QST de W1AW
Propagation Forecast Bulletin 28 ARLP028
From Tad Cook, K7VVV
Seattle, WA July 6, 2001
To all radio amateurs
SB PROP ARL ARLP028
ARLP028 Propagation de K7VVV
It has been a quiet week. There were no active or disturbed days, and solar activity declined. Average daily solar flux, compared the past week to the previous, declined over 51 points and average sunspot numbers were down more than 86 points.
Activity will probably reach a short-term minimum on Friday and Saturday, when solar flux is around 115, then it is expected to rise back to 130 by July 10, and near 200 around July 13-20. No increased geomagnetic activity is forecast, but the emergence of new sunspots could change that.
With summer here, MUF (Maximum Usable Frequency) is depressed. About the only openings on 10 meters are E-layer skip, which is somewhat sporadic and doesn't offer the longer distances of F layer communications. Twenty meters is open well into the night.
In last week's review of Field Day, it was claimed that 40 meters was the best all-around band. This drew a response from Chip, K7JA, who reported that around 0300-0400 UTC on Sunday morning (Saturday night on the West Coast, where he was) 40 meters went bad. He also reported that 15 and 20 meters were never good early in the day on Saturday. The 40- meter report in this bulletin was a resulted of information provided by a local Seattle area club that had an excellent 40-meter antenna, plus the numbers generated by W6ELprop looked good.
Judging from recent e-mail, it is probably time to repeat the occasional explanation of the indices reported in this bulletin.
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 (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. Ten 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. You can find an explanation of these numbers on the Web.
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.
Sunspot numbers for June 28 through July 4 were 143, 98, 115, 108, 130, 132 and 106, with a mean of 118.9. The 10.7- cm flux was 140.2, 139.9, 136.6, 135.4, 134.3, 131.9 and 127, with a mean of 135. Estimated planetary A indices were 5, 7, 10, 12, 8, 9 and 8 with a mean of 8.4.