver was a Dicke switching receiver used during the war to test silicon mixer crystals for radar applications. The radar system operated at a frequency of 2800MHz, which is a wavelength of 10.7cm.

The instrument was pointed in the direction of various celestial objects, including Jupiter, the Milky Way, aurora borealis, and the Sun. It was too insensitive to pick up any cosmic source apart from the Sun. However, as time passed, Covington and his colleagues realized that the Sun's emission at 10.7cm wavelength was varying. They did not expect this. Thinking at that time was that the solar emission at centimetre wavelengths would be simply black body emission from a ball of hot gas. This led to the question of whether this was a variation in the emission from the whole disc or that smaller, variable sources were present, perhaps associated with active regions and sunspot groups.

The poor angular resolution of the radio telescope (a few degrees) made it impossible to distinguish between these two possibilities. However, an opportunity to address the question offered itself on 23 November 1946, when an eclipse of the Sun occurred in the Ottawa area. The actual recording of the radio emission during that eclipse is shown in the Figure.

The observation showed convincing proof that strong contributions to the total emission at 10.7cm originated in the vicinity of sunspots. The eclipse record shows a strong dip in signal strength after 11:40 local time, when the Moon covered a large sunspot on the solar disc.

Covington then showed that the 10.7cm Solar Flux correlates with indices of solar activity such as sunspot number and total sunspot area, with the advantage over those indices that the measurements are completely objective, and can be made under almost any weather conditions. Since it is closely correlated with magnetic activity, it correlates closely with other activity indices and, since magnetic activity modulates the Sun's energy output, with solar irradiance.

The emission Covington had found is now known as the "Slowly-Varying" or S-component of solar radio emission. It was subsequently established, through both observation and theory, that the best wavelength to observe this component of solar radio emission is around 10cm. That Covington decided to make observations at 10.7cm wavelength was decided his using radar components designed to operate at that wavelength. The choice had nothing whatsoever to do with astronomical considerations, and must count as one of the more significant coincidences in astronomy.

The 10.7cm Solar Flux is currently one of the best indices of solar activity we have. It now forms a consistent, uninterrupted database covering more than 50 years. Only sunspot number counts cover a longer period, going back to at least the 17th Century. However, these data are subject to subjective effects in observation and evaluation, and are affected by the weather.

Between 1946 and 1990, the measurements were made in the Ottawa area — first at Goth Hill, a site south of Ottawa, and then at the Algonquin Radio Observatory. In 1990, following the closure of that observatory, the Solar Radio Monitoring was relocated to the Dominion Radio Astrophysical Observatory near Penticton British Columbia. At that point, what had been known for decades as the "Ottawa Flux" became the "Penticton Flux". In order to ensure the move did not affect data quality, the move was achieved by building a completely new solar flux monitor, installing it at Penticton, and then running the Penticton and Ottawa systems in parallel for more than six months. Then the Ottawa instrument was shut down and moved to Penticton, where it was set up to operate alongside the new instrument. Operating two instruments in parallel provides a quick check of measurement quality and a hot backup in the event of problems.

In 2003, the Solar Radio Monitoring Program became a joint program with the Canadian Space Agency and a component of the Canadian GeoSpace Monitoring Program, which brings together various geospace monitoring programs in one entity. This new operating arrangement has made possible a major upgrade program and should help ensure the value of this "Canadian Stethoscope on the Sun" for the foreseeable future.

Related Information

Institutes:

• NRC Herzberg Institute of Astrophysics