Propagation de K7RA

Over the past week, sunspot activity has again declined, with average daily sunspot number down by more than 7 points to 18.

Geomagnetic activity has been much more stable this week, with the planetary and mid-latitude A indexes less than half what they were the week before. The mid-latitude A index was actually down to zero on both weekend days, May 5-6. The next period of higher geomagnetic activity is expected around May 25-27.

Geophysical Institute Prague projects quiet conditions for May 11-12, quiet to unsettled May 13, unsettled May 14-15, and quiet again on May 16-17. For May 11-17, the US Air Force expects planetary A indices of 5, 5, 8, 8, 10, 10 and 5, a slightly different scenario.

May 6 was the halfway point between the vernal (spring) equinox and the summer solstice. This weekend, compared to the first day of spring, when observing propagation between the center of the United States and Europe, we expect much shorter openings on the low portion of the HF spectrum. This is because the hours of darkness are best for 80, 60 and 40 meters, and the first day of spring has a longer night than we currently see.

For much of the 24-hour day the MUF over this path would be higher than in March, except the six hours between 1430 - 2030 UTC, a period during most of which there is daylight on both ends of the path.

April 30 through May 3 saw sunspot numbers ranging from 32-38, more than 20 points higher than the usual daily readings of late. Several readers reported better propagation on those dates.

Randy Crews, W7TJ, of Spokane, Washington, said that 20 meters was still good for the Pacific and for stateside stations until 0600 UTC.

Vic Alfonsi, WA6MCL, of Corona, California, noted similar propagation until around 0400 UTC. Probably the reason for 20 appearing to close earlier in Southern California than in Washington State was because during spring and summer, the higher latitudes have more daylight.

On May 4, my sunset in Seattle at 47.7 degrees north latitude was around 0318 UTC (approximately the same as Spokane's), but Southern California at 34 degrees north was 45 minutes earlier at 0233 UTC. Seattle's longitude is about 4 degrees to the west of Los Angeles, which is another factor for a later sunset.

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 May 3 through 9 were 33, 19, 18, 14, 12, 12 and 18, with a mean of 18. 10.7 cm flux was 83.1, 82.5, 81, 78.1, 75.7, 73.4, and 72.1, with a mean of 78. Estimated planetary A indices were 4, 2, 3, 2, 18, 14 and 5, with a mean of 6.9. Estimated mid-latitude A indices were 2, 1, 0, 0, 11, 11 and 3, with a mean of 4.

Source: The American Radio Relay League

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