Spring 2003 Issue

Figure 6. In geomagnetic coordinates, Hawaii is nearly due north of Australia, and these transverse north-south TEP paths, often behave like conventional skip. Signals usually are clean and stable. East-west signals from Japan and South America commonly have heavy scatter modulation, at times resembling aurora, and display Doppler effects as well.

Part II of this article on 50-MHz F2 propagation is reprinted by permission from the Proceedings of the 34th Conference of the Central States VHF Society, July 2000.
 

By Jim Kennedy, KH6/K6MIO
Gemini Observatory, Hilo, Hawaii

Part I of this article in the Winter 2003 issue of CQ VHF pointed out that F-layer propagation depends upon a combination of many factors. Such variables include the amount of ionization present, the angle at which a radio wave encounters the ionospheric layer, and the presence or absence of irregularities in the layer. The F layer normally depends upon Extreme Ultraviolet (EUV) radiation from the Sun to produce its ionization, but near the geomagnetic equator, electrons may be driven from the E layer up into the F- layer by an interaction of E-layer winds and the Earth’s magnetic field. The absorption of F-layer electrons in the summer leads to higher ionization and MUFs (Maximum Usable Frequencies—ed.) in the winter, especially for east-west paths, while the more equal distribution of ions on both sides of the geomagnetic equator in the spring and fall favors north-south paths across the equator.

The Solar Cycles

For reasons that are still unknown, the general background magnetic field of the Sun reverses polarity every 11 years or so. Thus, the Sun experiences a 22-year magnetic polarity cycle of north to south to north again. This effect is accompanied by a cycle of solar activity that reaches a peak approximately every 11 years. The peak itself can be fairly broad, having significant effects for three or four years.

The solar activity cycle is seen in virtually every kind of signal we can receive from the Sun, from radio waves to x-rays. Not surprisingly, then, the amount of ionizing radiation impinging on the atmosphere varies with this same pattern, including the EUV that is the principal source of the F2 layer. Consequently, propagation is decidedly better near solar maximum, but the seasonal effects are still superimposed on the general enhancement seen during the solar maximum.
There is a second kind of variation because of the fact that the Sun rotates on its axis every 27 days or so, coupled with the fact that “activity” on the solar surface is generally confined to a few specific regions at any one given time. As a result, if the Sun is active at all, it is quite common for one side to be active and the other side to be relatively quiet. As the Sun rotates, there is often a very pronounced 27-day cycle in the radiation reaching the Earth.

It should be noted that the active solar longitudes change over time. The 27-day cycle of activity commonly repeats for several cycles, which is then briefly interrupted as old solar active regions fade and others emerge. When new active regions develop, typically at some other longitude, the cycle will be reestablished, but with a different phase. In other words, knowing that a particular two-week period was active last month is a pretty good predictor that the same two-week period will also be active. However, it is a very poor predictor of activity during the corresponding period six months from now.

During solar maximum and especially during periods of high activity, there is no doubt that the amount of EUV reaching the ionosphere increases substantially. In principle, this should mean better propagation. People have tried for some time to get direct measurements of the EUV radiation with an eye toward making short-range predictions of propagation conditions, but so far these have not been very successful.

Very little of the F2 producing EUV reaches the Earth’s surface, precisely because it is absorbed, making ions in the F layer. A number of spacecraft have carried EUV-sensing instruments, but generally these detectors are susceptible to damage from the very radiation they wish to measure. As a result, their sensitivity changes in time, making accurate, long-term, absolute measurements very difficult to obtain.
 

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50-MHz F2 Propagation Mechanisms Part II

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