The K7RA Solar Update

SEATTLE, WA, Sep 29, 2006 -- Average daily sunspot numbers were down this week by less than one point to 12.7. The average daily solar flux was down by five points from the previous week to 72. Geomagnetic indices were down a bit as well.

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, signaling 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, asks, "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. A lower density of charged particles means that the maximum usable frequency (MUF) 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

For more information concerning propagation and an explanation of the numbers used in this bulletin see the ARRL Technical Information Service Propagation page and the article "The Sun, the Earth, the Ionosphere: What the Numbers Mean, and Propagation Predictions," by Carl Luetzelschwab, K9LA. An archive of past bulletins is on the ARRL Web site. The propagation charts that used to appear in QST now are available on the QST Propagation Charts page on the ARRL Web site.

Weekly Sunspot Numbers

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.

Amateur solar observer Tad Cook, K7RA, Seattle, Washington, provides this weekly report on solar conditions and propagation. This report also is available via W1AW every Friday, and an abbreviated version appears in The ARRL Letter. Readers may contact the author via e-mail.

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