QST de W1AW
Propagation Forecast Bulletin 30 ARLP030
From Tad Cook, K7VVV
Seattle, WA July 23, 1999
To all radio amateurs
SB PROP ARL ARLP030
ARLP030 Propagation de K7VVV
Thanks to KH6BZF for writing a great bulletin last week while I was out of town.
Solar flux and sunspot numbers were down again this week, with the average sunspot numbers down nearly 60 points and the average solar flux down over 10 points. Conditions are improving though, with the predicted solar flux rising over the next few days to 160, 165 and 170 for Friday through Sunday. Planetary A indices are forecast at 10, 9 and 9 over the same days. Beyond the weekend the solar flux may rise as high as 180 over the next week, then fall to 165 by August 6, 155 around August 9, and 140 around August 12. The current high K and A index values should settle down this weekend.
W7MIC sent email recently asking about some of the numbers used in this bulletin. Here is a very basic explanation.
Amateur Radio operators who use HF generally like increased sunspots because it correlates with better worldwide radio propagation. When there are more sunspots, the sun puts out radiation which charges particles in the ionosphere. Radio waves bounce off of these charged particles, and the more dense these clouds of ions the better the HF propagation. When the ionosphere is more dense, higher frequencies will reflect off of the ionosphere rather than passing through to space. This is why every 11 years or so when this activity is higher 10 meters gets exciting. It is at a high enough frequency, right near the top of the HF spectrum, that radio waves propagate very efficiently when the sunspot count is high. Because of the wavelength, smaller antennas are very efficient on this band, so mobile stations running low power on 10 meters can communicate world wide on a daily basis when the sunspot cycle is at its peak.
The sunspot numbers used in this bulletin are calculated by counting the sunspots on the visible solar surface and also measuring their area. The solar flux is measured at an observatory in British Columbia using an antenna pointed toward the sun tuned to 2.8 GHz, which is a wavelength of 10.7 cm. Energy detected seems to correlate with sunspots and with the density of the ionosphere.
Other solar activity of concern to HF operators are solar flares and coronal holes, which emit protons. Since the charged ions in the ionosphere are negative, a blast of protons from the sun can neutralize the charge and make the ionosphere less reflective. These waves of protons can be so intense that they may trigger an event called a geomagnetic storm.
The Planetary A index relates to geomagnetic stability. Magnetometers around the world are used to generate a number called the Planetary K index. You can hear the Boulder K index updated every three hours on WWV, or by calling 303-497-3235.
A one point change in the K index is quite significant. A K index below three generally means good stable conditions, and above three can mean high absorption and poor reflection of radio waves. Each point higher than three is a big change in conditions.
Every 24 hours the K index is summarized in something called the A index. A one point change in A value is not very significant. A full day with the K index at 3 will produce an A index of 15, K of 4 means A of 27, K of 5 means A of 48, and K of 6 means A of 80. You can find an explanation of these numbers on the web at http://www.ngdc.noaa.gov/stp/GEOMAG/kp_ap.html.
The number reported here is the Planetary A index, which is sort of a worldwide average based on the K readings from a number of magnetometers. The numbers reported on WWV are the Boulder K and A index, measured in Colorado. Generally the higher the latitude of the measuring station, the higher the K and A indices reported. This is because the effects of geomagnetic instability tend to concentrate toward the polar regions of the globe.
You can get a lot more information by reading the chapter on propagation in any recent edition of the ARRL Handbook. Another good source is a book titled The New Shortwave Propagation Handbook, by George Jacobs, Ted Cohen and Robert Rose, published by CQ Magazine.
Note that this week we have two sets of solar statistics, covering the past two weeks. Users of the WA4TTK solar graphing software will want to create two files from this bulletin, each with just one of the following paragraphs. This will allow the software to seamlessly grab the data and enter it into the database in a two step process.
You can write to the author of this bulletin via K7VVV@arrl.net.
Sunspot numbers for July 8 through 14 were 143, 170, 174, 202, 199, 188 and 120 with a mean of 170.9. 10.7 cm flux was 149.1, 150.8, 156.3, 152.6, 154.1, 143.5 and 129.6, with a mean of 148, and estimated planetary A indices were 8, 6, 5, 6, 14, 6 and 6, with a mean of 7.3.
Sunspot numbers for July 15 through 21 were 109, 97, 97, 119, 126, 115 and 124 with a mean of 112.4. 10.7 cm flux was 129.5, 132, 136.7, 138.2, 141.3, 139.5 and 147.3, with a mean of 137.8, and estimated planetary A indices were 10, 5, 5, 4, 4, 6 and 13, with a mean of 6.7.