QST de W1AW
Propagation Forecast Bulletin 33 ARLP033
From Tad Cook, K7VVV
Seattle, WA August 10, 2001
To all radio amateurs
SB PROP ARL ARLP033
ARLP033 Propagation de K7VVV
Sunspot numbers and solar flux rose quite a bit this week. Sunspot numbers peaked on Sunday at 214, the highest since June 25. Average sunspot numbers rose over 83 points and average solar flux was up over 32 since the previous week. Sunday and Monday of this week were active geomagnetically. This increases absorption of HF radio signals, particularly in the higher latitudes. The planetary A index on both days was high at 23 and 21, and the College A index reported from Fairbanks, Alaska was 37 and 29. 37 indicates a severe storm, and there were three reporting periods on August 5 with K indices at 6, which is very high. For more information on the relationship between K and A indices, check http://www.ngdc.noaa.gov/stp/GEOMAG/kp_ap.html.
Conditions have been quiet for the past few days, but this Sunday, the second day of the Worked All Europe DX CW Contest, geomagnetic activity should rise again. The projected planetary A index for Friday through Monday is 10, 10, 25 and 20. Predicted solar flux for those same days is 155, 150, 150 and 145. Conditions should be good for the first day of the contest at least. The reason for the predicted rise in geomagnetic instability on Sunday is a solar wind blast from a coronal hole which should affect earth that day. But on Thursday a coronal mass ejection appeared, which could bump the A index even higher over the weekend.
Earlier this week was the halfway point from the summer solstice to the fall equinox. Fall is a great time for HF radio propagation, especially on the higher HF bands, and we should gradually see a drift from summer to fall conditions.
Both N1AW and KF2YP wrote to inquire about the source of the holographic images of the sun's backside, referred to in last week's Propagation Forecast Bulletin ARLP032. These are shown at http://spaceweather.com/ and an explanation of this technique for peering through the sun is at http://spaceweather.com/glossary/farside.html.
Since there is a lot of interest among contesters this weekend in propagation to Europe, we will include toward the end of this bulletin a few path projections for the Worked All Europe DX CW Contest. But first, there have been numerous requests to run the explanation again for some of the numbers used in this bulletin. It was last run a little over a month ago, but here it is again.
Amateur Radio operators who use HF generally like increased sunspots because they correlate with better worldwide radio propagation. When there are more sunspots, the sun puts out radiation which charges particles in the earth's ionosphere. Radio waves bounce off of (refract from) these charged particles, and the denser these clouds of ions, the better the HF propagation.
When the ionosphere is denser, higher frequencies will refract off the ionosphere rather than passing through to outer 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 shorter 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. There are also seasonal variations, and 10 meters tends to be best near the spring or fall equinox.
The sunspot numbers used in this bulletin are calculated by counting the spots on the visible solar surface and also measuring their area.
Solar flux is another value reported in this bulletin, and it is measured at an observatory in Penticton, British Columbia using an antenna pointed toward the sun hooked to a receiver tuned to 2.8 GHz, which is at a wavelength of 10.7 cm. Energy detected seems to correlate somewhat 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 refractive. These waves of protons can be so intense that they may trigger an event called a geomagnetic storm. In addition, energy from a solar flare may energize the D-layer of the ionosphere, which absorbs radio waves.
The Planetary A index relates to geomagnetic stability. Magnetometers around the world are used to generate a number called the Planetary K index.
A one-point change in the K index is quite significant. K index readings below 3 generally mean good stable conditions, and above 3 can mean high absorption of radio waves. Each point change reflects a big change in conditions.
Every 24 hours the K index is summarized in a number 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.
The geomagnetic number reported here is the Planetary A index, which is a worldwide average based on the K index 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 hear the Boulder K index updated every three hours on WWV, or by calling 303-497-3235.
Here are the path projections to Europe for this weekend. K9GY asked for them, and first are his projections for the best times to work Europe from the East Coast. 80 meters, 0200-0300z and 0400-0500z, 40 meters, 2300-0900z, 20 meters always open, 15 meters 0900-2300z, and 10 meters 1000-2000z.
Here are my projections.
Sunspot numbers for August 2 through 8 were 113, 140, 182, 214, 182, 177 and 191 with a mean of 171.3. 10.7 cm flux was 120.8, 131.6, 148.4, 156, 163.7, 166.3 and 166.9, with a mean of 150.5, and estimated planetary A indices were 8, 14, 9, 23, 21, 13 and 9 with a mean of 13.9.