ARRL

ARRL Propagation Bulletin ARLP038 (2001)

SB PROP @ ARL $ARLP038
ARLP038 Propagation de K7VVV

ZCZC AP38
QST de W1AW  
Propagation Forecast Bulletin 38  ARLP038
From Tad Cook, K7VVV
Seattle, WA  September 14, 2001
To all radio amateurs 

SB PROP ARL ARLP038
ARLP038 Propagation de K7VVV

Average daily sunspot numbers were up over 86 points this week over
last, and average solar flux rose 39 points. The daily sunspot
number peaked on Sunday at 291, the highest value since April 1.
Solar flux peaked on Tuesday at 249.7, also the highest since April
1. Solar flux is expected to level off over the next few days, to
235 on Friday, 225 on Saturday and Sunday, and around 215 on Monday.
A large complex of sunspots has just crossed the sun's central
meridian. Holographic images of the sun's far side show another
large sunspot, probably region 9591 which produced a large solar
flare at the end of August when it was on the sun's earth side.

There have been several coronal mass ejections since September 11,
but they were not earth-directed. Geomagnetic conditions have been
quiet this week, but on Thursday we are experiencing a rise in
activity. On Friday the predicted planetary A index is 20, and
conditions are expected to be unsettled after that.

K9LA recently wrote a better version of the explanation of the terms
and numbers that runs in this bulletin from time to time. He has
generously offered to share it, and it runs below. You can contact
k9la@gte.net with questions.

The Sun emits electromagnetic radiation and matter as a consequence
of the nuclear fusion process. Electromagnetic radiation at
wavelengths of 100-1000 Angstroms (ultraviolet) ionizes the F
region, radiation at 10-100 Angstroms (soft X-rays) ionizes the E
region, and radiation at 1-10 Angstroms (hard X-rays) ionizes the D
region. Solar matter (which includes charged particles - electrons
and protons) is ejected from the sun on a regular basis, and this
comprises the solar wind. On a ''quiet'' solar day the speed of this
solar wind heading toward Earth averages about 400 km per second.

The Sun's solar wind significantly impacts the Earth's magnetic
field. Instead of being a simple bar magnet, the Earth's magnetic
field is compressed by the solar wind on the side facing the sun and
is stretched out on the side away from the sun (the magnetotail,
which extends tens of Earth radii downwind). While the Sun's
electromagnetic radiation can impact the entire ionosphere that is
in daylight, charged particles ejected by the Sun are guided into
the ionosphere along magnetic field lines and thus can only impact
high latitudes where the magnetic field lines go into the Earth.

The Earth's magnetic field plays an important part in propagation.
When electromagnetic radiation from the Sun strips an electron off a
neutral constituent in the atmosphere, the resulting electron can
spiral along a magnetic field line. Thus the ionosphere is
critically dependent on the state of the Earth's magnetic field.
Variations in the Earth's magnetic field are measured by
magnetometers. There are two measurements readily available - the
daily A index and the 3-hour K index. The A index uses a linear
scale and goes from 0 (quiet) to 400 (severe storm). The K index
uses a quasi-logarithmic scale (which essentially is a compressed
version of the A index) and goes from 0 to 9 (with 0 being quiet and
9 being severe storm). Generally an A index at or below 15 or a K
index at or below 3 is best for propagation.

Sunspots are areas on the Sun associated with ultraviolet radiation.
Thus they are tied to ionization of the F region. The daily sunspot
number, when plotted over a month time frame, is very spiky.
Averaging the daily sunspot numbers over a month results in the
monthly average sunspot number, but it also is rather spiky when
plotted. Thus a more averaged, or smoothed, measurement is needed to
measure solar cycles. This is the smoothed sunspot number (SSN). SSN
is calculated using five and a half months of data before and after
the desired month, plus the data for the desired month. Because of
this amount of smoothing, the official SSN is about a half year
behind the current month.

Sunspots come and go in an approximate 11-year cycle. The rise to
peak (4 to 5 years) is usually faster than the descent to minimum (6
to 7 years). At and near the peak of a solar cycle, the increased
number of sunspots causes more ultraviolet radiation to impinge on
the ionosphere. This results in significantly more F region
ionization, and allows the ionosphere to refract higher frequencies
(15 meters, 12 meters, 10 meters, and even 6 meters) back to Earth
for DX contacts. At and near the minimum between solar cycles, the
number of sunspots is so low that higher frequencies go through the
ionosphere into space. Commensurate with solar minimum, though, is
less absorption and a more stable ionosphere, resulting in the best
propagation on the lower frequencies (160 meters and 80 meters).
Thus high SSNs are best for high frequency propagation, and low SSNs
are best for low frequency propagation.

Most of the disturbances to propagation come from solar flares and
coronal mass ejections (CMEs). The flares that affect propagation
are called X-ray flares due to their wavelength being in the 1-8
Angstrom range. X-ray flares are classified as C (the smallest), M
(medium size) and X (the biggest). Class C flares usually have a
minimal impact to propagation. Class M and X flares can have a
progressively adverse impact to propagation. The electromagnetic
radiation from these flares can cause the loss of all propagation on
the sunlit side of the Earth due to increased D region absorption.
In addition, big class X flares can emit very energetic protons that
are guided into the polar cap by the Earth's magnetic field. This
results in a polar cap absorption event (PCA), with high D region
absorption on paths passing through the polar areas of the Earth.

A CME is an explosive ejection of large amounts of solar matter, and
can cause the average solar wind speed to take a dramatic jump
upward - kind of like a shock wave heading toward Earth. When the
shock wave hits the Earth's magnetic field, it can cause large
variations in and distortions to the Earth's magnetic field. This is
seen as an increase in the A and K indices. Distortions to the
magnetic field can cause those electrons spiraling around magnetic
field lines to be lost into the magnetotail. With electrons gone,
maximum usable frequencies (MUFs) decrease, and return only after
the magnetic field returns to normal and the process of ionization
replenishes lost electrons.

Solar flares and CMEs are related, but they can happen together or
separately. Scientists are still trying to understand the
relationship between solar flares and CMEs. One thing is certain,
though - the electromagnetic radiation from a big flare, traveling
at the speed of light, can cause short-term radio blackouts on the
sunlit side of the Earth within about 10 minutes of the eruption.
The energetic particles ejected during a flare and the shock wave
from a CME can take up to a couple days to arrive in the vicinity of
Earth to cause their disruptions to propagation.

Each day the National Oceanographic and Atmospheric Administration
(NOAA) and the US Air Force jointly put out a Report on Solar and
Geophysical Activity (RSGA). These reports are archived at
http://www.sec.noaa.gov/ftpdir/forecasts/RSGA . Each daily report
consists of six parts.

Part IA gives an analysis of solar activity, including flares and
CMEs. Part IB gives a forecast of solar activity. Part IIA gives a
summary of geophysical activity. Part IIB gives a forecast of
geophysical activity. Part III gives probabilities of flare and CME
events. These five parts can be summarized as follows: normal
propagation (no disturbances) generally occurs when no X-ray flares
higher than class C are reported or forecasted, along with solar
wind speeds due to coronal mass ejections near the average of 400
km/sec.

Part IV gives observed and predicted 10.7 cm solar flux. A comment
about the daily solar flux - it has little to do with what the
ionosphere is doing on that day. This will be explained later.

Part V gives observed and predicted A indices. Part VI gives
geomagnetic activity probabilities. These two parts can be
summarized as follows: good propagation generally occurs when the
forecast for the daily A index is at or below 15 (this corresponds
to a K index of 3 and below).

WWV, at 18 minutes past the hour every hour, puts out a shortened
version of this report. It gives the previous day's 10.7 cm solar
flux, the previous day's A index, and the current 3-hour K index.
Current solar activity and geomagnetic field activity are also
given, along with forecasts for both of them. As in the RSGA report,
normal propagation (no disturbances) is expected when solar activity
is low and the geomagnetic field is quiet. A comment is appropriate
here - both the RSGA report and WWV give a status of solar activity.
This is not a status on the 11-year sunspot cycle, but rather a
status on solar disturbances. For example, if the solar activity is
reported as low, that doesn't mean we're at the bottom of the solar
cycle - it simply means the sun has not produced any major flares or
CMEs.

In order to predict propagation, much effort was put into finding a
correlation between sunspots and the state of the ionosphere. The
best correlation turned out to be between SSN and monthly median
ionospheric parameters. This is the correlation that our propagation
prediction programs are based on, which means the outputs (usually
MUF and signal strength) are values with probabilities tied to them.
They are not absolutes - understanding this is a key to the proper
use of propagation predictions.

Sunspots are a subjective measurement - they are counted visually.
It would be nice to have a more objective measurement, one that
actually measures the Sun's output. 10.7 cm solar has become this
measurement. But it is only a general measurement of the activity of
the Sun, since a wavelength of 10.7 cm is way too low in energy to
cause any ionization. Thus 10.7 cm solar flux has nothing to do with
the formation of the ionosphere. The best correlation between solar
flux and sunspots is smoothed 10.7 cm solar flux and smoothed
sunspot number - the correlation between daily values, or even
monthly average values, is not very acceptable.

Since our propagation prediction programs were set up based on a
correlation between SSN and monthly median ionospheric parameters,
the use of SSN or the equivalent smoothed 10.7 cm solar flux gives
the best results. Using the daily 10.7 cm solar flux, or even the
daily sunspot number, can introduce a sizable error into the
propagation prediction outputs due to the fact that the ionosphere
does not react to the small daily variations of the Sun. Even
averaging 10.7 cm solar flux over a week's time frame can contribute
to erroneous predictions. For best results, use SSN or smoothed
solar flux, and understand the concept of monthly median values. If
there was a good correlation between what the ionosphere is doing
today and today's solar flux or sunspot number, then we'd have a
daily propagation model as opposed to a monthly median model.

Sunspot numbers for September 6 through 12 were 204, 288, 281, 291,
217, 180 and 228 with a mean of 241.3. 10.7 cm flux was 222.2,
226.1, 249.5, 236.2, 244.5, 249.7 and 235.1, with a mean of 237.6,
and estimated planetary A indices were 8, 6, 7, 7, 5, 9 and 13 with
a mean of 7.9.
NNNN
/EX