ARRL

ARRL Propagation Bulletin ARLP001 (2002)

SB PROP @ ARL $ARLP001
ARLP001 Propagation de K7VVV

ZCZC AP01
QST de W1AW  
Propagation Forecast Bulletin 1  ARLP001
From Tad Cook, K7VVV
Seattle, WA  January 4, 2002
To all radio amateurs 

SB PROP ARL ARLP001
ARLP001 Propagation de K7VVV

Because this is the first bulletin of the new year, we'll spend some
time reviewing last year.

If we look at the average daily solar flux and sunspot numbers for
2001, it was really a very good year with lots of activity,
considering that the peak was supposed to be in 2000. Average daily
sunspot numbers for the years 1997-2001 were 30.7, 88.5, 136.3,
172.8 and 170.3. Average daily solar flux values for those same
years were 81, 117.7, 153.7, 179.5 and 181.6. Given those numbers,
both 2000 and 2001 look like peak years for the cycle.

Average quarterly sunspot numbers for 2001 were 147.3, 164.8, 170.4
and 198.1. Average quarterly solar flux for the same period was
164.4, 166.7, 175.5 and 219.1, so solar activity increased over the
year.

Over the past week, average sunspot numbers were up 13 points and
average solar flux was about the same. Sunday had unsettled
geomagnetic conditions, probably from a flare on Friday. Friday's
flare upset the 10.7 cm receiver at the Penticton observatory, which
read a solar flux of 655.6 for the day. This was adjusted downward
by NOAA SESC to 263.

Predicted solar flux for Friday through Monday is 220, 215, 210 and
210.

Carl Luetzelschwab K9LA has written another piece explaining basic
shortwave propagation principles, titled The Sun, the Earth, the
Ionosphere, What the Numbers Mean, and Propagation Predictions - a
brief introduction to propagation and the major factors affecting
it.

Here it is:

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 400km 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.

Additionally, 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 (it spirals around
the magnetic field line at the electron gyrofrequency). Thus the
Earth's magnetic field plays an important and critical role in
propagation. 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 is also rather spiky when
plotted. Thus a more averaged, or smoothed, measurement is needed to
measure solar cycles. This is the smoothed sunspot number (SSN).
The SSN is calculated using 6 months of data before and 6 months of
data after the desired month, plus the data for the desired month.
Because of this amount of smoothing, the official SSN is one half
year behind the current month.

Sunspots come and go in an approximate 11-year cycle. The rise to
maximum (4 to 5 years) is usually faster than the descent to minimum
(6 to 7 years). At and near the maximum of a solar cycle, the
increased number of sunspots causes more ultraviolet radiation to
impinge on the atmosphere. This results in significantly more F
region ionization, allowing the ionosphere to refract higher
frequencies (15, 12, 10, 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 and 80 meters). Thus in
general 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 solar 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 minimal impact to propagation. Class M and X flares can
have a progressively adverse impact to propagation. The
electromagnetic radiation from a class X flare in the 1-8 Angstrom
range can cause the loss of all propagation on the sunlit side of
the Earth due to increased D region absorption. Additionally, big
class X flares can emit very energetic protons that are guided into
the polar cap by the Earth's magnetic field. This can result 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 a large amount 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. If the
polarity of the Sun's magnetic field is southward when the shock
wave hits the Earth's magnetic field, the shock wave couples into
the Earth's magnetic field and can cause large variations in the
Earth's magnetic field. This is seen as an increase in the A and K
indices. In addition to auroral activity, these variations 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. Most of the time elevated A and K
indices reduce MUFs, but occasionally MUFs at low latitudes may
increase (due to a complicated process) when the A and K indices are
elevated.

Solar flares and CMEs are related, but they can happen together or
separately. Scientists are still trying to understand the
relationship between them. 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. Unfortunately we
detect the flare visually at the same time as the radio blackout
since both the visible light from the flare and the electromagnetic
radiation in the 1-10 Angstrom range from the flare travel at the
speed of light - in other words, we have no warning. On the other
hand, the energetic particles ejected from a flare can take up to
several hours to reach Earth, and the shock wave from a CME can take
up to several days to reach Earth, thus giving us some warning of
their impending disruptions.

Each day the Space Environment Center (a part of NOAA, the National
Oceanographic and Atmospheric Administration) and the US Air Force
jointly put out a Solar and Geophysical Activity Report. The current
and archived reports are at http://sec.noaa.gov/Data/near-earth.html
in the 'Daily or less' section in the 'Solar and Geophysical
Activity Report and 3-day Forecast' row. 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 first three 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 CMEs near the average of 400km/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 last 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 or 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. As in the Solar and
Geophysical Activity Report, normal propagation (no disturbances) is
expected when the solar activity is low and the geomagnetic field is
quiet. A comment is appropriate here - both the Solar and
Geophysical Activity Report and WWV give a status of solar activity.
This is not a status of the 11-year sunspot cycle, but rather a
status on solar disturbances (flares and CMEs). For example, if the
solar activity is reported as low, that doesn't mean we're at the
bottom of the solar cycle - it 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 over a month
time frame tied to them. They are not absolutes - they are
statistical in nature. 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. The 10.7 cm solar flux has
become this measurement. But it is only a general measure 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 10.7 cm solar flux and sunspots is the smoothed 10.7 cm
solar flux and the 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 predictions 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. To reiterate, for best results use SSN or
smoothed 10.7 cm solar flux, and understand the concept of monthly
median values. If there were a good correlation between what the
ionosphere is doing today and today's solar flux (or today's sunspot
number), then we'd have a daily propagation model as opposed to a
monthly median propagation model.

Sunspot numbers for December 27 through January 2 were 268, 263,
222, 218, 209, 222 and 241 with a mean of 234.7. 10.7 cm flux was
274.6, 263, 264.4, 246.6, 245.6, 232.2 and 231.1, with a mean of
251.1, and estimated planetary A indices were 6, 5, 10, 17, 11, 7
and 7 with a mean of 9.
NNNN
/EX