Thought many of you would enjoy reading this excellent introduction to the
very important topic of solar activity and propagation since it affects our
ham bands so much.
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Propagation 101
The Sun, the Earth, the Ionosphere: What the Numbers Mean, and Propagation
Predictions—a brief introduction to propagation and the major factors
affecting it.
By Carl Luetzelschwab, K9LA
[log in to unmask]
The sun emits electromagnetic radiation and matter as a consequence of the
nuclear
fusion process. Electromagnetic radiation at wavelengths of 10 to 100
nanometers
(extreme ultraviolet) ionizes the F region, radiation at 1 to 10 nanometers
(soft X-rays)
ionizes the E region, and radiation at 0.1 to 1 nanometers (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 Earth’s magnetic field. Instead
of being a
simple bar magnet, 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
ultimately 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 Earth’s
magnetic field plays an important and critical role in propagation.
Variations in Earth’s magnetic field are measured by magnetometers. There
are two
measurements readily available from magnetometer data--the daily A index and
the
three-hour K index. The A index is an average of the eight 3-hour K indices,
and 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 extreme 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 (monthly mean) sunspot number, but it is also rather
spiky when
plotted. Thus a more averaged, or smoothed, measurement is used to measure
solar
cycles. This is the smoothed sunspot number (R12). R12 is calculated using
six months of
data before and six months of data after the desired month, plus the data
for the desired
month. Because of this amount of smoothing, the official R12 is one-half
year behind the
current month. Unfortunately this amount of smoothing may mask any
short-term
unusual solar activity that may enhance (or hinder) propagation.
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
extreme
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 due to a quiet magnetic field, resulting in the
best propagation
on the lower frequencies (160 and 80 meters). Thus, in general, high
smoothed sunspot
numbers are best for high-frequency propagation, and low smoothed sunspot
numbers 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 0.1 to 0.8 nanometer range. X-ray flares are
classified by
magnitude 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 0.1 to 0.8
nanometer range can
cause the loss of all propagation on the sunlit side of 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 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 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 interplanetary magnetic field
is southward
when the shock wave hits Earth’s magnetic field, the shock wave couples into
Earth’s
magnetic field and can cause large variations in Earth’s magnetic field.
This is seen as an
increase in the A and K indices (elevated geomagnetic field activity).
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 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 Earth within about 10
minutes of 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 0.1 to 1
nanometer 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 in the
“Solar and
Geophysical Activity Report and 3-day Forecast section in the “Daily or less
section
under “Alerts and Forecasts at
http://sec.noaa.gov/Data/index.html. 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 and WWVH at 45 minutes past the
hour
every hour put out a shortened version of this report. A new format began
March 12,
2002. The new format gives the previous day’s 10.7-cm solar flux, the
previous day’s
mid-latitude A index, and the current mid-latitude three-hour K index. A
general
indicator of space weather for the last 24 hours and next 24 hours is given
next. This is
followed by detailed information for the three disturbances that impact
space weather:
geomagnetic storms (caused by gusts in the solar wind speed), solar
radiation storms (the
numbers of energetic particles increase), and radio blackouts (caused by
X-ray
emissions). For detailed descriptions of the WWV/WWVH messages, visit
http://sec.noaa.gov/Data/info/WWVdoc.html and
http://sec.noaa.gov/NOAAscales/.
Normal propagation (no disturbances) is expected when the space weather
indicator is
minor. A comment is appropriate here. Both the Solar and Geophysical
Activity Report
and WWV/WWVH give a status of general solar activity. This is not a status
of the 11year
sunspot cycle, but rather a status on solar disturbances (CMEs, particles,
and flares).
For example, if the solar activity is reported as low or minor, that doesn’t
mean we’re at
the bottom of the solar cycle; it means the sun has not produced any major
space weather
disturbances.
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
R12 (the smoothed sunspot number) 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 –”it is simply a proxy for the true ionizing radiation for each
region. 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
the smoothed sunspot number and monthly median ionospheric parameters, the
use of R12
or the equivalent smoothed 10.7-cm solar flux gives the best results. Using
the daily 10.7cm
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. To reiterate, for best results use the
smoothed sunspot
number or smoothed 10.7-cm solar flux, and understand the concept of monthly
median
values.
For short-term predictions, the use of the effective sunspot number (SSNe)
may be
helpful. In this method, an appropriate sunspot number is input to the
propagation
prediction software to force it to agree with daily ionosonde measurements.
Details of
this method can be found at
http://www.nwra-az.com/spawx/ssne24.html.
Dr. Ronald E. Milliman
Retired Professor of Marketing
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