By Russ Edmunds
Originally published on National
Radio Club web site
NOAA's Geophysical Alert Broadcasts are aired 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. The broadcast text is updated
every three-hours beginning at 0000 UTC. These broadcasts
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.
start by quoting the text of a WWV broadcast ( the text
of these broadcasts is also available on the Internet, but
more on that later ). Next, we'll look at the definitions
of the terms in italics. Some of these definitions are taken
from those provided on the website, albeit in some cases
with some editing:
indices for 4 January follow. Solar flux 175 and Boulder
A-index 11. The Boulder K-index at 1200 UTC on 5 January
was 0. Solar-terrestrial conditions for the last 24 hours
follow. Solar activity was low. The geomagnetic field was
quiet to active. The forecast for the next 24 hours follows.
Solar activity will be low to moderate. The geomagnetic
field will be quiet to unsettled."
Flux. 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. These emissions
have been shown to be proportional to sunspot activity,
and are also responsible for causing ionization in the earth's
upper atmosphere. These emissions produce the ionized 'layers'
involved in propagating radio signals over long distances.
Solar flux is reported in solar flux units (s. f. u.), which
range from a theoretical minimum of approximately 67 to
greater than 300.
index. A 3-hourly "equivalent amplitude" index of local
geomagnetic activity; "Ap" is used to designate an equivalent
planetary value based upon multiple local-site readings.
The "a" is derived from the 3-hourly K-index.
index. A 3-hourly quasi-logarithmic local index of geomagnetic
activity relative to an assumed quiet-day curve for the
recording site. Range is from 0 to 9.
Activity. Solar activity is a measure of energy releases
in the solar atmosphere, generally observed by X-ray detectors
on earth-orbiting satellites. Such energy is that which
can or might be anticipated to cause disruptions or degradations
to radio propagation on earth. Large solar x-ray outbursts
can produce sudden and extensive ionization in the lower
regions of the earth's ionosphere, which can rapidly increase
signal absorption there. Occurring on the sun-facing side
of the Earth, these 'sudden ionospheric disturbances' (
SID's ) can degrade radio communications for from minutes
to hours. Since the sun rotates once approximately every
27 days, periods of disruption may recur at about this interval
as a result.
are some additional definitions included in these reports
as indicated by conditions at the time relating to Solar
Active. Solar activity levels with at least one geophysical
event or several larger radio events per day
Proton Flare. Any flare producing significant fluxes
of protons in the vicinity of the earth.
Quiet. Solar activity levels with less than one energetic
event per day.
Solar Flare. A sudden eruption of energy on the solar
disk lasting minutes to hours, from which radiation and
particles are emitted. This is one type of energetic event,
which can subsequently result in increased geomagnetic activity.
Field. The magnetic field which surrounds the earth
where auroral conditions may result from specific types
of solar activity.
Geomagnetic Acivity. 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.
Range of A-index
0 - 7
Unsettled 8 - 15
Active 16 - 29
Minor Storm 30 - 49
Major Storm 50 - 99
Severe Storm 100 - 400
what the previous broadcast told us would be: Solar flux
is about average, the A-index indicates average conditions.
The K-index of 0 suggests that conditions might be improving.
The sun was relatively inactive during the preceding 24
hours, which indicates that no major disturbances are likely
the following day, either. The geomagnetic field conditions
were variable during the preceding period. Predicted solar
activity continues to suggest no disturbances, although
the geomagnetic field will continue to be variable. These
are very ordinary conditions, with the immediate next few
days likely to show a slight improvement.
are some additional definitions of terms used in the broadcasts
as conditions warrant:
Solar Wind. This is the outward flow of solar particles
and magnetic fields from the sun. The velocity of the solar
wind often increases dramatically as a result of certain
solar energetic events. This in turn can spark increased
activity in the earth's geomagnetic field, or even aurora.
A daily index of sunspot activity.
a predictor of future conditions, remember that it takes
roughly three days for activity on the sun to create an
adverse effect on earth, and that the best Trans-Atlantic
DX conditions are produced by several successive days of
A indices below 8, and no values over 30 within the past
2-3 weeks. On the other hand, heavy activity on the sun
doesn't always lead to disturbed conditions. Disturbances
are less likely when they are produced by sunspots or sunspot
groups located near the edges of the sun's disk ( or face
we noted earlier, you can access the text of the current
transmission via the Internet as well as by broadcast. The
URL for this is
From this page, select the option for "Latest Geophysical
Alert Message - WWV Broadcast". More detailed information
can be found under the options for "Report of Solar & Geophysical
Activity" and "Solar & Geophysical Activity Summary".