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 Previous: Surfin': Mapping Hams | ![Next (later) Story](/images/sty-up.gif "FCC Invites "Oppositions" on Two "Morse Proceeding" Reconsideration Petitions"){kind=small} Next: FCC Invites "Oppositions" on Two "Morse Proceeding" Reconsideration Petitions |
SEATTLE, WA, Apr 13, 2007 -- So far this month we've observed nine days in a row with zero sunspots, and all of our reporting week -- April 5 through 11 -- falls within this string of inactive solar days. As noted last week, comparing that week's average sunspot number (12.9) to this week's (0) doesn't mean much when activity is so low at the bottom of this sunspot cycle. Based on predicted smoothed sunspot numbers, if this month and the next are truly the solar minimum, we should probably see several weeks in a row with no sunspots. This should cheer those hoping for more activity and a return to worldwide propagation on 10 and 15, because recent observation shows the rise of any solar cycle is more rapid than its decline and, most important, the decline of the previous cycle.
Around the last solar minimum, between Cycle 22 and Cycle 23, sun watchers observed several long strings of days showing no sunspots. For instance, from February 10 to March 5, 1997, the average sunspot number was 2.1, and there were 20 days during that period with no sunspots. Another long stretch of inactivity occurred in the fall of 1996. Average daily sunspot number was 1.7 from September 5 to November 8, when 57 of 65 days the sun showed no spots. The longest continuous stretch with no sunspots at all in that period was 38 days following September 12, when a single sunspot was visible, and ending October 21, when another sunspot appeared. Both days had an official sunspot number of 11, and each time, the single sunspot was only visible for a single day.
This period was preceded and followed by three days of no sunspots. October 29 through November 8, 1996, also saw no sunspots, followed by December 24 through January 3, 1997.
Remember that an average sunspot number of one or two doesn't correspond to one or two actual sunspots. Because of the peculiar manner in which sunspot numbers are derived, the minimum non-zero sunspot number on any day is 11. This is because the number of sunspot groups is multiplied by 10, and the resulting number is added to the total number of visible spots. So two spots in one group is 12, three spots is 13, but three spots in two groups yields a sunspot number of 23.
To observe the rate that Cycle 22 fell toward minimum leading up to Cycle 23, it's useful to average sunspot numbers over each quarter. The average sunspot numbers for the years 1993 and 1994 were 79 and 48. The averages for the four quarters of 1995 were 47.4, 25.3, 21 and 21.3. The year 1996 had 13.1, 13, 12.4 and 14.2. For 1997, the quarterly averages were 11.3, 25.4, 37.2 and 48.2. The quarterly averages of sunspot numbers for the next two years, 1998 and 1999, were 62.9, 80.4 , 111.8, 99.1, 97.2, 147.2, 137.9 and 163.1. You can see that the new cycle rose quickly, beginning in the second quarter of 1997.
In practical terms, what is the difference between a couple of weeks of zero sunspots, and three months of a sunspot number around 163? Read on!
Every year Saad Mahaini, N5FF, of Richardson, Texas spends three weeks in Syria. Currently operating YK1BA, he will return to Texas on April 28. Plugging the numbers into a popular propagation prediction program this weekend when he talks to someone back home in the Dallas area, he might find spotty openings on 20 meters around 2100-2230 UTC and perhaps 0400 UTC. Less likely, 17 meters might be possible around 1700-2000 UTC, and far less likely, 15 meters from 1500-2100 UTC. He could also expect strong 40-meter signals from 0000-0400 UTC.
These are normative expectations. Your mileage may vary. Saad may experience fantastic propagation. These predictions are based on probability, and we can always be surprised.
But if the sunspot number leading up to this weekend had been about 163, he could expect stronger 40 meter signals over the same period, 20 meter propagation from 2100-0700 UTC with really strong signals from 0000-0430 UTC, and a good 15 meter opening from 1300 to 2300 UTC. He might even find 10 meters open over a shorter time during the same period. Or, the sun could be covered with spots, and a huge solar flare -- more likely during periods of higher solar activity -- could wipe out propagation in a radio blackout.
Coming soon, the prediction for the next period of unsettled geomagnetic conditions is for around April 20, with an expected planetary A index of 20. After that, a planetary A index of 25 is predicted for April 28. This same forecast (from the US Air Force, via NOAA) shows solar flux of 70 until April 16, when it rises to 75. This is a small shift, but it may signal the period during which we could see another sunspot, April 16-27.
David Moore tipped us off to an interesting article from the European Space Agency about the physics of solar wind and auroras.
Finally, one more report from the top of Cycle 19. John Hardin, W4NU, of Atlanta, Georgia, was first licensed as a Novice around the age of 16 as KN4JAG in June 1959. He upgraded to General a month later. He writes:
"The propagation was awesome. By 1960 I upgraded from a Knight Ocean Hopper receiver and Globe Scout 680-A with a 40 meter doublet to a Viking Valiant, NC-57B receiver, and a 3 el Hornet tribander on my parents' roof. 10 meters was open after midnight, 15 on into the morning and 20 meters all night. I could break the DX pileups with ease and ended up with DXCC in the early 1960s. I will never forget how crowded the 10 meter phone band was!"
For More Information
For more information concerning propagation and an explanation of the numbers used in this bulletin see the ARRL Technical Information Service Propagation page and the article "The Sun, the Earth, the Ionosphere: What the Numbers Mean, and Propagation Predictions," by Carl Luetzelschwab, K9LA. An archive of past bulletins is on the ARRL Web site. The propagation charts that formerly appeared in QST now are available on the QST Propagation Charts page on the ARRL Web site.
Weekly Sunspot Numbers
Sunspot numbers for April 5 through 11 were 0, 0, 0, 0, 0, 0 and 0 with, a mean of 0. The 10.7 cm flux was 70.9, 71, 71.2, 71.1, 69.9, 69.4, and 69.1, with a mean of 70.4. Estimated planetary A indices were 4, 4, 3, 3, 9, 7 and 4, with a mean of 4.9. Estimated mid-latitude A indices were 3, 2, 2, 2, 8, 6 and 3, with a mean of 3.7.
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.
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