Winter 2006 Issue


What’s There To Do in the World of VHF Propagation?

By Tomas Hood, NW7US/AAAØWA

In the last edition of this column, we took a look at dying sunspot Cycle 23 and how we’re expecting its end by 2007. With the approach of end of the cycle, is there anything (propagationally speaking) worth exploring on the VHF and UHF bands?

Because of the nature of the Earth’s orbit around our sun, we have two seasons each year when any adverse space weather has a greater influence on causing geomagnetic disturbances. The first is known as the Spring Equinoctial Season, and the second is known as the Autumnal Equinoctial Season (see figure 1). These are the two times during the course of the Earth’s orbit around the sun when the Earth is in just the right position to be most influenced by solar activity.

The Spring Equinoctial Season peaks between March and April of each year. Is it likely during this final year of the current Cycle 23 that we will have significant geomagnetic disturbances to trigger the sort of auroral activity known to bring VHF activity?

The answer lies in exploring past solar cycles and knowing the nature of space weather during the decline and end of a sunspot cycle. Specifically, the answer is tied to the existence of coronal holes. Last quarter’s column touched on the origin of the solar wind and the role coronal holes play in space weather.

One of the “atmospheric” layers around the sun is a region known as the corona. When a large area develops in the corona that is less dense than the surrounding area, a “coronal hole” opens up. These large-scale features are “open” magnetic-field regions that are sources of high-speed streams of solar electrons, protons, and ions (plasma).

Figure 1. This graph by IPS, Australia shows the two seasonal geomagnetic peaks during the equinoctial periods (see text). (Source: IPS, Australia)

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