Fall 2007 Issue


Solar Cycles
and Cycle 24 Predictions

Based on a paper presented at the 41st Central States VHF Society Conference in San Antonio, Texas on July 28–29, 2007, KH6/K6MIO begins this article with the F-layer and the Sun. He then discusses and summarizes the various predictions of solar Cycle 24 that have been made to date. Significant portions of this article also appeared in the Proceedings of the above-mentioned conference.

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

Figure 1. The Sun as seen by the National Solar Observatory (NSO) at Kitt Peak near Cycle 23 solar maximum on April 3, 2000. Note the two bands of sunspots north and south of the solar equator. (Credit: NSO/AURA/NSF)

On 6 meters, F2 propagation can produce dramatic results. The F-layer is ionized primarily by extreme ultraviolet (EUV) radiation from the Sun. The intensity of solar EUV is strongly dependent on the phase of the solar activity cycle. Unfortunately, the average level of solar EUV is not sufficient to raise the maximum usable frequency (MUF) above 50 MHz. Consequently, 6-meter F-layer propagation is confined almost entirely to the peak years of the solar activity cycle.1

Managing radio propagation, satellite health, and power-grid issues all lead to an interest in predicting future solar activity, on both short and long time scales. There are actually two intimately related “solar cycles”: the activity or “sunspot” cycle and the solar magnetic cycle.

Solar Activity Cycle

The sunspot cycle peaks roughly every 11 years. Sunspots are always found in pairs or groups. The spots and groups occur in two latitude bands, one north and the other south of the solar equator (Figure 1). They come and go within those latitude bands with end-to-end lifetimes of a few days to several weeks.

As will be shown in this article, sunspots are the visible effects of loops of powerful magnetic fields arising from within the Sun that have then floated up and bulged out above the Sun’s visible surface (Figure 2).The east-west leading spot(s) in a pair (or group) have the opposite magnetic polarity from that of the trailing spot(s). If the leading spots in the Southern Hemisphere band have one polarity, then the leading spots in the Northern Hemisphere band have the opposite polarity—that is, the direction of the field between the leading and following spots in the south are opposite of those in the north (Figure 3).

As the Sun rotates on its axis every 27 days or so, sunspot pairs and groups appear to march across the Sun from east to west, being visible for up to 14 days as they travel from limb to limb and then rotate out of sight around the far side. If they live long enough, they return to the near side about 14 days later.

Old Cycle, New Cycle

Each “new” activity cycle begins at the minimum after the preceding solar maximum. Near the minimum, the few remaining “old-cycle” spots are found in their two latitude bands, now very near the solar equator (about 5 degrees north and south). On the almost spotless Sun, the new cycle begins when spots begin to appear in two new bands about 30 degrees north and south—with opposite polarities from the old-cycle spots. Thus, the old and new cycles actually overlap each other for a period of time. Their latitude bands and polarities distinguish between the old- and new-cycle spots. Curiously, new-cycle spots generally do not appear at the same time in both the Northern and Southern Hemispheres (more later).As the cycle progresses, the new-cycle spots appear in increasing numbers, with their higher latitude bands slowly moving closer and closer to the equator. By solar maximum, the bands are centered on about 15 degrees north and south latitude. As the cycle wanes, the old-cycle spot count decreases and the two bands move to within about 5 degrees of the equator.

 

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