ARRL Propagation Bulletin ARLP025 (2010)

ARLP025 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 25  ARLP025
From Tad Cook, K7RA
Seattle, WA  June 25, 2010
To all radio amateurs 

ARLP025 Propagation de K7RA

Sunspot numbers were lower for the past week, compared to the
previous period.  Average daily sunspot numbers fell nearly 11
points to 16.1.

ARRL Field Day is this weekend, and it looks like we could have
unsettled to active geomagnetic conditions.  NOAA predicts solar
flux for June 25-27 at 75, 75 and 76, and planetary A index at 8, 15
and 12.  Beyond that, the predicted solar flux for June 28 through
July 1 is 76, 77, 77 and 72, with planetary A index of 10, 10, 12
and 15.

Geophysical Institute Prague calls for unsettled conditions June 25,
unsettled to active June 26, unsettled June 27-28, quiet to
unsettled June 29, and unsettled June 30 and July 1.

The predicted solar flux for this weekend is slightly higher than it
has been this week.  In fact, solar flux has been below the 75-76
range since June 11-13.

Of course, we want solar flux and sunspot numbers to be high, with
geomagnetic indices such as planetary A index to be low.

In Propagation Forecast bulletins ARLP024 and ARLP023 we talked
about solar flux, both observed and adjusted.  The adjusted values
are based on what the measurements would be if the distance from
Earth to Sun were at one AU or Astronomical Unit, which is the
average distance.

ARLP023 mentioned an alleged increasing discrepancy between sunspots
and solar flux, which seems apparent when you look at the
information at and see the gradually
declining solar flux.  But actually this is probably due to the
changing distance between Earth and Sun, since these are observed
numbers, not adjusted.

As noted in ARLP024, the observed values would be adjusted down in
January and up recently to get the adjusted values, and this would
probably yield a more constant value in the chart in the previous
paragraph.  If you want, you could chart it out yourself using the
numbers at, Just compare the observed
values in the "fluxobsflux" column with the adjusted values in the
"fluxadjflux" column.

A note from Carl Luetzelschwab, K9LA points out that sunspot numbers
and solar flux really don't correlated that well.  On his web page click on the "Timely Topics" link
and then the June 23 link, "The Variability of the Sun."  Carl's web
page has a great deal of good information explaining propagation

MSNBC has an interesting article about sunspots in popular culture
at and toward the end of the piece is
even some useful information about the state of the current cycle.

If you would like to make a comment or have a tip for our readers,
email the author at,

For more information concerning radio propagation, see the ARRL
Technical Information Service web page at, For an explanation of the
numbers used in this bulletin, see An archive of past
propagation bulletins is at,  Find more good
information and tutorials on propagation at,

Monthly propagation charts between four USA regions and twelve
overseas locations are at

Instructions for starting or ending email distribution of ARRL
bulletins are at

Sunspot numbers for June 17 through 23 were 14, 16, 28, 13, 14, 14,
and 14 with a mean of 16.1. 10.7 cm flux was 70.4, 70.5, 68.9, 70,
72, 73.3 and 74 with a mean of 71.3. Estimated planetary A indices
were 10, 5, 4, 3, 5, 6 and 4 with a mean of 5.3. Estimated
mid-latitude A indices were 8, 4, 2, 2, 6, 5 and 2 with a mean of