Predicting the Terrestrial Impact of Solar Flares

(Originally published in QST Canada, 1992)

Paul M Dunphy, VE1DX
3351 #7 Highway,
Lake Echo, NS
B3E 1C6

Most amateurs are aware of the solar terrestrial indices that are broadcast by radio station WWV at 18 minutes past each hour. Following these numbers, there is a prediction of solar and geomagnetic activity for the next 24 hours. Most of us, at one time or another, have listened to the computerized voice at 10, 15 or 20 MHz say something like "Solar terrestrial indices for 16 March: The solar flux was 169 and the K index was 10 . . . the estimated Boulder A index at 18:18 UTC was 3 . . . conditions for the next 24 hours follow: The solar flux will be low. The geomagnetic field will be quiet to unsettled . . ."

These short-term predictions, with a few exceptions, are usually fairly accurate. The 2800 MHz solar flux is by far the easiest. It is a measure of the slowly varying background radiation generated by the thermal processes occurring within sunspots. It is directly related to the number of sunspots groups on the earthward surface of the sun. The sun has a rotation period of about 28 days. Sunspot groups pass across the region of the sun facing earth in about 14 days. This rotation allows scientists to forecast with reasonable certainty where the more active sunspot regions will be and thus the intensity of the solar flux.

Geomagnetic activity is more difficult to forecast. Terrestrial magnetic storms are caused by solar flares. Solar flares are massive explosions in the sun's atmosphere over active sunspot groups. Certain sunspot groups have characteristics that make flaring likely, but it is difficult to predict when and if a flare will occur. Researchers use the location and types of radio emissions from these groups to make fairly accurate estimates on the probability of flare activity. If a flare occurs, accurate judgments of its impact on radio propagation and ionospheric conditions are also possible.

Flare Emissions
When the flare does takes place, it can be detected at earth within 8.3 minutes because the resultant electromagnetic radiation travels at near light speed. While flares do not normally produce enough light to be visible, but there is a rapid increase in X-ray and ultra-violet radiation. A large flare may cause a short term HF blackout due to abnormally high levels of ionization on the sunlit side of the earth.
Usually conditions return to normal within a few minutes. Somewhere between 15 minutes and several hours later flare ejected, high-energy, protons and alpha particles reach earth. These particles typically attenuate signals, but sometimes there is a short-term propagation enhancement during the first 12 to 18 hours after a flare. The serious geomagnetic impact is not seen at earth until 24 to 48 hours later. This is when the slower moving solar particles arrive. The quantity of solar material expelled during a flare and how much reaches earth are the reasons for the varied effect on the terrestrial magnetic field. Geomagnetic storms induced by a single flare usually only last 24 to 36 hours.

One of the observations scientists use to predict how severe the effects of a flare will be is its location on the sun's surface in relation to the earth/sun trajectory. If a sunspot region near the edges of the solar disk (the portion of the sun visible at earth) produces the flare, it is unlikely significant quantities of solar matter will reach earth. The ejected particles will, for the most part, be propelled into space and have little or no effect on terrestrial conditions. On the other hand, if the flare occurs near the center of the solar disk, a large number of particles will reach earth and upset the geomagnetic field. Scientists can predict the potential impact of a flare by calculating the angle at which it occurs relative to earth. They can often determine low, moderate or high particle interaction simply by the flare's location. This prediction can be made soon after the flare has been detected.

Intensity and Duration
Equally important to the flare's location is the duration and intensity of the explosion. Typical flares last only a few minutes, usually not longer than a half an hour. Devastating rogue flares, identified as a class X proton type, may last many hours. The amount of energy contained in the solar particles when they reach earth will determine how severe the effect. Stations located worldwide are continually monitoring the number of particles that enter the atmosphere, how deeply they penetrate, and how quickly the ionosphere is disturbed. Particles that travel the distance between the sun and earth in 24 hours or less are capable of producing substantial terrestrial effects. Scientists can therefore judge how conditions will progress by observing the duration of the flare and monitoring how long it takes the solar particles to reach earth. A flare that lasts only a few minutes is not likely to cause much of a change in the earth's magnetic field. If solar particles are not detected until 36 or more hours after the flare, they are also unlikely to cause significant geomagnetic effects. On the other hand, a flare that lasts an hour or more and generates increased particle activity at earth within 24 hours is likely to result in magnetic disturbances, auroras, sporadic-E and polar cap absorption.

Solar Radio Emissions
Yet another way scientists predict the intensity of solar flares and their potential terrestrial impact is by monitoring solar radio waves. While it may take solar particles up to two days to reach earth, changes in certain solar radio emissions are observed before, during and shortly after a flare.

The sun emits radio waves across the entire spectrum. Some of these emissions are associated only with flares. The observation of when they occur, how long they last, and how they change with time is perhaps one of the better ways to predict and monitor flare activity. There are two classes of radio emissions that are particularly significant. They are sweep frequency events and noise bursts. A sweep frequency event is a term used to describe the radio emissions as observed at earth. These emissions consist of intensified regions of the radio spectrum that drift from higher to lower frequencies. A very strong correlation exists between certain types of sweep frequency events, noise bursts and post flare geomagnetic activity. By calculating the rate of sweep frequency events and the strength of related noise bursts, the severity of terrestrial effects can be predicted with moderate accuracy. The lag time between radio emission variation and magnetic storms can allow scientists to make their forecasts 2 to 3 days in advance.

Computer Analysis
We live in an age of computers with essentially unlimited data storage and computing ability. As a result, researchers are able to save and analyze solar data, including flare characteristics and conditions that precede and follow them. They continue to improve computerized models of flare morphology by using data from previous flares to predict their hypothetical terrestrial impact. These results are compared with the actual observations and computer software is enhanced accordingly.

As software and computer technology improves, so will the accuracy of these predictions. Statistical comparison of the behavior of complex sunspot groups with similar historical data will become the forecaster's most valuable technique in predicting when and if a flare will occur. Similarly, mathematical modeling of post flare solar radio emissions and related observations will improve the accuracy of magnetic storm prediction.

The methods mentioned above imply that solar flare induced terrestrial magnetic activity can be predicted with a great deal of certainty. This would be the case if flare occurrences were infrequent. This is not so, particularly in the years around the solar maximum. We sometimes see ongoing sequences of flares with little or no interspersed quiet periods. There are frequently several magnetically complex sunspot regions on the solar disk at the same time. These regions can all have the potential for major flare production. When several flares occur within a short time, prediction of their terrestrial effects becomes more difficult. Nonetheless, the reliability of the predictions broadcast by WWV can be quite high. As we have discussed, they are the result of thorough observation and analysis.

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