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ARRL Propagation Bulletin ARLP017 (2004)

ARLP017 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 17  ARLP017
From Tad Cook, K7RA
Seattle, WA  April 23, 2004
To all radio amateurs 

ARLP017 Propagation de K7RA

Solar flux and sunspot numbers rose this week, and geomagnetic
indices were down. Although still headed down the far side of solar
cycle 23, rising sunspot counts and lower geomagnetic disturbances
are great for HF propagation. Over the next few days, (April 23-25)
solar flux should stay around 120, and is expected to drop to around
115, 115 and 110 for April 26-27.

The planetary A index my rise to around 15 or higher on Saturday and
Sunday (April 24-25) due to a solar wind stream, especially if the
interplanetary magnetic field points south. Currently the field is
pointing north, protecting earth from solar wind. There is a
possibility of effects from a coronal hole reaching earth this
Sunday, April 25.

In the longer immediate term projection, returning activity as the
sun rotates relative to earth could cause a larger rise in
geomagnetic activity around May 3. A holographic helioseismic image
of the sun's far side from earlier this week reveals a sunspot group
currently out of earth's view. Of course, this should rotate back
into view soon assuming these spots don't fade away entirely. An
explanation of helioseismic holography and imaging of the sun's far
side is at, A daily
picture of earth's side of the sun is at,

This week's bulletin has a new index added to the numbers at the end
of the bulletin. It follows the planetary A index and weekly average
that is always presented, and is an estimate of the middle latitude
A index. The planetary A index factors in data from magnetometers
worldwide. During periods of greater geomagnetic activity, this
number is higher than the middle latitude index because of reporting
from higher latitudes, where geomagnetic disturbance tends to become
more intense. The middle latitude numbers reflect conditions
experienced by the majority of our readers in North America who live
below the 50th parallel. Data from both sets of reported numbers
come from a NOAA ftp site at,

As promised last week, essays by Carl Luetzelschwab, K9LA, about
using free propagation software are now posted to the ARRL web site
on the Technical Information Service page devoted to propagation.
The W6ELprop article, in PDF format, is at and you can read the
VOACAP piece, also in PDF format, at We sometimes present
path projections in this bulletin, and use W6ELprop to make the
predictions. But you can have the flexibility of making your own
projections between any locations, seasonal variations or solar
activity by downloading the free software and plugging in your own

For more information concerning propagation and an explanation of
the numbers used in this bulletin see the ARRL Technical Information
Service propagation page at,

Sunspot numbers for April 15 through 21 were 60, 53, 55, 92, 108, 96
and 98 with a mean of 80.3. 10.7 cm flux was 96.7, 96.9, 98.2, 109,
113.4, 110.7 and 112.7, with a mean of 105.4. Estimated planetary A
indices were 6, 12, 9, 11, 6, 4 and 5, with a mean of 7.6.
Estimated mid-latitude A indices were 3, 10, 6, 6, 4, 4 and 6, with
a mean of 5.6.


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