ZCZC AP12
QST de W1AW
Propagation Forecast Bulletin 12 ARLP012
From Tad Cook, K7RA
Seattle, WA March 21, 2003
To all radio amateurs
SB PROP ARL ARLP012
ARLP012 Propagation de K7RA
This has been quite a week for geomagnetic storms. The average daily planetary A index--a measure of geomagnetic activity averaged over 24 hours from the planetary K indices in the same period--was about 20 points higher this week than last. This was caused by a near-constant strong solar wind accompanied by solar flares.
The worst, or most active, day was Monday, when the planetary A index was 39 and the higher latitude college A index (measured in Alaska) was 78. The college K index was as high as 8, indicating a severe geomagnetic storm near the polar region.
For an explanation of the relationship between K and A indices, check the NOAA Geomagnetic kp and ap Indices Web page. Looking at the chart you can see that if the K index of 8 persisted for 24 hours, this would produce an A index of 207--almost unheard of. Fortunately, mid-latitude numbers weren't as bad.
Solar flux and sunspot numbers declined this week over last. Average daily sunspot numbers dropped nearly 82 points to 92, and solar flux was down by more than 14 points.
This trend is expected to continue, with solar flux below 100 through the middle of next week. Unsettled to active geomagnetic conditions also should continue, at least over the next few days. Solar flux and sunspot numbers were better over the last few years around this date. During the comparable weeks in the past years, average daily sunspot numbers were 134.3 in 2002, 101.7 in 2001 and 183.3 in 2000. Average daily sunspot numbers were 180.9 in 2002, 144.2 in 2001 and 207.8 in 2000.
Today is the vernal equinox--a fancy way of saying it's the first day of spring. Spring is a great time for HF propagation, with all points on Earth today getting about 12 hours of sunlight. There actually is some variation in the actual length of the daylight period, depending on the observer's latitude. This is explained on the US Naval Observatory Web site.
For more information about propagation and an explanation of the numbers used in this bulletin see the ARRL Web site Propagation page.
Sunspot numbers for March 13 through 19 were 88, 114, 124, 121, 80, 64 and 53, with a mean of 92. The 10.7-cm flux was 134.2, 138.9, 130.7, 128.6, 164.1, 118.4 and 108.2, with a mean of 131.9. Estimated planetary A indices were 15, 25, 24, 23, 39, 26 and 14, with a mean of 23.7.