Winter 2004 Issue

PROPAGATION

Radio Aurora

By Tomas Hood,* NW7US

A coronal mass ejection’s plasma cloud comes toward the Earth. (Courtesy NASA)

One man’s garbage is another’s treasure. Space weather and the state of the Earth’s geomagnetic field might be thought of in the same way. That which degrades HF (high frequency) radio propagation might create conditions for useful VHF (very high frequency) radio propagation. During times of minor to severe geomagnetic storm activity, the ionosphere loses its ability to refract HF. At the same time, however, these geomagnetic storms often trigger auroral substorms that create areas of ionization capable of reflecting VHF signals. This mode of propagation, called radio aurora, offers a challenging yet exciting opportunity to increase your grid-square count.

Many years of auroral observations reveal that peak periods of radio aurora occur close to the equinoxes—that is, during the months of March and April, and again in September and October. Of the two yearly peaks, the greater peak, in terms of the number of contacts reported, occurs during October. However, some of the strongest levels of geomagnetic storms are in the spring. The minimum yearly activity occurs during the months of June and July, with a lesser minimum during December.

When active aurora is seen in the auroral zone, a strong magnetic disturbance is usually also observed there. These disturbed magnetic fields often are much stronger than those of a geomagnetic storm, but are strictly local, fading away quickly as one moves equator-ward. This suggests that the currents that disturb the magnetic fields flow somewhere nearby—probably near the auroral arcs. The Norwegian physicist Kristian Birkeland (whose portrait appears on Norwegian currency) carefully observed auroral disturbances and concluded that the currents flow parallel to the ground, along the auroral formation. Because electrical currents must flow in a closed circuit, and because these magnetic disturbances seemed to be caused by processes taking place in distant space, Birkeland proposed that the currents came down from space at one end of an arc and returned to space at another end.

In 1910 Birkeland performed a series of experiments to reproduce many of the characteristics of the aurora that he had observed during his expeditions. He placed an electromagnetic sphere, coated with fluorescent paint, inside a vacuum chamber and projected a beam of electrons at the sphere. This enabled him to view the trajectories of streaming electrons. Birkeland was able to accurately reproduce how solar wind would make its way into the Earth’s magnetic poles, and was able to simulate the auroral ovals near the Earth’s magnetic poles.

Finally, in 1954, auroral electrons were actually observed by sensors aboard a rocket launched into an aurora by Meredith, Gottlieb, and Van Allen, of Van Allen’s team at the University of Iowa. The Van Allen team discovered the Earth’s radiation belts, now called the Van Allen Belts
 

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