don't I hear anything?
Alerts tells you all about it
This User's Guide is primarily based on information made available
by the United States National Oceanic and Atmospheric Administration's
Space Environment Services Center in Boulder, Colorado. The Geophysical
Alert Broadcasts are primarily intended for users in North America
and the Pacific since that region is where radio stations WWV
and WWVH are best received. However, data contained in the broadcasts
are useful worldwide and propagation conditions can be such that
either or both of these stations can be received surprisingly
well throughout the planet.
would like to express our thanks and appreciation to the people
at the Space Environment Services Center for their cooperation
and assistance in the production of this User's Guide.
Comments and suggestions concerning this User's Guide only are
always welcome direct to the author:
David A. Rosenthal
P. O. Box 1502
Ridgecrest, CA 93556
U S A
One of the handiest (and cheapest) methods to better understand
the current state of shortwave radio propagation conditions is
by monitoring the Geophysical Alert Broadcasts made at 18 minutes
past each hour over the U. S. National Institute of Standards
and Technology radio station WWV in Ft. Collins, Colorado and
at 45 minutes past each hour via WWHV on the island of Kauai in
Hawaii WWV broadcasts continuously on shortwave frequencies of
2.5, 5, 10, 15, and 20 MHz and WWVH broadcasts on 2.5, 5, 10 and
15 MHz. Their signals are audible throughout North America and
the Pacific Region and often the rest of the world, depending
upon radio propagation conditions.
Geophysical Alert Broadcasts outline the current nature of the
solar-terrestrial environment. They are produced by the National
Oceanic and Atmospheric Administration's Space Environment Services
Center (SESC). This center operates a worldwide network of sensors
which continuously observe conditions between the earth and the
sun. A listener familiar with the types of information presented
can gain a surprising amount of insight into how the natural phenomena
primarily responsible for long-distance HF radio communication
are currently affecting it at the moment as well as in the near
three-hours beginning at 0000 UTC, the Geophysical Alert Broadcasts
are concerned with two primary types of Earth-sun interaction:
electromagnetic radiation and geomagnetic activity (which includes
effects from solar sub-atomic particle emissions).The effects
of each are summarized below:
The sun's electromagnetic spectrum is a continuum of radiation
spanning not only infrared, visible, and ultraviolet wavelengths,
but the radio portions, x-rays and beyond. Sensors on the Earth
and in space continuously observe specific portions of the sun's
energy spectrum to monitor their levels and give scientists indications
of when significant events occur.
in this category are all electromagnetic in nature, that is, they
move at the speed of light. Events detected on the sun in these
wavelengths begin to affect the Earth's environment around 8 minutes
after they occur.
In addition to electromagnetic radiation, the sun constantly ejects
matter in the form of atomic and subatomic particles. Consisting
typically of electrons, protons, and helium nuclei, this tenuous
gas is accelerated to speeds in excess of the sun's gravitational
escape velocity and thus moves outward into the solar system.
The collective term for the gas and the particles making them
up is the Solar Wind. The sun's approximately 27-day rotation
period results in the clouds being slung outward in an expanding
spiral pattern which, at the earth-sun distance, overtakes the
earth from behind as it moves along in it's orbit.
As the clouds
encounter the earth, the geomagnetic field and the earth's atmosphere
prevents the solar wind particles from striking the planet directly.
Magnetic interactions between the clouds and the geomagnetic field
cause the solar wind particles to flow around the field, forming
a shell-like hollow with the earth at the center. The hollow,
known as the earth's Magnetosphere, is actually distorted
into a comet shape with the head of the comet always pointing
directly into the solar wind and the tail directly away. In the
absence of significant solar activity, the solar wind is uniform
with a velocity of approximately 400 km/second. Under these conditions,
the earth's magnetosphere maintains a fairly steady shape and
orientation in space.
occur on the sun, some clouds of solar particles can be blasted
away at tremendous velocities. As these higher speed solar particle
clouds encounter the earth's magnetosphere, they perturb it, changing
the intensity and direction of the earth's magnetic field. This
is analogous to a weather vane in gusty wind; sudden higher speed
gusts can strike it and cause it to move around. Moreover, changes
in solar wind density and velocity can cause the Earthís
surface and are referred to as a "sudden impulse" (SI).
activity, including solar particle-caused variations in the geomagnetic
field are carefully monitored by instruments both on the Earth
and in space. High levels of geomagnetic activity act indirectly
to degrade the ability of the ionosphere to propagate HF radio
signals. So they are of interest to users of that portion of the
radio frequency spectrum. Like the electromagnetic radiation portions
of the sun's output, geomagnetic activity comprises another family
of interactions observed and reported by groups such as IPS and
The Geophysical Alert Broadcasts consist of three primary sections
to describe the Solar-terrestrial environment: The most current
information, then a summary of activity for the past 24 hours,
and finally a forecast for the next 24 hours. The actual wording
of each section of the broadcast is explained below with a brief
description of what is being reported. Similar wording is also
used in other broadcasts, so the WWV example is relevant to other
"Solar-terrestrial indices for (UTC Date) follow: Solar flux (number)
and (estimated) Boulder A index (number) . Repeat, solar flux
(number) and (estimated) Boulder A index (number) . The Boulder
K index at (UTC time) on (UTC Date) was (number) repeat (number)
final A index is not available until 0000 UTC, the word "estimated"
is used for the 1800 and 2100 UTC announcements.
Solar Flux is a measurement of the intensity of solar radio emissions
at a frequency of 2800 MHz made using a radio telescope located
in Ottawa, Canada. Known also as the 10.7 cm flux (the
wavelength of the radio signals at 2800 MHz), this solar radio
emission has been shown to be proportional to sunspot activity.
In addition, the level of the sun's ultraviolet and X-ray emissions
is primarily responsible for causing ionization in the earth's
upper atmosphere. It is these emissions which produce the ionized
"layers" involved in propagating shortwave radio signals over
flux number reported in the broadcast is in solar flux units (s.
f. u.) and is recorded daily at Ottawa at 1700 UTC to be forwarded
to the SESC. Solar flux readings range from a theoretical minimum
of approximately 67 to actually-observed numbers greater than
300. Low solar flux numbers dominate during the lower portions
of the 11-year sunspot cycle, rising as the cycle proceeds with
the average solar flux a fairly reliable indicator of the cycle's
long-term behavior. 1 s. f. u. = 10-22Watts/meter2 Hz = 104 jansky.
The A index is an averaged quantitative measure of geomagnetic
activity derived from a series of physical measurements. Magnetometers
measure differences between the current orientation of the magnetosphere
and compare it to what it would be under "quiet" geomagnetic conditions.
But there is more to understanding the meaning of the Boulder
A index reported in the Geophysical Alert Broadcasts. The Boulder
A index in the announcement is the 24 hour A index derived from
the eight 3-hour K indices recorded at Boulder. The first estimate
of the Boulder A index is at 1800 UTC. This estimate is made using
the six observed Boulder K indices available at that time (0000
to 1800 UTC) and the SESC forecaster's best prediction for the
remaining two K indices. To make those predictions, SESC forecasters
examine present trends and other geomagnetic indicators. At 2100
UTC, the next observed Boulder K index is measured and the estimated
A index is reevaluated and updated if necessary. At 0000 UTC,
the eighth and last Boulder K index is measured and the actual
Boulder A index is produced. For the 0000 UTC announcement and
all subsequent announcements the word "estimated" is dropped and
the actual Boulder A index is used.
The underlying concept of the A index is to provide a longer-term
picture of geomagnetic activity using measurements averaged
either over some time frame or from a range of stations over the
globe (or both). Numbers presented as A indices are the result
of a several-step process: first, a magnetometer reading is taken
to produce a K index for that station (see K index below); the
K index is adjusted for the station's geographical location to
produce an a index (no typographical error here, it is
a small case "a") for that 3-hour period; and finally a collection
of a indices is averaged to produce an overall A index for the
timeframe or region of interest.
A and a
indices range in value from 0 to 400 and are derived from K-indices
based on the table of equivalents shown in the Appendix.
The K index is the result of a 3-hourly magnetometer measurement
comparing the current geomagnetic field orientation and intensity
to what it would have been under geomagnetically "quiet" conditions.
K index measurements are made at sites throughout the globe and
each is carefully adjusted for the geomagnetic characteristics
of its locality. The scale used is quasi logarithmic, increasing
as the geomagnetic field becomes more disturbed. K indices
range in value from 0 to 9.
In the Geophysical
Alert Broadcasts, the K index used is usually derived from magnetometer
measurements made at the Table Mountain Observatory located just
north of Boulder, Colorado. Every 3 hours new K indices are determined
and the broadcasts are updated.
Conditions for the
past 24 hours
"Solar-terrestrial conditions for the last 24 hours follow:
Solar activity was (Very low, Low, Moderate, High, or Very high)
, the geomagnetic field was (Quiet, Unsettled, Active, Minor storm,
Major storm, Severe storm) ."
Solar activity is a measure of energy releases in the solar atmosphere,
generally observed by X-ray detectors on earth-orbiting satellites.
Somewhat different from longer-term Solar Flux measurements, Solar
Activity data provide an overview of X-ray emissions that exceed
prevailing levels. The five standard terms listed correspond to
the following levels of enhanced X-ray emissions observed or predicted
within a 24-hour period:
events less than C-class.
(1 to 4) M-class x-ray events.
(5 or more) M-class x-ray events, or isolated (1 to 4) M5
or greater x-ray events.
(5 or more) M5 or greater x-ray events.
event classes listed correspond to a standardized method of classification
based on the peak flux of the x-ray emissions as measured by detectors.
Solar x-rays occupy a wide range of wavelengths with the portion
used for flare classification from 0.1 through 0.8 nm. The classification
scheme ranges in increasing x-ray peak flux from B-class events,
through C- and M-class, to X-class events at the highest end (see
In the Geophysical
Alert Broadcasts, solar activity data provides an overview of
x-ray emissions which might have effects on the quality of shortwave
radio propagation. Large solar x-ray outbursts can produce sudden
and extensive ionization in the lower regions of the earth's ionosphere
which can rapidly increase shortwave signal absorption there.
Occurring on the sun-facing side of the Earth, these sudden
ionospheric disturbances are known as "shortwave fadeouts"
and can degrade short wave communications for from minutes to
characterized by the initial disappearance of signals on lower
frequencies with subsequent fading up the frequency spectrum over
a short period (usually less than a hour). Daytime HF communication
disruptions due to high solar activity are more common during
the years surrounding the peak of the solar cycle. The sun rotates
once approximately every 27 days, often carrying active regions
on its surface to where they again face the Earth; periods of
disruption can recur at about this interval as a result.
Thumb: The higher the solar activity, the better the conditions
on the higher frequencies (i.e. 15, 17, 21, and 25 MHz). During
a solar X-ray outburst, the lower frequencies are the first to
suffer. Remember too that that signals crossing daylight paths
will be the most affected. If you hear announcements on broadcast
radio stations (e.g. Radio Netherlands) or via WWV/WWVH of such
a solar disturbance try tuning to a HIGHER frequency. Higher frequencies
are also the first to recover after a storm. Note that this is
the opposite to disturbances indirectly caused by geomagnetic
As an overall assessment of natural variations in the geomagnetic
field, six standard terms are used in reporting geomagnetic activity.
The terminology is based on the estimated A index for the 24-hour
period directly preceding the time the broadcast was last updated:
terms correspond to the range of a and A indices previously explained
in the A INDEX section. Increasing geomagnetic activity corresponds
to more and greater perturbations of the geomagnetic field as
a result of variations in the solar wind and more energetic solar
Using the earlier analogy, imagine the geomagnetic field to be
like a weather vane in an increasingly violent windstorm. As the
winds increase, the weather vane is continually buffeted by gusts
and oscillates about the direction of the prevailing wind. Essentially,
the reported geomagnetic activity category corresponds to how
violently the geomagnetic field is being knocked about.
For shortwave radio spectrum users, high geomagnetic activity
tends to degrade the quality of communications because geomagnetic
field disturbances also diminish the capabilities of the ionosphere
to propagate radio signals. In and near the auroral zone, absorption
of radio energy in the ionosphereís D region (about 80
km high) can increase dramatically , especially in the lower portions
of the HF band. Signals passing through these regions can become
disturbances in the middle latitudes can decrease the density
of electrons in the ionosphere and thus the maximum radio frequency
the region will propagate. Extended periods of geomagnetic activity
known as geomagnetic storms can last for days. The impact
on radio propagation during the storm depends on the level of
solar flux and the severity of the geomagnetic field disturbance.
geomagnetic storms, worldwide disruptions of the ionosphere are
possible. Called ionospheric storms, short wave propagation
via the ionosphereís F region (about 300 km high) can be
affected. Here, middle latitude propagation can be diminished
while propagation at low latitudes is improved. Ionospheric storms
may or may not accompany geomagnetic activity, depending on the
severity of the activity, its recent history, and the level of
the solar flux.
thumb: Oversimplification is dangerous in the complex field
of propagation. We know much less about the "radio weather" than
ordinary weather. In general though, for long distance medium-wave
listening, the A index should be under 14, and the solar activity
low-moderate. If the A-index drops under 7 for a few days in a
row (usually during sunspot minimum conditions) look out for really
excellent intercontinental conditions (e.g. trans Atlantic reception).
geomagnetic storms, signals from the equatorial regions of the
world are least affected. On the 60 and 90 metre tropical bands
you can expect interference from utility stations in Europe/North
America/Australia to be lower. Sometimes, this means that weaker
signals from the tropics can get through, albeit they may suffer
fluttery fading. Signals on the higher frequencies fade out first
during a geomagnetic storm. Signals that travel anywhere near
the North or South Pole may disappear or suffer chronic fading.
Forecast for the
next 24 hours
for the next 24 hours follows:
Solar activity will be (Very low, Low, Moderate, High, or Very
The geomagnetic field will be (Quiet, Unsettled, Active, Minor
storm, Major storm, Severe storm)."
The quantitative criteria for the solar activity forecast are
identical to the "Conditions for the past 24 hours" portion of
the broadcast as explained previously except that the forecaster
is using all available measurement and trend information to make
as informed a projection as possible. Some of the key elements
in making the forecast include the number and types of sunspots
and other regions of interest on the sun's surface as well as
what kinds of energetic events have occurred recently.
six standardized terms are used as in the "Conditions for the
past 24 hours" portion of the broadcast with the forecast mainly
based on current geomagnetic activity, recent events on the sun
whose effects could influence geomagnetic conditions, and longer-term
considerations such as the time of year and the state of the sunspot
Information by phone
The Geophysical Alert Broadcast is also available as a telephone
recording. This is useful if you cannot hear WWV or WWHV on the
air due to poor propagation. The message can be reached by calling
+1 303 497 3235. This message lasts about 40 seconds, and is updated
every 3 hours. If you're trying some long distance listening,
and neither WWV nor WWVH are audible in your area, the short call
to Boulder may save a lot of time. At off-peak rates it can be
very economical, even long-distance.
a similar service is offered by the Ionospheric Prediction Service
near Sydney. Their propagation message is available free of charge
by dialing +61 22 69 86 14.
Information on the
Chances are you have access to the Web. In that case, current
solar activity is available on-line. Check our Hitlist for details
since this is kept up to date.
A 3-hourly "equivalent amplitude" of geomagnetic activity for
a specific station or network of stations expressing the range
of disturbance of the geomagnetic field. The a index is scaled
from the 3-hourly K index according to the following table:
flare class. Ranking of a flare based on its x-ray output.
Flares are classified according to the order of magnitude of the
peak burst intensity (I) measured at the earth in the 0.1 to 0.8
nm wavelength band as follows:
0.1 to 0.8 nm band
I < 10-5
I < 10-4
is used to indicate the level within each class. For example:
M6 = 6 X
10-5 Watts/square metre
If you have
any further specific questions about propagation, here are some
further sources of information:
Space Environment Services Center
Boulder, Colorado 80303
U S A
IPS Radio and Space Services,
P.O. Box 702,
Darlinghurst NSW 2010