ARRL

ARRL Propagation Bulletin ARLP039 (2000)

SB PROP @ ARL $ARLP039
ARLP039 Propagation de K7VVV

ZCZC AP39
QST de W1AW  
Propagation Forecast Bulletin 39  ARLP039
From Tad Cook, K7VVV
Seattle, WA  September 29, 2000
To all radio amateurs 

SB PROP ARL ARLP039
ARLP039 Propagation de K7VVV

Solar flux and sunspot numbers were up over the past week, while
average geomagnetic indices were lower, which is always a happy
condition for HF radio enthusiasts. Solar flux peaked at 232.2 on
Friday and sunspot numbers peaked at 255 on Sunday. Average sunspot
numbers for the week were up nearly 87 points, and average solar
flux rose by almost 39 points, when compared to the previous week.

The sunspot number is calculated by counting the visible sunspots
and factoring in their size, so a significant factor was sunspot
9169, reported in last week's Propagation Forecast Bulletin ARLP038
as one of the largest seen in many years. It is now fading as it
rotates off of the visible solar disk. We were lucky not to have a
great deal of flare activity from this magnetically complex spot.

K4WY sent a web reference concerning this particular sunspot. Check
http://antwrp.gsfc.nasa.gov/apod/ap000925.html .

The most active geomagnetic day over the past week was Tuesday, when
the planetary A index was 21. Planetary K index was 4 during most of
the day, but Alaska's College K index, which is usually higher
because of the polar region proximity, was as high as 6.  College A
index was 37 for the day.

Friday and Saturday were the quietest geomagnetic days, with A
indices in the single-digits, Planetary K indices at 2 and 3, and
mid-latitude K indices at 1 and 2. Fortunately for HF enthusiasts,
this was also the period when the sunspot count and solar flux were
the highest, which often is not the case.

Geomagnetic indices should remain stable over the next few days,
with planetary A indices predicted at around 10. On Monday through
Wednesday the A index is forecast at 12, 15 and 12, probably based
on the previous solar rotation. This indicates an unsettled to
active geomagnetic conditions, with higher absorption of HF radio
signals, particularly in the higher latitude or polar paths.

Solar flux is expected to decline over the next few days, with
Saturday at 190 and Sunday around 180. For the short term, flux
values should reach a minimum near 155 around October 7-9, then head
above 200 again around mid-month.

We have now passed the autumnal equinox, and are experiencing Fall
HF conditions. 10 and 12 meter operators should expect great
propagation, at least when the K index as reported by WWV is 3 or
less. Openings follow the sunlight, with propagation to the east in
the morning and toward the west later in the day. 15 meters should
offer plenty of worldwide openings as well, but also later into the
evening after 10 meters has closed. Worldwide 20 meter openings
should be available around the clock. As the northern hemisphere
moves further from the summer season, 160 and 80 meters should
improve with shorter days and less of the static commonly associated
with summer.

Judging by recent email, it is time to repeat the occasional
explanation of the various numbers and indices that are cited in
this weekly bulletin, which appears below. Questions and comments
are always welcome at k7vvv@arrl.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 (refract from) these charged particles, and the denser these
clouds of ions, the better the HF propagation.

When the ionosphere is denser, higher frequencies will refract from
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 refractive.
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
reading 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 September 21 through 27 were 198, 248, 216, 255,
215, 223 and 233 with a mean of 226.9. 10.7 cm flux was 225.1,
232.2, 225.2, 224.5, 225.6, 223.6 and 204.7, with a mean of 223, and
estimated planetary A indices were 9, 7, 7, 10, 16, 21 and 11 with a
mean of 11.6.
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/EX