QST de W1AW
Propagation Forecast Bulletin 35 ARLP035
From Tad Cook, K7VVV
Seattle, WA August 27, 1999
To all radio amateurs
SB PROP ARL ARLP035
ARLP035 Propagation de K7VVV
All solar indices were up this week, but so were geomagnetic numbers. This meant that although there was more ionizing radiation from the sun to produce a reflective ionosphere for HF signals, proton activity from flares and coronal holes kept conditions unstable.
When protons strike charged particles in the ionosphere, their positive charge neutralizes the negative ionization. In addition, the lower D layer, which tends to absorb HF radio signals expands, and there is increased absorption in the polar regions as well.
Sunspot number averages for this week were up over 20 points compared to last week, and solar flux averages were up by over 43 points. The only really quiet day in terms of geomagnetic stability was August 21, when the planetary A index was below 10 and the K index over most periods was only 2. The worst days were August 20 and 23, when the A index was 33, and the K index was 6. As one moves higher in latitude, these effects get worse, and the College A index from Alaska on those days was 65 and 67, with K indices as high as 7. This is a severe geomagnetic storm toward the polar regions.
Unsettled conditions are forecast for the next few weeks. There are frequent periods when the A index should be above 10, and around September 12-16 an A index of 20 is predicted. We have been in a period of rising solar flux, and the values for this Friday through Sunday are forecast at 220, 215 and 215. Flux values should stay above 200 through the end of August, then drop to below 170 around September 3. Solar flux should bottom out around 125 on September 9 or 10, then rise up above 170 by September 18. Of course conditions could get better if new sunspots rotate into view.
On August 21 at 0737 UTC KH6HME worked W1LP/MM on 2 meter SSB, a distance of almost 3,000 miles. W1LP was in grid DL51CE.
K8KZ wrote from Michigan to ask why propagation on 10 and 15 meters disappears for the most part during summer months, and he wonders if the late night 20 meter propagation will disappear this winter.
In the summer, heating takes place in the F layers of the ionosphere, causing it to expand during daylight hours. Since the MUF (Maximum Usable Frequency) is based on the density of the ionosphere, expansion by heating makes it less dense, so the MUF is lower. 10 meters tends to get left out, but 20 does not.
But on 20 meters in the summer, propagation into the night (particularly toward the west) is enhanced because the days are longer. During daylight the ionosphere is receiving more ionizing radiation from the sun, so 20 is great from North America toward the Pacific on summer nights.
In the winter, the days are shorter, so we don't get that long propagation after sunset. Also, because with less daylight the F layers are not heated as much, MUFs are higher, as the layers are more dense.
The bands are great for worldwide propagation in the Fall and Spring because the conditions are more even between the southern and northern hemispheres, providing more opportunities for worldwide propagation of HF signals. The equinox is coming up in less than a month on September 22. Look for great fall conditions, especially if some new sunspots appear.
K8KZ is in Farmington Hills, Michigan, and he wonders if the solar flux could go high enough this winter to allow evening propagation on 20 meters. Running some samples on a propagation program, we can look at the path from K8KZ to KH6 on August 21, when the solar flux was 172.7. 20 meter signals should have been very strong from 0100-0730 UTC, possibly through the night, and quite good again in the morning from 1130-1600 UTC.
If we do another projection, this time in the fall on November 21 and assume a optimistic solar flux of 250, the signals are good throughout the day, but fade quickly after 0200 UTC. Projecting forward to the last day of the year, and assuming a very optimistic flux of 300, evening signals fade around the same time. If the flux is 200, they fade a little faster. So the answer appears to be that even with optimum conditions, the summertime seasonal effects don't extend into winter.
Sunspot numbers for August 19 through 25 were 68, 65, 79, 78, 110, 103 and 127 with a mean of 90. 10.7 cm flux was 134.7, 151.6, 161.2, 172.7, 187.5, 202 and 208.4, with a mean of 174, and estimated planetary A indices were 22, 33, 6, 11, 33, 29 and 8, with a mean of 20.3.
The path projections for this week are from K8KZ in Farmington Hills, MI. This should still be good from anywhere nearby, such as Detroit, Ann Arbor, Southfield, Pontiac, Lansing, Toledo, Ohio or Southwest Ontario.