ARRL

ARRL Propagation Bulletin ARLP036 (2007)

SB PROP @ ARL $ARLP036
ARLP036 Propagation de K7RA

ZCZC AP36
QST de W1AW  
Propagation Forecast Bulletin 36  ARLP036
From Tad Cook, K7RA
Seattle, WA  August 31, 2007
To all radio amateurs 

SB PROP ARL ARLP036
ARLP036 Propagation de K7RA

Sunspot numbers pulled up from 0 this week, but barely.  Average
daily sunspot numbers rose over nine points to 12.9.  Geomagnetic
conditions were quiet. Conditions have been quiet for so long that
we may not appreciate this, as many of us wish for more solar
activity and sunspots.

It wasn't long ago when we had more sunspots, but the accompanying
geomagnetic activity made conditions difficult.  Check the Planetary
A index in October 2002, April, June, September and December 2003,
and into January 2004.  For 2003, check
http://www.sec.noaa.gov/ftpdir/indices/old_indices/2003_DGD.txt.
For any other year, just change 2003 toward the end of the URL, and
to check sunspot numbers change DGD to DSD at the end of the URL.

If you read ARRL Propagation Bulletins from back then, in addition
to complaints about the higher geomagnetic activity, you'll find
stories of better VHF propagation during periods of geomagnetic
disturbance.  You can see old bulletins back through 1995 at,
http://www.arrl.org/w1aw/prop/.

This morning the Australian Ionospheric Prediction Service sent out
a warning of increased geomagnetic activity centered on September 1
due to a wind stream from a solar coronal hole.  They predict today,
August 31, will be quiet with increasing activity late in the day,
unsettled to active conditions with possible minor storm on
Saturday, September 1, and mostly unsettled conditions September 2.

Geophysical Institute Prague predicted earlier that August 31 would
be quiet to unsettled, September 1 unsettled to active, unsettled
conditions September 2-3, quiet September 4-5, and unsettled to
active again on September 6.

Over the same period the US Air Force predicts a Planetary A index
of 15, 25, 12, 12, 8, 5 and 15 for August 31 through September 6.
From the same prediction, it looks like September 8-17 may see a
return of 0 sunspot days.

In response to last week's mention of ionospheric sounders, Pat
Dyer, WA5IYX of San Antonio, Texas wrote in with some useful
information and links.

At http://digisonde.haystack.edu/Search.html you can view recent
ionographs from the MIT Haystack Observatory at Millstone Hill in
Westford, Massachusetts. In the typical Lowell Digisonde graph, we
can see echoes from the ionosphere over a range of 1-10 MHz, for
radio energy beamed straight up and swept through the radio
spectrum.  Frequency is shown along the bottom, on the x-axis, and
the vertical axis is calibrated in kilometers.  This is the
calculated height of the part of the ionosphere that is returning
echoes.

On the left side at the top we see foF2, which is the approximate
frequency of the strongest echoes. Along the bottom, below the
graph, is a list of "D" and "MUF."  This is the calculated Maximum
Usable Frequency for the patch of ionosphere over the observatory,
related to distance.  So stations 1000 km apart, with the
observatory in the middle, would see the MUF value associated with
that distance reflected by the patch of sky above the observatory.

Currently I am looking at a chart for August 31 at 0730z.  For the
distances of 100, 200, 400, 600, 800, 1000, 1500 and 3000 km I am
seeing MUF values of 2.7, 2.7, 2.8, 3.0, 3.1, 3.4, 4.3 and 6.4 MHz.
This means that stations 1500 km apart, or 750 km on each side of
the observatory, could communicate well on 80 meters, but not at
1000 km or shorter distances.

WA5IYX gives an example at,
http://www.qsl.net/wa5iyx/ionogram/01701400.gif.

Referring to the example, Pat writes, "The x-axis is the swept
ionosonde freq (MHz) of the pulse sent straight up (VI, vertical
incidence) with (if any) echoes displayed.

"The short red dashes just under the x-axis are freq segments where
their ionosonde is shut off to avoid QRMing the existent services
there."

He continues, "The y-axis is the "virtual height" in km as
determined by the ms time delay for the echo (1 ms round trip is
approximately 300/2 km).  It's called virtual since (with the
exception of Es) the reflection takes place over a depth of 10s of
km, with the pulse actually being slowed down just before and after.
This is what gives those striking curves to the F-region traces.
For F1 it's the lower E region and for F2 the F1 - they are still
slowing (retarding) down the pulse, less so as the freq rises,
giving the appearance that the F2 layer is at first high, then
lowers, and then rises again.  Only the latter is really indicative
of a true height change as the pulse goes higher (deeper) into the
layer before being reflected as the freq is raised (and eventually
fully penetrating it)."

"foF2 is the highest VI freq reflection of the ordinary wave from
the F2 region. A plasma (which is what the ionosphere is) in a
magnetic field splits an incident radio wave into two components
(birefringence) - the ordinary (o) and extraordinary (x) wave.  On
VI ionograms these echoes are separated by the natural gyrofrequency
(the spin/sec that a free electron exhibits in a given strength
magnetic field).  So that separation will vary across the earth as
the magnetic field (in long-lat as well as height).  For Lowell it's
about 1 MHz difference."

Pat goes on to say, "In my Lowell sample, that 27.58 MUF is the
calculated maximum oblique frequency that sampled portion of the
F-layer should be able to return (in their case over a 3000 km
range).  These are calculated from the Secant Law using the foF2 and
F2 values.  Their current display format lists (at the bottom left)
the interpolated MUFs for ranges of 100, 200, 400, 600, 800, 1000,
1500, and 3000 km. (This isn't visible in his example, but the
actual MUFs not shown are 4.4, 4.4, 4.6, 4.8, 5.2, 5.7, 7.3 and 11.2
MHz)."

He closes by saying, "A super-complex explanation of Dynasondes is
at,
http://www.ngdc.noaa.gov/stp/IONO/Dynasonde/tutorial/tutorial.html."

Thanks, Pat!

If you would like to make a comment or have a tip for our readers,
email the author at, k7ra@arrl.net.

For more information concerning radio propagation, see the ARRL
Technical Information Service at,
http://www.arrl.org/tis/info/propagation.html. For a detailed
explanation of the numbers used in this bulletin, see,
http://www.arrl.org/tis/info/k9la-prop.html. An archive of past
propagation bulletins is at, http://www.arrl.org/w1aw/prop/ .
Monthly propagation charts between four USA regions and twelve
overseas locations are at, http://www.arrl.org/qst/propcharts/.

Sunspot numbers for August 23 through 29 were 12, 12, 14, 13, 12, 14
and 13 with a mean of 12.9. 10.7 cm flux was 70.8, 71.6, 71.5, 70.1,
69.2, 70.1, and 69.6 with a mean of 70.4. Estimated planetary A
indices were 2, 2, 8, 10, 12, 11 and 4 with a mean of 7. Estimated
mid-latitude A indices were 1, 1, 6, 10, 10, 9 and 4, with a mean of
5.9.
NNNN
/EX