Propagation de K7RA

Average daily sunspot numbers were down again this week, but only by less than one point, to 12.7. Average daily solar flux was down five points from the previous week to 72. Geomagnetic indices were down, also by a small amount.

The Air Force projects that today, September 29, geomagnetic conditions will be quiet, with a planetary A index of 8, rising to 15, then 20 on Sunday, and dropping back to 8 on Monday, October 2. Currently the interplanetary magnetic field is pointed south, which signals that Earth is vulnerable to solar flares or solar wind from coronal holes. Sunspots 913 and 914 are rotating into view, and sunspot number and solar flux should rise slightly by Monday.

Geophysical Institute Prague projects unsettled conditions for September 29, quiet conditions on September 30, unsettled to active conditions on October 1, unsettled October 2, quiet to unsettled for October 3, then quiet conditions on October 4-5.

Bill Huntimer, KI0CW in Dell Rapids, South Dakota wrote to ask, "Would you explain what sunspots do to radio interference?"

Sunspots can be great for HF radio propagation. The more sunspots we see, the greater the density of charged particles in the ionosphere, which reflect or refract radio waves. Higher densities mean higher frequencies can propagate worldwide, instead of just passing through the ionosphere and off into space. When there are sunspots, there is also the possibility of other solar activity, such as solar flares, and holes in the Sun's corona, which can spew out a solar wind of charged particles. These particles, as well as x-rays from flares, may disturb the Earth's magnetic field, and cause charged particles in the ionosphere to re-combine, canceling the charge. Lower density of charged particles means that the Maximum Usable Frequency is lower. In addition, polar cap absorption occurs, so propagation over the poles becomes poor.

Sometimes in the media, we may hear of interference to broadcast TV or FM radio signals related to sunspots. The interference can be from distant stations, when listeners are accustomed to hearing only local stations. The interference is actually due to improved propagation.

For more information concerning radio propagation, see the ARRL Technical Information Service at,
http://www.arrl.org/tis/info/propagation.html.

For a detailed explanation of the numbers used in this bulletin, see http://www.arrl.org/tis/info/k9la-prop.html.

An archive of past propagation bulletins is at, http://www.arrl.org/w1aw/prop/ .

If you would like to make a comment or have a tip for our readers, email the author at, k7ra@arrl.net.

Sunspot numbers for September 21 through 27 were 11, 17, 13, 13, 11, 13 and 11 with a mean of 12.7. 10.7 cm flux was 71.4, 72.3, 70.4, 69.8, 70.2, 70.7, and 72, with a mean of 71. Estimated planetary A indices were 4, 3, 9, 23, 6, 6 and 4 with a mean of 7.9. Estimated mid-latitude A indices were 2, 1, 7, 15, 6, 5 and 2, with a mean of 5.4.

Source: The American Radio Relay League