ARRL

ARRL Propagation Bulletin ARLP037 (2003)

SB PROP @ ARL $ARLP037
ARLP037 Propagation de K7RA

ZCZC AP37
QST de W1AW  
Propagation Forecast Bulletin 37  ARLP037
From Tad Cook, K7RA
Seattle, WA  September 12, 2003
To all radio amateurs 

SB PROP ARL ARLP037
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
SpaceRef.com. You can read it at
http://www.spaceref.com/news/viewpr.html?pid=12504.

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.
NNNN
/EX