Geophysical Alerts

One of the methosd to obtain information about the propagation conditions respect to the variations in the solar activity is through the so-called geophysical alerts. These alerts are updated each three hours, being transmited via the time signal stations WWV and WWVH, respectively at the minutes 18 and 45 each full hour, with each transmission lasting 45 seconds.

Geophysical alerts present three basic sets of data that allow the description of the solar-terrestrial environment: the most recent information, a summary of activity for the the past previous 24 hours, and a forecast for the next 24 hours. Next, the basic layout of the transmisisons is presented, as well the meaning ofthe parameters and terms adopted. Os alertas geofísicos apresentam três conjuntos de dados que permitem a descrição do ambiente solar-terrestre: a informação mais recente, um resumo da atividade nas 24 horas anteriores, e uma previsão para as 24 horas seguintes. A seguir, são é apresentado o formato básico das transmissões, assim como o significado dos parâmetros e termos utilizados.

Layout of the transmission, for each set of data

• Current information:

  "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]."

• Activity in the past 24 hours:

  "Solar activity was {very, low, moderate, high, very high}, the geomagnetic field was {quiet, unsettled, active, minor storm, major storm, severe storm}."

• Forecast for the next 24 hours:

  "Solar activity will be {very, low, moderate, high, very high}, the geomagnetic field will be {quiet, unsettled, active, minor storm, major storm, severe storm}."

Solar Flux, A-Index, K-Index

The data that refers to the current information are the solar flux index, the A index, the K index, besides the current UTC time and date. Higher values of the solar flux and lower values of A e K indicate the best conditions, typically.

Solar Flux

Measure of the intensity of solar radio emisisons at a frequency of 2.8 GHz (or 10.7 cm, in wavelenghts, then known also as the 10.7cm Radio Flux). 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 responsible by the ionization of the upper layers of the ionosphere, resulting in the ionized layers involved in the long-distance radio propagation. The solar flux is expressed in terms of solar flux units (s.f.u.) and its range varies from a theoretical minimum of approximately 67 to observed values greather than 300. Low values dominate the lower portions of the 11-year suspot cycle, rising as the cycle proceeds, being the average solar flux a good estimative of the long-term behavior of this cycle. A solar flux unit equals to 10^-22 W/m² /Hz.

A Index

Averaged quantitative measure of the geomagnetic activity from a series of physical measures. The differences between the current orientation of the magnetosphere and the orientation under "quiet" geomagnetic conditions are compared by magnetometers. The A index provides a long-term picture of the geomagnetic activity by using measures averaged either over a certain time frame or from stations over the terrestrial globe, or both. Thevalues of the A index are obtained in several steps. Initiallty, a magnetometer gives a K index value for the station. The K index is adjusted according the geographic localization of the station in order to produce a a index for a period of three hours. Finally, a overall A index is obtained from a collection of a indiices. The a index is obtained from the 3-hour K index according to the following table:

In the geophysical alerts it is announced the Boulder A index, whose value is the 24 hour A index obtained from the 3-hour K index recorded at Boulder. Its first estimative occurs at 18 UTC, by using the six Boulder K indices observed in the current day, and a forecast for the two remainig K indices. At 21 UTC the next observed K index is measured, and the estimated value of the A index is updated. It is used the expression "estimated" for the announcements at 18 UTC and 21 UTC, because the index value is estimated. For the 0 UTC announcement and all subsequent announcements the actual Boulder A index is used, until the announcement at 18 UTC, whena a new estimate is realized, and so on.

K Index

It results from the comparison of the current intensity and orientation of the geomagnetic fiels with the values obtained in "quiet" conditions. The measures are realized by magnetometers for a 3-hour period, in sites through the world, being each measure adjusted according to the geomagnetic characteristics of its locality. The values of theK index vary from 0 to 9, in a quasi-logarithm scale, increasing as the geomagnetic fields becomes more disturbed:

Solar and Geomagnetic Activity

Both the conditions for the past 24 hours and the forecast for the next 24 hours are reported by terms that identify the solar activity status and the geomagnetic field status. The difference, in each status, is in the set of data used when determining the appropriate term. For example, for the forecast for the next 24 hours are used data regarding to the current activity, recent events, and the status of the sunspot cycle. Next, the meaning of each status, and of each term used.

Solar Activity

MEasure of the energy released in the solar atmosphere, usually observed by X-ray detectors located in earth-orbiting satellites. Different longer-term solar flux measures, the solar activity data provide a general view of the X-ray emisisons that exceeds certain levels.

The five terms used correspond to the following levels of the 24 hour observed (or predicted) X-ray emissions:

The solar activity data provide an overview of X-ray emissions which might have effects in the quality of the shortwave propagation. Large X-ray eruptions can produce sudden and extensive ionization in the lower regions of the ionosphere, that can increase rapidy the absortion of shortwave signals, leading to shortwave fadouts, mainly in the low frequencies.

A flare (resulting from a solar explosion), can be 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 wavelenght band. A multiplier is used in order to indicate the level in each class. For example: M6=6x10-5 W/m2. The following table shows the different classes in terms of the peak value.

class peak (W/m2) B I < 10-6 C 10-6 < I < 10-5 M 10-5 < I < 10-4 X I > 10-4

Geomagnetic Activity

The natural variations in the geomagnetic field can be evaluated by using a set of six standardized terms. These terms correspond to the variation of the A and a indices referred previously. The increase in the geomagnetic activity corresponds to greather perturbations ofthe geomagnetic field as a result of variations in the solar wind and more energetic solar particles emissions.

The six terms, determined according to the range of the 24 hour A index (presented in the right column) are presented below:

For shortwave radio emisisons, high geomagnetic activity tends to degrade the quality of communications because geomagnetic field disturbances all diminish the capabilities of the ionosphere to propagate radio signals. In and near the auroral zone, absorption of radio energy in the layer D of the ionosphere can increase dramatically, especially in the lower portions of the high-frequency spectrum. In the middle latitudes, the geomagnetic disturbances can decrease the density of electrons in the ionosphere as well the MUF (maximum usable frequency) value.

Extended periods of geomagnetic activity, known as geomagnetic storms, can last for days. The impact on rario propagation during the storm depends on the level od solar flux and the severith of the geomagnetic field disturbance. During some geomagnetic storms worldwide disruptions of the ionosphere are possible, being called ionospheric storms. Shortwave propagation via F layer of the ionosphere can be affected. The propagation in low latitudes can be improved, in contrast withe the propagation in middle latitudes. 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. During minor geomagnetic storms the radio signals coming from the equatorial regions are least affected., especially in the 60 and 90 metre tropical bands. Signals on the higher frequencies fade out first during a geomagnetic storm.

Other Sources of Information

There are other sources of information besides the WWV and WWVH transmissions. On the Web, there are several sites that gives the actual status of the solar activity and the geomagnetic activity. An example is the web site atividade geomagnética. Um exemplo é o sítio N3KL. A solar activity monitor, that can be included in any web page, gives the current solar conditions, in terms of the status of the solar X-ray and the geomagnetic field. Computer programs as RLDB (Radio Listener's Database) also provides the values of the solar flux and the A and K indices, collecting this information from the NOAA web site.

References:

• The Solar Guide: How to Use Geophysical Alert Broadcasts: on-line guide from Radio Netherlands web site, with information about how to interpret and to use the data available in the geophysical alerts transmissions.