ARRL

Tad Cook K7VVV Explains Propagation Numbers

W1AW Propagation Forecast Bulletin 29--July 21, 2000

This has been quite a week for solar activity, with blasts of solar wind dominating space weather news. Saturday was the big day for HF radio blackouts and aurora, with the planetary A index jumping to an incredible 152 and the mid-latitude A index at 148. The planetary K index, updated every three hours, was at 9 for three readings on Saturday. A K index reading of 9 over a 24-hour period would be equivalent to an A index of 300. This is big, really big. These numbers are associated with an extreme geomagnetic storm that was nearly off the scale. On Friday one of the most powerful solar flares of the current cycle triggered a storm of protons directed toward earth. The Solar and Heliospheric Observatory recorded a full halo coronal mass ejection heading toward earth at greater than one-million meters per second. Check out animations of this event at http://sohowww.nascom.nasa.gov/data/CME/. There are only a few events of this magnitude in the average solar cycle. The only factor that probably impeded a spectacular aurora visible far down into the U.S. was that this is the summer season. A dark evening sky would reveal a rich tapestry of northern lights. Of course, it being winter in the southern hemisphere, bright displays were reported in Australia and New Zealand. The author made a weak attempt Saturday night, walking barefoot into the middle of the street and trying to peer north past city lights, but no auroral glow was visible. Seattle is north of the 47th parallel, and although there were no local reports of aurora, there were observations as far south as 40 degrees in Europe, Asia, and parts of Eastern North America. If you point your web browser toward http://www.sec.noaa.gov/info/kp-aurora.html you will see a nice map from the June, 1968 issue of Sky and Telescope which shows how far south aurora may be visible depending on the planetary K index. What is not clear from this map is whether it shows how far south the edge of the aurora extends, or how far south it is visible when looking north. Another coronal mass ejection emerged on Wednesday, July 19, but the predicted effect is uncertain because the ejection may not be aimed squarely at earth. On Thursday the planetary K index went up to 6 for several hours, but by the end of the UTC day it was 3. The planetary A index for Thursday was 43, and the College A index, recorded in Alaska, was 57. The latest forecast shows the planetary A index rising to 50 on Friday, then dropping to 20 and 15 and then 10 on Saturday through Monday. Solar flux peaked for the recent short term at 252.9 on Thursday, and is expected to drop to 245, 235, 230 and 225 on Friday through Monday. The next short term minimum is predicted around July 28 at 170, followed by another peak above 200 around August 6-9. The author has received many more inquiries recently asking for explanations of the various parameters reported in this bulletin. Although the explanations were repeated six weeks ago, it is probably time to run them again, and they follow this paragraph. Feel free to send questions to the author via k7vvvarrl.net. 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 these charged particles, and the denser these clouds of ions, the better the HF propagation. When the ionosphere is denser, 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. 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 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 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 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 July 13 through 19 were 240, 243, 229, 268, 335, 343 and 342 with a mean of 285.7. 10.7 cm flux was 231.9, 203.9, 213.1, 218.9, 228.3, 261.9 and 249.9, with a mean of 229.7, and estimated planetary A indices were 33, 35, 152, 46, 9, 13 and 15, with a mean of 43.3. NNNN /EX