Register Account

Login Help

ARRL Propagation Bulletin ARLP006 (2005)

ARLP006 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 6  ARLP006
From Tad Cook, K7RA
Seattle, WA  February 11, 2005
To all radio amateurs 

ARLP006 Propagation de K7RA

All solar activity indicators rose this week, though not by much.
Average daily sunspot number for the week rose 4.4 points to 41.3,
average daily solar flux was up 11.6 points to 85.1, and average
planetary A index rose 4 points to 15.1. Average daily mid-latitude
A index rose 2.2 points to 10.6.

Monday through Wednesday, February 7-9 had unsettled to active
geomagnetic activity with Tuesday the most disturbed day. The
planetary A index was 34 that day and mid-latitude A index was 27.
At the 0300z February 8 reading, the planetary K index reached a
high of 6, and the highest reading for Alaska's college K index came
at 1800z that day when it reached a very high value of 8. There was
a K index of 7 on the previous day, and the college A index for
February 7-10 was 50, 71, 45 and 30, all high values.

This is why Alaskan operators oftentimes report such poor HF
conditions during periods when the rest of the United States hardly
notices. For instance, between 1200-1500z on February 8, the Alaska
K index readings were 6, 6, and 8, indicating a severe geomagnetic
storm. But the mid-latitude K index, which affects most of the rest
of us was only 2, 3 and 3, quite moderate numbers. Of course for the
rest of us, a high latitude path (such as the one to Europe from
Seattle) would also be affected, even though each end of the path is
at a more moderate latitude.

The college A and K index is measured at Fairbanks, Alaska, and the
mid-latitude indices that we use are from Fredericksburg, Virginia.
The planetary K and A index is the mean-standardized index from 13
geomagnetic observatories between 44 and 60 degrees northern or
southern latitude.

That single A index for the day is calculated from the eight K index
readings, and the scales are quite different, the K index being
quasi-logarithmic. A single point change in the K index is quite
significant, but a several point change in the A index is not. A day
with all eight K index readings of 2 would yield an A index for the
day of 7, but all-day K index of 3 would produce an A of 15, and K
of 4 all day would yield an A index of 27. You can check the
relationship between the A and K readings on NOAA's National
Geophysical Data Center site at,

A week-long graph showing Boulder (another mid-latitude
observatory), Fredericksburg, planetary and college K index is at Notice from the graphs
that the Boulder measurement is often slightly higher (at least it
was in the week prior to this bulletin) than the Fredericksburg
values. There is only a two-degree difference in latitude, Boulder
being at 40 degrees north, and Fredericksburg at 38 degrees north.

The higher A and K numbers this week were caused by a robust solar
wind stream. The Interplanetary Magnetic Field pointed south, so the
earth was vulnerable to these particles. When the IMF points north,
the earth is shielded, and it didn't point north until February 10,
a quiet day.

Last week this bulletin said that solar flux values should rise to
130 by February 7, and stay there for about a week. This was because
of the return of sunspot 720, which was quite large the last time it
faced earth. When 720 returned a few days after the bulletin, it was
much smaller than before. Currently solar flux is expected to stay
around 115 for February 11-12, then drop a few points for the next
few days. Solar flux could drop below 100 around February 19, but
the further out we look, the prediction becomes less meaningful.

Roger Bonuchi, AC9Y of Plainfield, Illinois sent an email this week
asking about GOES Solar X-Ray Flux as shown at, He wonders if this relates
to the 10.7 cm solar flux we report in this bulletin. They actually
are measurements of electromagnetic radiation in different parts of
the spectrum. X-rays have wavelengths shorter than ultraviolet
light. 10.7 cm solar flux is at a much lower frequency, around 2.8

See the relative positions of x-rays, light waves and radio waves at
a physics study aid at,

10.7 cm solar flux correlates to some degree with sunspots, and
typically the higher solar flux or sunspot numbers result in a
higher MUF, or Maximum Usable Frequency. If you use a propagation
program such as W6ELprop (free at,
over most paths the MUF will go higher when the sunspot numbers or
solar flux are higher.

X-rays are correlated with events such as coronal mass ejections or
solar flares, and the measurements shown in the graph that Roger
sent are done by the GOES satellite. You'll notice on that same page
there is a link to the D-region absorption prediction at, which should show
greater absorption when the x-ray flux is higher.

Allen Robbins, W7AM of Salem, Oregon asked about the scales heard on
the WWV geophysical alerts. You can see the text of the latest WWV
broadcast alert at A
couple of web pages that explain the numbers and scales for radio
blackouts, solar radiation storms and geomagnetic storms are at, and

If you would like to comment or have a tip, email the author at,

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 February 3 through 9 were 23, 22, 22, 47, 62, 53
and 60 with a mean of 41.3. 10.7 cm flux was 83, 82.1, 94.6, 97,
103.1, 108.2 and 108.6, with a mean of 96.7. Estimated planetary A
indices were 8, 3, 4, 9, 23, 34 and 25 with a mean of 15.1.
Estimated mid-latitude A indices were 5, 2, 1, 6, 19, 27 and 14,
with a mean of 10.6.


Instragram     Facebook     Twitter     YouTube     LinkedIn