|
||||
|
     |
|
|
|
Solar Cycle 23
INTRODUCTION Solar Cycle 23, which began in May 1996, reached its peak in 2000 and is now beginning to tail off, although we are still in the "solar maximum" period, which spans about 4 years and includes the actual maximum in terms of sunspot number. The highest recorded monthly sunspot number, 170.1, was close to the original prediction although Cycle 23 was less active overall than expected. Cycle 23 will continue until the next minimum sometime in 2006 when Cycle 24 should begin Importance of Solar Cycle Prediction Predicting the activity level of a solar cycle is very important for many reasons. An active sun can cause geomagnetic storms that disrupt communications and power systems on Earth. In particular, planning for space missions requires a knowledge of expected solar activity levels years in advance. The total radiation dose that a satellite receives depends to a significant extent on the charged particles and electromagnetic radiation emitted from the sun. One large solar flare can permanently reduce the solar array power by 5%, even as much as 10%. Both NIMIQ-1 and ANIK-F1 were designed to operate throughout Solar Cycle 23 and into part of Cycle 24, as will be NIMIQ-2 AND ANIK-F2. Historical Background Galileo made the first observations on sunspots in the Western Hemisphere in 1610, shortly after he started observing the sun with his new telescope. Records of visual observations of solar activity exist from 1700. Daily observations were started at the Zurich Observatory in 1749 and, with the addition of other observatories, continuous observations were recorded starting in 1849. For a plot of solar cycles since 1700, see Fig. 1. The solar cycle numbering system comes from a scheme that dates from the mid-19th century, following the creation in 1848 of the " sunspot number" concept of measuring solar activity, by Rudolf Wolf of the Zurich Observatory. Wolf's sunspot number (now called the International sunspot number or the Zurich number) represents a blend of actual numbers of individual spots and numbers of groups of spots on the sun. On average, the sunspot number varies from a minimum through a maximum to the next minimum in approximately 11 years. Because the solar magnetic fields reverse at the peak of each 11-year cycle, each solar activity cycle actually spans a 22-year "Hale cycle." Cycle 23 is the last half of the current Hale cycle (composed of Cycles 22 and 23) that began in 1986. Overall, the sun is hotter during solar maximum than during the minimum, although sunspots themselves are darker (and thus cooler) than the surrounding visible surface of the sun. They are associated with active regions that are strong emitters of ultraviolet light, X-rays, and ionized particles. The Earth's climatic variations may be linked to level of sunspot activity. The "Little Ice Age" corresponded with a 70-year period, 1645-1715 (the "Maunder minimum"), when very few sunspots were seen. Also, there is strong statistical correlation between current trends in surface temperatures to solar activity (Wilson, Journal of Geophysical Research (Atmospheres). The "space age" covers only the last four cycles, 19-22. Cycle 19 peaked at a sunspot number of 201 in 1958, just as the first satellites were being launched; so far, it holds the record for solar activity. Sunspot maximums for the other three cycles were 110 (Cycle 20), 164 (Cycle 21), and 158 (Cycle 22). For a plot of sunspot numbers for the four most recent solar cycles and the predicted shape/peak of Cycle 23, see Fig. 2. Solar Cycle Prediction Methodology A number of techniques are used to predict the amplitude of a Solar Cycle. Prediction of the maximum is fairly reliable once the cycle is underway (about 3 years after the minimum). Prior to that time the predictions are less reliable but nonetheless equally important. The size of a coming cycle maximum has been found to be related to the length of the previous cycle, the level of activity at solar minimum, and the size of the previous cycle. Among the most reliable techniques are those that use the measurements of changes in the Earth's magnetic field at, and before, solar minimum. Of these "geomagnetic precursor" techniques, three are predominant. The earliest is from Ohl and Ohl (Solar-Terrestrial Predictions Proceedings, Vol. II. 258, 1979), who found that the value of the geomagnetic 'aa' index at its minimum was related to the sunspot number during the ensuing maximum. The primary disadvantage of this technique is that the minimum in the geomagnetic aa index often occurs slightly after sunspot minimum so the prediction is not available until the sunspot cycle has started. Dr. Joan Feynman of NASA JPL developed a second method, by separating the geomagnetic aa index into two components. A part of the aa index is directly proportional to sunspot number. This component varies directly with the current solar cycle and is related to sporadic events like solar flares and prominences. The maximum in this component occurs at sunspot minimum and is proportional to the sunspot number during the following maximum. This method allows for a prediction of the next sunspot maximum at the time of solar minimum. A third method is due to Richard Thompson (Solar Physics 148, 383, 1993), who found a relationship between the number of days during a sunspot cycle in which the geomagnetic field was "disturbed" and the amplitude of the next sunspot maximum. This method has the advantage of giving a prediction for the size of the next sunspot maximum well before sunspot minimum. NASA's Marshall Space Flight Center employs these three methods, in combination with several others, to predict the size and shape of the sunspot cycle using a technique that weights the different predictions according to their reliability. They predicted a maximum sunspot number of 168 ±15 for Cycle 23, which agrees very well with the actual maximum of 170.1. For the latest data on Cycle 23, plotted against the prediction, see Fig. 3. Note that the prediction is a "smoothed" curve, whereas the actual month-to-month variation in actual sunspot number is quite wide. This variation is normal. It can be seen that, on average, the actual sunspot number was below the original prediction. An updated prediction was issued in January 2000. How the Satellite Design Accounts for Solar Cycles/Flares Accounting for radiation damage over a satellite's mission life, particularly deterioration in the solar array power-generating capacity, is a fundamental design requirement. Although the onset and peak of a solar cycle can be predicted to the nearest year or two, the occurrence of sunspots, and especially solar flares, can not. The total dose delivered over a solar cycle is dominated by a small number of very high fluence events. The number of these events in a given cycle is not constant; they are more frequent in some cycles than others, but seldom occur in the quiet portion. Thus, although there may be unpredictable drops in array output from time to time, there must be complete assurance that the total drop due to solar flares over a given solar cycle will not be more than a certain amount, which is allowed for in the power budget. The data base for the solar proton environment is limited to the last four solar cycles (19-22). Various sources have used this data base to model the solar flare/proton environment. JPL have the most up-to-date model, which was published in 1993 and uses data from 1963-91. Goddard Space Flight Center produced an earlier model from cycle 20 which is still widely used, and they have also analyzed cycles 20-22 statistically. There is a wide variation of solar flare/proton events over these four cycles, as determined by different sources, partly because differing criteria can be used to define an event. There is better agreement on the integral (cumulative) flux over each cycle. The correlation between sunspot activity and flare production, although not random, is not definitive. For this reason, currently available solar flare proton predictors, such as that published by JPL (i.e. the JPL'91 model, which is used by Telesat), do not depend on sunspot number to derive a dose prediction. Instead, they provide an expected total (integrated) dose for a given mission duration within a certain probability. As far as protection is concerned, it is not practical within the capability of present technology to provide complete protection against solar flare protons. Solar cells are partially shielded by cover glass, which cuts off the low energy end of the proton spectrum, but it is impractical to shield against all energy levels because the weight penalty would be too high. Electronic equipment includes 50% margin above the nominal lifetime dose when analyzing circuits for worst-case operation. This margin is more than adequate to compensate for any uncertainties in the space environment. Grateful acknowledgment to NASA Goddard and Marshall Space Flight Centers for sunspot historical data and Cycle 23 prediction, as well as some of the above material.
|
|
|
Send E-mail to
TSN@The-Saudi.Net with questions or
comments about The Saudi Network.
We are Looking for Business Sponsorship or Marketing Partnership |
