ARLP039 Propagation de K7VVV:
September 29, 2000

Propagation Forecast Bulletin 39 ARLP039
From Tad Cook, K7VVV
Seattle, WA September 29, 2000
To all radio amateurs

ARLP039 Propagation de K7VVV

Solar flux and sunspot numbers were up over the past week, while average geomagnetic indices were lower, which is always a happy condition for HF radio enthusiasts. Solar flux peaked at 232.2 on Friday and sunspot numbers peaked at 255 on Sunday. Average sunspot numbers for the week were up nearly 87 points, and average solar flux rose by almost 39 points, when compared to the previous week.

The sunspot number is calculated by counting the visible sunspots and factoring in their size, so a significant factor was sunspot 9169, reported in last week's Propagation Forecast Bulletin ARLP038 as one of the largest seen in many years. It is now fading as it rotates off of the visible solar disk. We were lucky not to have a great deal of flare activity from this magnetically complex spot.

K4WY sent a web reference concerning this particular sunspot. Check

The most active geomagnetic day over the past week was Tuesday, when the planetary A index was 21. Planetary K index was 4 during most of the day, but Alaska's College K index, which is usually higher because of the polar region proximity, was as high as 6. College A index was 37 for the day.

Friday and Saturday were the quietest geomagnetic days, with A indices in the single-digits, Planetary K indices at 2 and 3, and mid-latitude K indices at 1 and 2. Fortunately for HF enthusiasts, this was also the period when the sunspot count and solar flux were the highest, which often is not the case.

Geomagnetic indices should remain stable over the next few days, with planetary A indices predicted at around 10. On Monday through Wednesday the A index is forecast at 12, 15 and 12, probably based on the previous solar rotation. This indicates an unsettled to active geomagnetic conditions, with higher absorption of HF radio signals, particularly in the higher latitude or polar paths.

Solar flux is expected to decline over the next few days, with Saturday at 190 and Sunday around 180. For the short term, flux values should reach a minimum near 155 around October 7-9, then head above 200 again around mid-month.

We have now passed the autumnal equinox, and are experiencing Fall HF conditions. 10 and 12 meter operators should expect great propagation, at least when the K index as reported by WWV is 3 or less. Openings follow the sunlight, with propagation to the east in the morning and toward the west later in the day. 15 meters should offer plenty of worldwide openings as well, but also later into the evening after 10 meters has closed. Worldwide 20 meter openings should be available around the clock. As the northern hemisphere moves further from the summer season, 160 and 80 meters should improve with shorter days and less of the static commonly associated with summer.

Judging by recent email, it is time to repeat the occasional explanation of the various numbers and indices that are cited in this weekly bulletin, which appears below. Questions and comments are always welcome at

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 from 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. 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 sunspots on the visible solar surface and also measuring their area. Solar flux is measured at an observatory in British Columbia using an antenna pointed toward the sun tuned to 2.8 GHz, which is at 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 refractive. 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 reading below 3 generally means good stable conditions, and above 3 can mean high absorption and poor reflection 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 at

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.

Currently we are near the peak of the solar cycle, so conditions are generally better because of the increased ionization of the ionosphere. But along with the increased sunspots come more solar flares and coronal holes, producing disturbed conditions.

Sunspot numbers for September 21 through 27 were 198, 248, 216, 255, 215, 223 and 233 with a mean of 226.9. 10.7 cm flux was 225.1, 232.2, 225.2, 224.5, 225.6, 223.6 and 204.7, with a mean of 223, and estimated planetary A indices were 9, 7, 7, 10, 16, 21 and 11 with a mean of 11.6.