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Geomagnetic activity broadcasts
By Russ Edmunds
Originally published on National Radio Club web site

The 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.

We'll 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:
"Solar-terrestrial 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."

Solar 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.

A 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.

K 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.

Solar 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.

Following are some additional definitions included in these reports as indicated by conditions at the time relating to Solar Activity:
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.
Geomagnetic 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.
Category Range of A-index
Quiet 0 - 7
Unsettled 8 - 15
Active 16 - 29
Minor Storm 30 - 49
Major Storm 50 - 99
Severe Storm 100 - 400

So, 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.

Below 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.
Sunspot Number. A daily index of sunspot activity.

As 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 ).

As 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".

 Solar stuff
Index guide
User guide to WWV broadcasts.
Index guide
One more guide on WWV broadcasts.
A-index clues
How to use A index effectively on MW.
MW Notes
Propagation notes for MW DXers.
Solar plotter
Get your own solar index plotter.

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