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ARRL Propagation Bulletin ARLP004 (2010)

ARLP004 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 4  ARLP004
From Tad Cook, K7RA
Seattle, WA  January 29, 2010
To all radio amateurs 

ARLP004 Propagation de K7RA

Average daily sunspot numbers for this reporting week, Thursday
through Wednesday, rose over 9 points to 28.  Average solar flux
slipped over 2 points to 81.9.  Geomagnetic indices were a tiny bit

Predicted solar flux for January 29-31 is 76, 80 on February 1-2, 82
on February 3-5, and 88-89 for the following five days.  We don't
see any geomagnetic upset predicted until February 16, with the
planetary A index only rising to 10, which is hardly an upset.

On the STEREO image at we can see
sunspot 1041 in the Sun's southern hemisphere, around 30 degrees
past the central meridian.  The unseen area (not yet visible to the
STEREO mission) of the Sun is exactly at 45 degrees wide today, and
shrinking.  A sunspot is moving toward the eastern horizon, and
looks to be less than 55 degrees away from first visibility.  If it
takes roughly 27.5 days for a complete solar rotation relative to
Earth, we can divide 27.5 days by 360 degrees, then multiply the
result by 55 to estimate that the sunspot may begin to emerge in a
little over four days.

This week we received a very interesting email from Richard Grubb,
W0QM an Associate of the NOAA Space Weather Prediction Center in
Boulder, Colorado.  Richard pointed out that the third paragraph of
last week's bulletin Propagation Forecast Bulletin ARLP003 may have
been confusing.

He writes, "The term Sudden Ionospheric Disturbance (SID) is used
today to replace Short Wave Fadeout (SWF) which was coined many
years ago in the 1930s when the immediate effects of a solar flare
on the ionosphere were first observed and correlated.

The flare produces a sudden large increase in the flux of solar
x-ray and far EUV radiation (see which reaches the
Earth at the speed of light.

See for
more information.

The radiation is absorbed in the ionosphere at D region heights
(70-90 km) and produces an increase in ionization that acts to
absorb signals that would otherwise be propagated via the F and E
regions. The effect can be drastic and very obvious for large

These effects are quite independent of the polarity of the
Interplanetary Magnetic Field (IMF).

The IMF comes into play when there is a change in the strength and
density of the solar wind reaching the outer boundary of the Earth's
magnetic field. These changes can be due to solar coronal mass
ejections (CMEs) that are generated at the Sun by explosive events
often accompanying a flare. A CME, as a shock wave and accompanying
sudden density enhancement takes hours to reach the Earth from the

Other changes in the solar wind at the Earth occur as a boundary of
the co-rotating spiral structure of the solar wind passes the Earth.
The effects that these solar wind changes have on the Earth's
magnetic field and the ionosphere are strongly dependant on the
polarity of the magnetic field embedded in the solar wind plasma.

If the north-south component is such that the fields couple (like
attracting magnets) then the effect can be large, or small in the
opposite case. The IMF does not protect the Earth against Solar
Flares per se, but against the disturbances in the solar wind that
may be solar flare produced.

SIDs of course only occur in the sunlit hemisphere of the Earth. To
cause an SID the flare can be anywhere above the visible solar rim.
However flare or event position is very important in determining
whether an event generated in the solar wind propagates to Earth.
Events occurring on the eastern (left-hand side viewed from Earth)
are generally better coupled for propagation to the Earth because
the solar wind is dragged out into a spiral structure by the solar

On January 20th we had both effects going on. There were several M
class flares, the largest at 1755z, which would have produced a
measurable SID; however, earlier in the day a co-rotating stream
interacted with the Earth and produced a disturbance in the
geomagnetic field that would also have created an ionospheric effect
via the change in the distribution of the trapped radiation in the
magnetosphere. The entire process of interaction between the solar
wind and the Earth's magnetosphere/ionosphere system that can
produce geomagnetic storms is complex and still the subject of
active research. However the broad outlines are clear.

As hams we probably want to have a simple picture of the solar
effects on the propagation we observe. However the distinction
between the prompt (SID) and the delayed solar wind carried
disturbances is important and needs to be kept clear.

It is the right hand (western) side of the Sun as viewed from Earth
that is most strongly coupled to the Earth for events propagating
through the interplanetary medium. Small events tend to follow the
spiral magnetic field to the Earth. Large events can effectively
force their way to the Earth from center or eastward locations on
the Sun.

The Stanford site shows how to
monitor SIDs using simple VLF receiver monitors. One of the reasons
that the term SWF was largely dropped in favor of SID was because,
apart from the HF absorption, the prompt effects of the x-ray
radiation from the Sun is a the change in propagation of VLF signals
via the D region which can be readily monitored by the changes in
amplitude and phase at a remote receiver. We used to use VLF
monitors for solar flare detection at the SWPC (SESC) in the '50s
and early '60s before we had the x-ray monitors on the GOES.

See also and"

Thanks to Dick for a very enlightening message.

K6SGH has a very gripping and suspenseful account of battling the
Santa Barbara Jesusita Fire in May 2009 as it raced toward his rural
home and ham station perched on a canyon rim.  Check it out at

Someone named D. Moore has sent us many interesting articles over
the years, and this week he posted an article on the new Solar
Dynamics Observatory, which you can read at,  Several
months ago he sent an article on NASA's MESSENGER spacecraft which
we neglected until now, and you can see it at,  Yes, being in
all caps, MESSENGER is an acronym, although a tortured one.  It is
derived from MErcury Surface, Space ENvironment, GEochemistry and
Ranging probe.  The name has a connection to Greek mythology,
because Mercury was the messenger of the gods.

Don't miss the CQ World Wide 160 Meter CW Contest this weekend.  Low
solar activity and weak solar wind generally mean good conditions
for this event.

Take a break from the contest tonight and observe the biggest full
moon of the year as it reaches perigee, the point of closest
approach to Earth.  It is full at 0618z January 30, which is 10:18
PM PST January 29 here on the Left Coast.  You can check local
moonrise time with the calculator at,  Try to catch
it low in the sky early on so you can see it through foreground
objects, maximizing the moon illusion. It should be huge. Thanks to for the tip.

We had hoped to get to some questions this week about why
propagation on 75 meters often goes "long," and local signals are
attenuated, but that will have to wait, although we touched on the
subject in Propagation Forecast Bulletin ARLP036 last year (see

Finally, this edition of the bulletin is dedicated to Howard S.
Pyle, "YB of W7OE," born 112 years ago yesterday, January 28, 1898
(he became a Silent Key on November 29, 1972).  When I was a kid he
was a well known author of amateur radio books and magazine
articles, mostly in 73 or CQ.

I met him one day on HF CW when I was 14, and he was local to me in
Seattle, on Mercer Island.  He only used CW, never used directional
antennas, and always operated barefoot.

YB told many great stories about working as the radio operator at a
mine in Alaska (I think the callsign there was YB), later on board
ship, and as an inspector for the FCC.  He encouraged me to write,
and sent a letter of introduction to Jim Fisk, W1DTY, at that time
(1968) the editor of 73 Magazine, who later founded Ham Radio
magazine. To my surprise, they published my article and the next one
I sent them as well.  YB had been on the air since he was 9 years
old in 1907, when he first operated a spark gap transmitter.

He also inducted me into the low power operator's organization, QRP
Amateur Radio Club International, which is still going strong (see  At that time the club had very modest
goals, and encouraged members to run 100 watts or less!  YB said
that Novice class licensees were all qualified, because we were
restricted to 75 watts, crystal controlled CW on HF.

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 a detailed
explanation of the numbers used in this bulletin, see An archive of past
propagation bulletins is at

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

Instructions for starting or ending email distribution of this
bulletin are at

Sunspot numbers for January 21 through 27 were 17, 30, 40, 32, 34,
28, and 15 with a mean of 28. 10.7 cm flux was 82.6, 82.4, 84.6,
84.6, 81.2, 79.8, and 77.8 with a mean of 81.9. Estimated planetary
A indices were 4, 2, 4, 4, 3, 3 and 2 with a mean of 3.1. Estimated
mid-latitude A indices were 4, 2, 4, 2, 3, 1 and 0 with a mean of


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