Winter 2010 Issue

VHF PROPAGATION

The Sun is Alive!

By Tomas Hood,* NW7US

Figure 1. One of the largest active sunspot regions yet observed in the new sunspot Cycle 24, NOAA 1035, seen here in the Michelson Doppler Imager (MDI) intensitygram (IGR) on December 18, 2009. (Source: Solar and Heliospheric
Observatory [SOHO])

The last decade closed out with a welcomed sign that our nearest star was no longer inactive. From November 2009 until press time (early January 2010), sunspot activity ruled the solar disc. December was a very active month, with only 10 days without official sunspots. December 9 ended 16 days of zero spots that started at the end of moderately-active November. Sunspot region 1034 (as numbered by The National Oceanic and Atmospheric Administration, NOAA), small but belonging to the new Cycle 24, emerged near the eastern limb of the Sun. This small region resulted in an initial sunspot count on December 9 of 13. By December 12, it appeared to be fading, yet on December 13 it increased in spots with a count of 14.

Then on December 14 another new sunspot region numbered 1035 emerged, kicking the sunspot count up to 28. By December 15, its size was seven times wider than Earth! Over the next several days through December 18 this new Cycle 24 sunspot group rapidly increased in size, becoming one of the biggest yet in the new cycle. On December 16, the complex magnetic structures within this sunspot region triggered a coronal mass ejection (CME) toward Earth. This massively huge cloud of solar plasma (billions of tons!) arrived about three days later, but did not cause any geomagnetic disturbance.

Coronal mass ejections are the fuel for auroral activity, and that is welcomed activity to the VHF weak-signal DX hound. When active sunspot regions breed CMEs, the possible result is geomagnetic storms that counter any positive effect that the increased solar activity may have on radio signal propagation on the frequencies below 6 meters. At the same time, the CME unleashes a plasma cloud that rides the solar wind and then, if the unleashed ejection is directed into the orbital path of Earth, causes aurora. Auroral activity occurs at the E-region of the ionosphere, and “clouds” of highly-ionized clouds form that in turn may reflect radio signals in VHF and sometimes even UHF spectrum.

By December 19, the Sun kicked into high-gear with the total sunspot count climbing to 43, the highest yet in the new sunspot Cycle 24. This pushed the 10.7-cm flux up to 87 on December 17! While the increase in sunspot activity and the higher daily 10.7-cm flux (remaining in the mid-80s) are not yet high enough to energize the ionosphere for F-region VHF propagation, it signals an encouraging up-tick in sunspot cycle activity.

Speaking of size, the size of active sunspot regions is given as units, each unit being one millionth of the Sun’s visible hemisphere (this unit does not have a specific name). The Active Region 1034 that emerged on December 9 measured ten of these units, or 10 millionths of the visible solar disc. By December 11, it grew to 20 millionths. With the new sunspot region, 1035, emerging on December 14, the total area of all active regions only totaled 30 millionths. However, 1035 quickly grew in size. By December 20, the total area of all sunspot regions equaled a huge 330 millionths of the visible Sun!

By the New Year’s Eve, three additional sunspot regions emerged—1036, 1037, and 1038. Region 1037 quickly ended, but the others continued to help keep things exciting. Additionally, Active Region 1039 emerged on December 27 and continued to rotate across the solar disc until it rotated around out of view on January 6, 2010. On January 7, region 1036 appeared to be rotating back into view! Perhaps now we can start to accept the idea that the new cycle is well under way. With that comes overall improvement on higher frequencies in the high-frequency shortwave spectrum. Soon, with this up-tick in sunspot activity the F-region of the ionosphere will begin to offer VHF propagation.

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