WWV Geophysical Alert Broadcasts are given at 2.5, 5, 10, 15, and 20 MHz at 18 minutes after the hour. The alerts give current conditions for long distance HF propagation. There are three sections to the alerts: Current Information, a Summary of Activity

Current Information

WWV will announce as follows: "Solar-terrestrial indices for (UTC Date) follow: Solar flux (number) and Boulder A-Index (number). The Boulder K-Index at (UTC Time) on (UTC Date) was (number)."

Solar Flux is a measurement of intensity of solar radio emissions at 2800 MHz made using a radio telescope located at Ottawa, Canada. Solar flux is proportional to sunspot activity, and is responsible for causing ionization in the earth's upper atmosphere. Solar flux readings range from a theoretical minimum of approximately 67 to actually-observed numbers greater than 300. The higher numbers occur during the years surrounding the peak of the 11 year sunspot cycle.

The A-Index is an averaged measure of geomagnetic activity derived from a series of physical measurements using magnetometers. These physical measurements are made every 3 hours and are called the K-Index. K-Index values range from 0 to 9, and follow a quasi logarithmic scale. the K-Index is adjusted for the station's geographical location (Boulder, CO) to produce an a-Index whose value ranges in value from 0 to 400. Finally, the A-Index is determined by averaging the past 8 hours of a-Index values, such that the A-Index has a value from 0 to 400 also.

Conditions for the Past 24 Hours

WWV will announce as follows: "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 in the solar atmosphere, observed by X-ray detectors on earth-orbiting satellites. Classification of solar activity is as follows:

These classes range from B, C, M, and X, with the X-class having the highest x-ray peak flux.

Solar activity data gives an overview of x-ray emissions which might have effects on the quality of shortwave radio reception. Large solar x-ray outbursts can produce extensive ionization in the lower regions of the ionosphere, which can quickly cause the ionosphere to absorb HF signals for minutes to hour at a time. Initially, the lower frequencies fade out, then the higher frequencies fade out within a period of time usually less than an hour. Daytime HF communication disruptions due to high solar activity are more common during the years surrounding the peak of the solar cycle.

The higher the solar activity, the better the conditions on the higher frequencies (15, 17, 21, and 25 MHz. If WWV indicates a high solar disturbance, try listening to higher frequencies, since they are the first to recover after a stornm.

Geomagnetic Field Activity is categorized as follows:

High geomagnetic activity tends to degrade the quality of communications.

For long distance medium-wave listening, the A-Index should be under 14 and the solar activity low to moderate. If the A-Index falls under 7 for a few days in a row (usually during sunspot minimum conditions) look for really excellent intercontinental conditions. If the A-Index is less than 10 and the K-Index is around 3, nearly ideal HF conditions exist.

Signals on the higher frequencies fade out firsts during a geomagnetic storm.

Forcast for the Next 24 Hours

WWV will announce as follows: "Solar activity will be (Very Low, Low, Moderate, High, or Very High). The geomagnetic field will be (Quiet, Unsettled, Active, Minor Storm, Major Storm, Severe Storm)."

The solar activity forecast is made considering the number and types of sunspots and other regions of inteest on the sun's surface, as well as the kinds of energetic events that have occurred recently.

The Geomagnetic activity forecast is 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 cycle.

For a detailed account of NIST activities, including WWV information see the following website:

http://tf.nist.gov/timefreq/general/pdf/1383.pdf