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The K7RA Solar Update
SEATTLE, WA, Sep 9, 2005--Sunspot 798--last seen in August--returned this week with a vengeance, releasing several powerful X-class solar flares over the past couple of days. The solar flare of September 7 was considered among the largest on record. Although these flares were not aimed squarely pointed at Earth, glancing blows could cause greater geomagnetic activity. This week's flares came on the heels of a high-speed solar wind last weekend that provoked geomagnetic instability--hence the high A index values--although conditions quieted down after Sunday.
Currently the interplanetary magnetic field points north, which could offer some protection. The forecast for the planetary A index for Friday through Monday, September 9-12, is 25, 20, 10 and 20. Predicted solar flux for the same days is 100 for Friday, September 9, then values around 110 for the following week. A little higher sunspot activity is welcome for the next couple of weeks as we head toward the autumnal equinox, usually a better time for HF propagation.
Randy Crews, W7TJ, asked about sunspot and solar flux numbers at the bottom of the solar cycle, which is expected around the end of 2006 or early 2007. Of course, there is enough daily variation that we won't really know when the current cycle bottoms out until well after that time has passed. We only know for sure after looking at a smoothed curve where daily numbers are averaged over many months.
Of course we will see many days, sometimes weeks, with no sunspots at all. Solar flux drops below 70 to a background level, around 67. A typical low period was September and October 1996, in which the average sunspot number was 2.6, and average solar flux was 69.3. Perhaps we'll see these low numbers again some 12 to 18 months from now.
For more information concerning radio propagation and an explanation of the numbers used in this bulletin see the ARRL Technical Information Service Propagation page. An archive of past bulletins is on the ARRL Web site.
Sunspot numbers for September 1 through 7 were 24, 28, 14, 12, 12, 12 and 11, with a mean of 16.1. The 10.7 cm flux was 79.2, 77.1, 74.2, 74.6, 75, 83.4, and 117, with a mean of 82.9. Estimated planetary A indices were 21, 33, 32, 26, 14, 9 and 15 with a mean of 21.4. Estimated mid-latitude A indices were 11, 24, 20, 18, 9, 6 and 18, with a mean of 15.1.
Amateur solar observer Tad Cook, K7RA, Seattle, Washington, provides this weekly report on solar conditions and propagation. This report also is available via W1AW every Friday, and an abbreviated version appears in The ARRL Letter. Readers may contact the author via e-mail.
Page last modified: 10:38 AM, 09 Sep 2005 ET
Page author: awextra@arrl.org
Copyright © 2005, American Radio Relay League, Inc. All Rights Reserved.
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