ARRL

ARRL Propagation Bulletin ARLP016 (2001)

SB PROP @ ARL $ARLP016
ARLP016 Propagation de K7VVV

ZCZC AP16
QST de W1AW  
Propagation Forecast Bulletin 16  ARLP016
From Tad Cook, K7VVV
Seattle, WA  April 12, 2001
To all radio amateurs 

SB PROP ARL ARLP016
ARLP016 Propagation de K7VVV

High solar activity continued this week.  Although the actual
sunspot numbers and solar flux values have generally declined, new
active areas on the sun have rotated into earth's view, bringing
flares and coronal holes, and with them geomagnetic storms and
dramatic auroral displays.

Average sunspot numbers for the week declined over 100 points, and
average solar flux was off over 62 points.  Again this week, on
April 5 and 6, the 10.7 cm solar flux value had to be adjusted
because the observatory in Penticton, British Columbia was
overwhelmed by energy from solar flares.  Daily values for last
Thursday and Friday were flare enhanced at 398.7 and 563.5, but were
adjusted downward and reported by NOAA as 210 and 192.

As this bulletin is being written on Thursday, two clouds of charged
particles are headed our way.  The last is from another X-class
solar flare.  This one erupted near sunspot group 9415 at 1025 UTC,
and caused a radio blackout across the Atlantic Ocean, Europe, and
parts of Western Asia.

On Wednesday energy from a pair of coronal mass ejections hit earth
and triggered a severe G4-category geomagnetic storm between around
1500-1800 UTC.  G4 category is severe, one step below the top G5
category, which is extreme.  This may cause problems with power
distribution systems, with surges in voltage and tripping of circuit
breakers.  On Wednesday the planetary K index hit 8, and the
planetary A index was 60.  Middle latitude A index was 69.

On the bands expect long periods of very little HF propagation, but
look for auroral propagation on VHF.  Current prediction is for
solar flux to decline below 150 this weekend, bottoming out around
140 on Sunday.  Then it is expect to rise to 180 around April 20,
and peak around 185 after April 23 and toward the end of the month.
Of course, with the recent upsurge in activity, we could see higher
values when last month's active regions rotate into view again.
Because this bulletin is being written earlier in the day, we do not
yet know the planetary A index for Thursday, but current estimates
are 20 for Friday, 50 for Saturday and 40 for Sunday.

Last week we mentioned with great excitement the free release of
W6Elprop, available at http://www.qsl.net/w6elprop/ .  W6EL has
incorporated a couple of minor fixes since last week, so users
should download the new version.

W6ELprop is a neat program for Windows computers to run analysis of
propagation over different paths, frequencies, seasons and periods
of the solar cycle.  One interesting exercise is to look at seasonal
effects on 10 meter propagation.

If we look at 28 MHz from Seattle to Japan on the first day of
spring, March 20, using a solar flux of 153 and K index of 3, we see
fairly good conditions with signals 21 db over noise (assuming 100
watts and a dipole) from 2300-0100 UTC.  The rating assigned to the
path is B, meaning it should be open 50-75 percent of the time.

Use the same parameters for one week later, on March 27 and the path
has degraded to C and D, indicating a much poorer chance for
communication.  Run the same parameters a month after the equinox,
and it shows no 28 MHz opening.

But now let us examine effects of latitude.  Hams in Seattle are
often jealous of operators at lower latitudes, who seem to get more
openings on higher frequencies.  Run the same numbers from San
Diego, a 16 percent longer path at over 9200 km, and the 28 MHz
opening has an A rating from 2130-0200 UTC, and doesn't drop below a
B rating until 0400 UTC.  Run the San Diego path again a week later
and it is almost as good.  Run the path again a month after the
equinox, and there is still a chance for propagation.  No wonder
Seattle hams refer to themselves as ''Sufferin' Sevens''.

These numbers are not absolutes, but are certainly interesting for
comparison.  To run the same exercise, you will need to update the
atlas in W6Elprop for San Diego.  I used a latitude of 32.71 and
longitude of 117.15.

Sunspot numbers for April 5 through 11 were 214, 136, 153, 188, 185,
170 and 178 with a mean of 174.9. 10.7 cm flux was 210, 192, 179.5,
169.2, 164.8, 169.7 and 159.6, with a mean of 177.8, and estimated
planetary A indices were 19, 12, 16, 41, 19, 9 and 60 with a mean of
25.1.
NNNN
/EX