ARRL

ARRL Propagation Bulletin ARLP042 (1999)

SB PROP @ ARL $ARLP042
ARLP042 Propagation de K7VVV

ZCZC AP42
QST de W1AW  
Propagation Forecast Bulletin 42  ARLP042
From Tad Cook, K7VVV
Seattle, WA  October 15, 1999
To all radio amateurs 

SB PROP ARL ARLP042
ARLP042 Propagation de K7VVV

Solar activity is up by quite a bit this week.  Average solar flux
values increased by nearly 30 points over last week, and average
sunspot numbers were up by almost 80 points.  The reporting week,
which runs Thursday through Wednesday, began with a stable
geomagnetic field and A indices in the single digits, but quickly
changed after the weekend to storm conditions.  The worst conditions
were probably on Tuesday, October 12, when the planetary A index
reached 34, and the highest planetary K index was 6.  Of course it
was worse in the higher latitudes, with the College A index from
Alaska at 61 and K indices has high as 7.

The author has been operating on 10 meter SSB recently, and when the
A indices were high, sometimes the only signals audible were from
the southern hemisphere.  I wrote to Bob Brown, NM7M, to ask about
this.  Dr. Brown taught physics at UC Berkeley, and has spent his
retirement years using amateur radio and studying HF propagation.
Bob wrote that the geomagnetic disturbances of the past week weren't
really big storms, and that part of the reason for observed
trans-equatorial propagation is because of the absence of other
propagation modes.  ''In the absence of high latitude propagation due
to lowered MUFs or auroral absorption, the old standby,
trans-equatorial propagation, looks better than ever,'' Bob wrote.
He also said that there are effects which may increase HF
propagation at low latitudes, but only during really big geomagnetic
storms.

The projected solar flux values for the next three days, Friday
through Sunday, are 200 for each day, and the planetary A index is
forecast at 25, 20 and 15.  Solar flux is expected to drift lower
after the weekend, reaching 150 around October 20 and a minimum of
125 from October 23-39.  Predicted disturbed days, when the A index
is 25 or more, are October 24 and 27 and November 6-8.

Recently I have received more requests for an explanation of some of
the numbers in this bulletin.  Here it is.

Amateur Radio operators who use HF generally like increased sunspots
because it correlates with better worldwide radio propagation.  When
there are more sunspots, the sun puts out radiation which charges
particles in the ionosphere.  Radio waves bounce off of these
charged particles, and the more dense these clouds of ions, the
better the HF propagation.  When the ionosphere is more dense,
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.

The sunspot numbers used in this bulletin are calculated by counting
the sunspots on the visible solar surface and also measuring their
area.  The solar flux is measured at an observatory in British
Columbia using an antenna pointed toward the sun tuned to 2.8 GHz,
which is 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
higher than 3 is a big change in conditions.

Every 24 hours the K index is summarized in something 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.

Sunspot numbers for October 7 through 13 were 184, 170, 235, 195,
163, 191 and 210 with a mean of 192.6.  10.7 cm flux was 129.4,
151.2, 153.2, 160.5, 166.6, 183.6 and 191, with a mean of 162.2, and
estimated planetary A indices were 6, 8, 6, 28, 23, 34 and 26, with
a mean of 18.7.
NNNN
/EX