QST de W1AW
Propagation Forecast Bulletin 45 ARLP045
From Tad Cook, K7VVV
Seattle, WA November 10, 2000
To all radio amateurs
SB PROP ARL ARLP045
ARLP045 Propagation de K7VVV
Solar flux has been declining over the past week, although because of the rise toward the end of the previous week, the average reported in this bulletin and the last was the same. Last week's average solar flux was 186.8, and this week's was 186.7. There was quite a bit of geomagnetic activity this week, with the most activity centered around Monday, November 6. On that day the planetary A index was 42, and the planetary K index was 6 over several periods. This indicates a severe geomagnetic storm. Alaska's College A index, because of the higher latitude, was 102, and the College K index was 7 and 8 over several periods.
It looks like more fireworks for this weekend. A forecast of stable, quiet conditions would be nice for the Japan International DX Phone Contest and the RTTY and OK/OM tests, but no such luck. A powerful solar flare at 2330z on November 8 triggered a radiation storm that is producing a flux of high-energy protons 100,000 times as powerful as normal levels. This is the fourth largest radiation storm since 1976. To make matters potentially worse, depending on the trajectory, there was a bright coronal mass ejection following the solar flare.
The latest forecast shows the planetary A index for Friday through Monday at 15, 15, 50 and 20, but the severe space weather could hit earth as early as Friday night. Check WWV or the Space Environment Center data over the phone at 303-497-3235. When the K index hits 4 or 5, you'll know that the disturbance has hit.
Solar flux is expected to decline to a minimum around 150 from November 12-16, and should rise above 180 after November 24. Solar flux is expected to peak around 200 on November 28.
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. The explanation appears below. Questions and comments are always welcome via email at firstname.lastname@example.org.
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 more dense, 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 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. K index readings below 3 generally mean 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 http://www.ngdc.noaa.gov/stp/GEOMAG/kp_ap.html.
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.
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, such as what we will probably see this weekend.
Sunspot numbers for November 2 through 8 were 213, 196, 159, 173, 158, 120 and 171 with a mean of 170. 10.7 cm flux was 196.3, 198.8, 194.7, 186.4, 178.1, 179.9 and 172.8, with a mean of 186.7, and estimated planetary A indices were 4, 5, 23, 10, 42, 39 and 15 with a mean of 19.7.