Solar Activity

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:

Very Low X-ray events less than C-class. Low  C-class x-ray events. Moderate Isolated (1 to 4) M-class x-ray events. High Several (5 or more) M-class x-ray events, or isolated (1 to 4) M5 or greater x-ray events. Very High Several (5 or more) M5 or greater x-ray events.

The x-ray 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 APPENDIX).

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 hours.

They are 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.

Rule of 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 storms.

Geomagnetic Activity
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

These standardized 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 particle emissions.

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 ionosphereìs 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 unusable.

Geomagnetic 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.

During some geomagnetic storms, worldwide disruptions of the ionosphere are possible. Called ionospheric storms, short wave propagation via the ionosphereìs 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.

Rule of 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).

During minor 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

'The forecast for the next 24 hours 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).'

Solar Activity

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.

Geomagnetic Activity

The same 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 cycle.