ARRL

ARRL Propagation Bulletin ARLP011 (2011)

SB PROP @ ARL $ARLP011
ARLP011 Propagation de K7RA

ZCZC AP11
QST de W1AW  
Propagation Forecast Bulletin 11  ARLP011
From Tad Cook, K7RA
Seattle, WA  March 18, 2011
To all radio amateurs 

SB PROP ARL ARLP011
ARLP011 Propagation de K7RA

Average daily sunspot numbers this week plummeted 45 points to 69,
and average daily solar flux was off over 26 points to 113.1.  In a
brief report for the ARRL Letter we said that average flux was off
nearly 25 points to 114.5.  The average changed because the
observatory in Penticton reported the solar flux for Wednesday,
March 16 as 104.9, but later it was adjusted down to 95 by NOAA's
Space Weather Prediction Center.

This happens sometimes when energy from a Coronal Mass Ejection
overwhelms the 2.8 GHz (10.7 cm wavelength) receiver at the Dominion
Radio Astrophysical Observatory in Penticton, British Columbia.  A
good example was on March 7, 2011 when the noon solar flux reading
was 938.6.  This was an obvious outlier, caused by a solar flare out
of sunspot group 1166 that was positioned near optimal
geoeffectiveness. As a result, the official number was adjusted down
to 122.  But Wednesday's number didn't seem so obviously out of
line, and the timing didn't seem to line up with the only event I
knew of, an eruption from sunspot group 1169, which was nearly over
the solar horizon.

When I saw an image at http://www.spaceweather.com showing the CME
on the western limb with a time stamp of 2048z, I assumed this meant
the event occurred 48 minutes after the 2000z flux reading.  But it
turned out to be a long duration event.

Rob Steenburgh, KA8JBY works at the Space Weather Prediction Center
in Boulder, and looked into this for us.  He checked with the
forecasters, who wrote:

"The Penticton noon flux did come at 105 but was flare enhanced. We
estimated a value of 95 based on the morning flux.

"The long duration event (LDE) began at 16/1752Z, max at 16/2034Z
and end at 16/2324Z. It peaked at C3.7 from Region 1169 (N18W75).
The Penticton noon reading is taken between the hours of 1900 -
2000Z, so right in the middle of the LDE.  It was an extremely
impressive looking event on GOES-15 SXI."

So I didn't realize that the noon reading was actually taken over
the course of the hour before noon, not just a snapshot right at
local noon (2000z).  Rob clarified this, and said there are four
readings taken over the hour.  One is ignored (the outlier, or the
one most different from the other three) and the rest are averaged
to produce the noon solar flux.  So that estimate for Wednesday was
95, and the measured value for Thursday was 90.1, which seems
consistent with the declining sunspot numbers for Wednesday and
Thursday, 50 and 45, and the shrinkage of total sunspot area, 300
and 170 millionths of a solar hemisphere.

Last Friday, March 11 the planetary A index reached 40 due to a
powerful X1.5 class solar flare at 2323z on March 9.  This is
another definite indication that solar Cycle 24 is ramping up.
After four years of no X class flares, there have been two in the
last month.

The past few days have seen very quiet geomagnetic conditions, with
the K index in most locations at 0.

The latest forecast shows quiet conditions with planetary A index
around 5 until March 27-30, when predicted planetary A index is
expected to rise to 7, 7, 19 and 7.

Geophysical Institute Prague predicts quiet conditions March 18-19,
unsettled March 20, quiet to unsettled March 21, unsettled March 22,
quiet to unsettled March 23, and quiet on March 24.

Predicted solar flux for March 18-25 is 85, 80, 85, 90, 95, 100,
105, 110, and 85 on March 26-29.

This Sunday is the Spring Equinox in the Northern Hemisphere, a
welcome sign for DXers everywhere, when both the northern and
southern hemispheres are bathed in an equal amount of solar
radiation.

In the bulletin preview yesterday, we promised to explain how the
geomagnetic A index for the day is calculated using the eight K
index values throughout the day.  Here is how it works.

The K index value is not linear, and is based on magnetic
measurements in units called nano-Teslas.  K index of 0 corresponds
to a three-hour measurement of 0-5 nT, 1 is 5 >nT, K of 2 is >10, 3
is >20, 4 is >40, 5 is >70, 6 is >120, 7 is >200, 8 is >339, and 9
is >500 nT.  The idea of the A index is to take an average of the K
index to express a value for the whole day, but because the K index
is not a linear scale, it is not realistic to just average the eight
K index measurements.  Instead, we convert each K index into a
linear scale called the A index, then average those, and that yields
the A index value for the day, which is always expressed in whole
numbers.

The equivalent A values for each K value are, K of 1 is 3, 2 is 7, 3
is 15, 4 is 27, 5 is 48, 6 is 80, 7 is 140, 8 is 240 and 9 is 400.

So looking at http://www.swpc.noaa.gov/ftpdir/indices/DGD.txt, note
the high-latitude A index (measured at the University of Alaska at
Fairbanks) on March 11, 2011 is 56, a high value.  The eight K index
values are 4, 4, 5, 5, 5, 7, 6 and 4.  Those convert to A values of
27, 27, 48, 48, 48, 140, 80 and 27.  The sum of those is 445.
Divide by eight, and the average is 55.625, which rounds out to 56,
the A index for the day.

Bert Ingalls, KH6HI, operates a 6-meter beacon in Hawaii on the
island of Oahu.  The KH6HI/B beacon is on 50.0645 MHz and runs 20
watts into a pair of horizontally polarized loop antennas stacked
vertically at about 2/3 wavelength, or 12 feet apart.  It is at a
commercial broadcast site in the Waianae mountain range (see
http://www.6meterbeaconproject.org/kh6hi.html) at 2,500 feet with an
unobstructed shot to the horizon in all directions.

On Saturday, March 12, LW3EX in Buenos Aires reported hearing the
beacon at 2349z with an RST of 539.  Between grid squares BL01 and
GF05 is a distance of about 7,600 miles.  Bert wrote, "I believe the
following report is the first TE/F2 to South America from Hawaii for
Cycle 24.  It occurred at 1:49 PM local time which is typical for
TE/F2 in that direction during previous solar cycles.  I would
expect that propagation to ZL/VK and other South Pacific islands
will begin to occur as well providing the solar flux continues to
remain at current or higher levels."

Bob Forsman, WK5X of Stuarts Draft, Virginia wrote about the recent
report of possibly very long distance Argentina to North Carolina 2
meter propagation, and notes that this type of report isn't
unprecedented.  He found a couple of reports from 1979 and 1981 in
the March 2010 "World Above 50 MHz" column in QST, and included
these with some of his own comments.

"1) The December 1981 "The World Above....." column details a
September 23, 1981 (evening) occurrence of 2-Meter propagation
between CP5CL (Cochabamba, Bolivia) and the WB5QFM 2-Meter repeater
near Arcadia, Louisiana. It is interesting that this path, along
with last week's path, both occurred during an equinoctial (but
opposite) period. The South American station was full quieting into
the Louisiana repeater for 30 minutes, and then dropped to about 3 X
5 for a couple of hours.

"2) LU1DMA reports a QSO on 2-Meter SSB with Steve, WB9YWN (now
AF9X) on March 1, 1979 (last week's report was from March 2). After
tracking down AF9X, it was determined that Steve was likely
operating either maritime mobile from the vicinity of Bermuda, or
fixed from Charleston, South Carolina. These paths are not
dissimilar to the path reported last week."

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 web page at
http://arrl.org/propagation-of-rf-signals. For an explanation of the
numbers used in this bulletin, see
http://arrl.org/the-sun-the-earth-the-ionosphere. An archive of past
propagation bulletins is at
http://arrl.org/w1aw-bulletins-archive-propagation. Find more good
information and tutorials on propagation at
http://mysite.ncnetwork.net/k9la/index.html.

Monthly propagation charts between four USA regions and twelve
overseas locations are at http://arrl.org/propagation.

Instructions for starting or ending email distribution of ARRL
bulletins are at http://arrl.org/bulletins.

Sunspot numbers for March 10 through 16 were 88, 105, 78, 64, 51,
47, and 50, with a mean of 69. 10.7 cm flux was 131.3, 123.1, 120.7,
112.9, 107.4, 101.5, and 95, with a mean of 113.1. Estimated
planetary A indices were 20, 40, 13, 6, 6, 0, and 0, with a mean of
12.1. Estimated mid-latitude A indices were 14, 18, 10, 5, 2, 0, and
1, with a mean of 7.1.
NNNN
/EX