QST de W1AW
Propagation Forecast Bulletin 37 ARLP037
From Tad Cook, K7VVV
Seattle, WA September 6, 2002
To all radio amateurs
SB PROP ARL ARLP037
ARLP037 Propagation de K7VVV
Average daily sunspot numbers for the week were higher than last week's, and the average solar flux was slightly lower. Geomagnetic indices were unsettled for most of the week. Then on Wednesday there was a surprising jump in geomagnetic activity. This began Tuesday evening in North America after the interplanetary magnetic field near Earth unexpectedly turned south.
Earth's magnetic field forms a bubble around the planet that helps protect against effects from the solar wind. Where the interplanetary magnetic field meets Earth's magnetic field is called the magnetopause. Earth's magnetic field points north at this point. If the interplanetary magnetic field points south at the magnetopause, it partially cancels Earth's magnetic field at this point of contact and lets in the solar wind. This is what happened on Wednesday, and the results were impressive auroral displays and a planetary K index of six.
Over the next few days solar flux is expected to rise, with values over 200 by Sunday, then peaking around 230 by the end of next week. Don't forget that the autumnal equinox is soon. This year it will be on September 23 at 0448 UTC. High-frequency propagation is improving as we move from summer to fall.
There is an interesting and informative space physics text on the net, which has lots of information on the sun, the magnetosphere, solar wind, and many other topics. You can find it at the "Space Physics Textbook" Web site. N4RYX wrote to ask about some basic propagation information. As we've mentioned in the past, see "The Sun, the Earth, the Ionosphere: What the Numbers Mean, and Propagation Predictions--a brief introduction to propagation and the major factors affecting it," by Carl Luetzelschwab, K9LA and the Propagation page on the ARRL Web site.
In the past we've mentioned the WM7D solar resource page. There, you'll find links to some interesting historical charts of sunspot cycles. Now W3DF writes to remind us of his "90 Day Solar Activity Summary" historical solar activity charts. Note that it links to a cycle 19-23 comparison and many other interesting views of solar data.
Sunspot numbers for August 29 through September 4 were 146, 150, 153, 187, 227, 266 and 215, with a mean of 192. The 10.7-cm flux was 169.3, 170, 180.3, 180.5, 173.8, 171.4 and 171.3, with a mean of 173.8. Estimated planetary A indices were 10, 10, 13, 13, 14, 10 and 42, with a mean of 16.