ARRL

ARRL Propagation Bulletin ARLP029 (2000)

SB PROP @ ARL $ARLP029
ARLP029 Propagation de K7VVV

ZCZC AP29
QST de W1AW  
Propagation Forecast Bulletin 29  ARLP029
From Tad Cook, K7VVV
Seattle, WA  July 21, 2000
To all radio amateurs 

SB PROP ARL ARLP029
ARLP029 Propagation de K7VVV

This has been quite a week for solar activity, with blasts of solar
wind dominating space weather news. Saturday was the big day for HF
radio blackouts and aurora, with the planetary A index jumping to an
incredible 152 and the mid-latitude A index at 148. The planetary K
index, updated every three hours, was at 9 for three readings on
Saturday. A K index reading of 9 over a 24-hour period would be
equivalent to an A index of 300. This is big, really big.

These numbers are associated with an extreme geomagnetic storm that
was nearly off the scale. On Friday one of the most powerful solar
flares of the current cycle triggered a storm of protons directed
toward earth. The Solar and Heliospheric Observatory recorded a full
halo coronal mass ejection heading toward earth at greater than
one-million meters per second. Check out animations of this event at
http://sohowww.nascom.nasa.gov/data/CME/.

There are only a few events of this magnitude in the average solar
cycle. The only factor that probably impeded a spectacular aurora
visible far down into the U.S. was that this is the summer season.
A dark evening sky would reveal a rich tapestry of northern lights.
Of course, it being winter in the southern hemisphere, bright
displays were reported in Australia and New Zealand. The author made
a weak attempt Saturday night, walking barefoot into the middle of
the street and trying to peer north past city lights, but no auroral
glow was visible. Seattle is north of the 47th parallel, and
although there were no local reports of aurora, there were
observations as far south as 40 degrees in Europe, Asia, and parts
of Eastern North America. If you point your web browser toward
http://www.sec.noaa.gov/info/kp-aurora.html you will see a nice map
from the June, 1968 issue of Sky and Telescope which shows how far
south aurora may be visible depending on the planetary K index.
What is not clear from this map is whether it shows how far south
the edge of the aurora extends, or how far south it is visible when
looking north.

Another coronal mass ejection emerged on Wednesday, July 19, but the
predicted effect is uncertain because the ejection may not be aimed
squarely at earth. On Thursday the planetary K index went up to 6
for several hours, but by the end of the UTC day it was 3. The
planetary A index for Thursday was 43, and the College A index,
recorded in Alaska, was 57.

The latest forecast shows the planetary A index rising to 50 on
Friday, then dropping to 20 and 15 and then 10 on Saturday through
Monday. Solar flux peaked for the recent short term at 252.9 on
Thursday, and is expected to drop to 245, 235, 230 and 225 on Friday
through Monday. The next short term minimum is predicted around July
28 at 170, followed by another peak above 200 around August 6-9.

The author has received many more inquiries recently asking for
explanations of the various parameters reported in this bulletin.
Although the explanations were repeated six weeks ago, it is
probably time to run them again, and they follow this paragraph.
Feel free to send questions to the author via k7vvvarrl.net.

Amateur Radio operators who use HF generally like increased sunspots
because they correlate with better worldwide radio propagation.
When there are more sunspots, the sun puts out radiation which
charges particles in the earth's ionosphere. Radio waves bounce off
of these charged particles, and the denser these clouds of ions, the
better the HF propagation. When the ionosphere is denser, higher
frequencies will reflect off of the ionosphere rather than passing
through to space. This is why every 11 years or so when this
activity is higher, 10 meters gets exciting. 10 meters is at a high
enough frequency, right near the top of the HF spectrum, that radio
waves propagate very efficiently when the sunspot count is high.
Because of the wavelength, smaller antennas are very efficient on
this band, so mobile stations running low power on 10 meters can
communicate world wide on a daily basis when the sunspot cycle is at
its peak. There are also seasonal variations, and 10 meters tends to
be best near the spring or fall equinox.

The sunspot numbers used in this bulletin are calculated by counting
the sunspots on the visible solar surface and also measuring their
area. Solar flux is measured at an observatory in British Columbia
using an antenna pointed toward the sun tuned to 2.8 GHz, which is
at a wavelength of 10.7 cm. Energy detected seems to correlate with
sunspots and with the density of the ionosphere.

Other solar activity of concern to HF operators are solar flares and
coronal holes, which emit protons. Since the charged ions in the
ionosphere are negative, a blast of protons from the sun can
neutralize the charge and make the ionosphere less reflective.
These waves of protons can be so intense that they may trigger an
event called a geomagnetic storm.

The Planetary A index relates to geomagnetic stability.
Magnetometers around the world are used to generate a number called
the Planetary K index. You can hear the Boulder K index updated
every three hours on WWV, or by calling 303-497-3235.

A one point change in the K index is quite significant. A K index
below 3 generally means good stable conditions, and above 3 can mean
high absorption and poor reflection of radio waves. Each point
change reflects a big change in conditions.

Every 24 hours the K index is summarized in a number called the A
index. A one point change in A value is not very significant. A full
day with the K index at 3 will produce an A index of 15, K of 4
means A of 27, K of 5 means A of 48, and K of 6 means A of 80. You
can find an explanation of these numbers on the web at
http://www.ngdc.noaa.gov/stp/GEOMAG/kp_ap.html.

The number reported here is the Planetary A index, which is a
worldwide average based on the K readings from a number of
magnetometers. The numbers reported on WWV are the Boulder K and A
index, measured in Colorado. Generally the higher the latitude of
the measuring station, the higher the K and A indices reported.
This is because the effects of geomagnetic instability tend to
concentrate toward the polar regions of the globe.

Currently we are near the peak of the solar cycle, so conditions are
generally better because of the increased ionization of the
ionosphere. But along with the increased sunspots come more solar
flares and coronal holes, producing disturbed conditions.

Sunspot numbers for July 13 through 19 were 240, 243, 229, 268, 335,
343 and 342 with a mean of 285.7. 10.7 cm flux was 231.9, 203.9,
213.1, 218.9, 228.3, 261.9 and 249.9, with a mean of 229.7, and
estimated planetary A indices were 33, 35, 152, 46, 9, 13 and 15,
with a mean of 43.3.
NNNN
/EX