QST de W1AW
Propagation Forecast Bulletin 42 ARLP042
From Tad Cook, K7VVV
Seattle, WA October 15, 1999
To all radio amateurs
SB PROP ARL ARLP042
ARLP042 Propagation de K7VVV
Solar activity is up by quite a bit this week. Average solar flux values increased by nearly 30 points over last week, and average sunspot numbers were up by almost 80 points. The reporting week, which runs Thursday through Wednesday, began with a stable geomagnetic field and A indices in the single digits, but quickly changed after the weekend to storm conditions. The worst conditions were probably on Tuesday, October 12, when the planetary A index reached 34, and the highest planetary K index was 6. Of course it was worse in the higher latitudes, with the College A index from Alaska at 61 and K indices has high as 7.
The author has been operating on 10 meter SSB recently, and when the A indices were high, sometimes the only signals audible were from the southern hemisphere. I wrote to Bob Brown, NM7M, to ask about this. Dr. Brown taught physics at UC Berkeley, and has spent his retirement years using amateur radio and studying HF propagation. Bob wrote that the geomagnetic disturbances of the past week weren't really big storms, and that part of the reason for observed trans-equatorial propagation is because of the absence of other propagation modes. ''In the absence of high latitude propagation due to lowered MUFs or auroral absorption, the old standby, trans-equatorial propagation, looks better than ever,'' Bob wrote. He also said that there are effects which may increase HF propagation at low latitudes, but only during really big geomagnetic storms.
The projected solar flux values for the next three days, Friday through Sunday, are 200 for each day, and the planetary A index is forecast at 25, 20 and 15. Solar flux is expected to drift lower after the weekend, reaching 150 around October 20 and a minimum of 125 from October 23-39. Predicted disturbed days, when the A index is 25 or more, are October 24 and 27 and November 6-8.
Recently I have received more requests for an explanation of some of the numbers in this bulletin. Here it is.
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. 10 meters 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 3 generally means good stable conditions, and above 3 can mean high absorption and poor reflection of radio waves. Each point higher than 3 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 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.
Sunspot numbers for October 7 through 13 were 184, 170, 235, 195, 163, 191 and 210 with a mean of 192.6. 10.7 cm flux was 129.4, 151.2, 153.2, 160.5, 166.6, 183.6 and 191, with a mean of 162.2, and estimated planetary A indices were 6, 8, 6, 28, 23, 34 and 26, with a mean of 18.7.