QST de W1AW
Propagation Forecast Bulletin 23 ARLP023
From Tad Cook, K7VVV
Seattle, WA June 9, 2000
To all radio amateurs
SB PROP ARL ARLP023
ARLP023 Propagation de K7VVV
A large coronal mass ejection on June 6 is raising havoc with HF propagation. Geomagnetic conditions were rough on Wednesday, with K indices of 3 and 4, but the big effect was measured on Thursday, when the planetary K index was 7 at 0900 and 1200z, followed by 6 at 1500z. The planetary A index for Thursday was 53, while the College A index (in Alaska) was 79. This indicates a severe geomagnetic storm, which should disrupt HF communications but may provide interesting auroral communication opportunities for VHF enthusiasts. Regarding visible aurora effects, the chances over North America are declining on Friday morning, although earlier in the day there was an intense aurora visible over Asia.
To add to the excitement, there was another coronal mass ejection on Wednesday, June 7. The latest word has solar wind providing another disruption on Saturday, June 10.
Planetary A index should rise on Friday to 75, then drop to 40 on Saturday, 25 on Sunday, 18 on Monday and 15 on Tuesday. Solar flux is expected to rise over the same period, to 185 on Friday, 190 on Saturday, 200 on Monday and 210 on Tuesday. Solar flux is expected to peak over the short term around 245 on June 16.
Last week's bulletin mentioned monitoring WWV for the latest solar and geophysical numbers, and both WB6RIB and W9LYN wrote to suggest the URL of ftp://ftp.sel.noaa.gov/pub/latest/wwv.txt for the latest text of the WWV bulletin that appears at 18 minutes after every hour.
WB7QXU wrote to inquire about the significance of the various numbers presented weekly in this bulletin. A basic explanation is presented periodically, but has not appeared since last October. We repeat it here now.
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 more dense, 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.
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 comes more solar flares and coronal holes, producing disturbed conditions.
Sunspot numbers for June 1 through 7 were 128, 126, 125, 139, 132, 133 and 145 with a mean of 132.6. 10.7 cm flux was 148, 187.2, 165.9, 169.7, 171, 186.4 and 180.3, with a mean of 172.6, and estimated planetary A indices were 12, 10, 12, 12, 26, 16 and 14, with a mean of 14.6.