ARLP037 Propagation de K7RA:
September 12, 2003

Propagation Forecast Bulletin 37 ARLP037
From Tad Cook, K7RA
Seattle, WA September 12, 2003
To all radio amateurs

ARLP037 Propagation de K7RA

Daily sunspot numbers took a dive this week, with the average dropping 47 points from last week to 56.1. Solar flux declined a little over 18 points. Sunspot numbers on Tuesday and Wednesday, September 9 and 10, were quite low, 43 and 42. This is quite a contrast from a year ago, when Tuesday and Wednesday of the same week had sunspot numbers of 226 and 213.

Coincidentally, last year's bulletin for the week September 5th through 11th showed a comparison from the year before, when conditions were even better. The 2002 bulletin reports that the solar flux the year before was 42 points higher and sunspot numbers were greater by 34.

A nice thing this week was lower geomagnetic indices, which were best on September 7-8. On September 7 the normally high College A index (measured in Fairbanks, Alaska) was all the way down to 2, which is very quiet. Over the local evening time in Fairbanks, the K index was 0 for 18 hours straight! It was either 0 or 1 for a continuous 36 hours. We have been inside a strong solar wind this week, but the interplanetary magnetic field has been pointing north, which protects the earth's magnetic field and keeps A and K indices low.

We are drawing closer to the fall equinox, only about 10 days off. This is a prime time for HF DX, because the solar radiation reaching earth is equal in northern and southern hemispheres. The day is exactly 12 hours long, regardless of whether you are on the equator or at either pole.

This weekend is the Worked All Europe SSB Contest. Let's see how the vast difference in sunspot numbers might affect propagation this week when compared to a year ago.

Using the September 10, 2002 sunspot number of 226 with the W6ELprop software, plotting a 20-meter path from Chicago to Germany shows it closes about four and a half hours later than it would on the same date with a sunspot number of 42. At 42, 15 meters has a low probability of opening from 1630-2130z, but with the higher numbers, conditions on 15 meters look excellent from 1300-2330z.

On 75 meters, we see the opposite effect. With the lower numbers, peak signal strength is several decibels higher than it would be last year. A good opening this year would be from 2300-0700z, and last year from 2330-0600z.

Over the weekend, expect stable geomagnetic conditions. Solar flux should rise above 100, peaking around 120 from September 17-19.

David Moore sent in an interesting article about solar wind from You can read it at

Sunspot numbers for September 4 through 10 were 79, 57, 60, 54, 58, 43, and 42, with a mean of 56.1. 10.7 cm flux was 112.2, 108, 104.9, 107.8, 98.8, 95.9, and 99.3, with a mean of 103.8. Estimated planetary A indices were 19, 16, 12, 10, 9, 19, and 19, with a mean of 14.9.