ARLP017 Propagation de K7RA:
April 23, 2004

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 values.

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.