The Sunspot Cycle

ES 767 Quaternary Geology

Bradley D Johnson

28 November 2011
Table of Contents
Abstract Definition of Sunspot
History of Sunspot Observations The Solar Cycle and Sunspots
Sunspots and Earth Changes Predictions
Conclusions References

Abstract

Dark, highly magnetized features known as sunspots which regularly appear on the Sun’s surface have been observed and recorded by civilizations for thousands of years. The development of the telescope in the early 17th century ushered in a new age of sunspot observations and paved the way for a nearly continuous record of spot activity for the last 400 years. The past few centuries have led to the progression of standards for observing sunspots and record keeping, as well as led to a greater understanding of the phenomenon. Contemporary research is focused on the forces which drive the 22-year solar cycle, composed of two 11-year sunspot cycles, and the possible effects these occurrences may be having on the Earth. Predicting the number of sunspots likely to occur during any given cycle has become critical to scientists concerned with understanding terrestrial consequences of solar activity and many methods have been created, combined and refined in attempt to predict sunspot activity.

Definition of Sunspot

Sunspots typically appear as a relatively small, dark and pore-like feature on the surface or 'photosphere' of the sun. The average size of a sunspot, when it first appears, is approximately 1600 kilometers in diameter (1000 miles) and the feature usually lasts for a few hours. Some sunspots, however, continue to grow and quickly become many times larger than the Earth and can last for more than a month. The sunspots which continue to grow and become the largest typically occur in pairs surrounded by several smaller spots. A black region, called an "umbra" (meaning shadow) is found at the center of all sunspots and is encircled or rimmed by a lighter region called the "penumbra" (meaning almost shadow).

In relation to the rest of the photosphere, sunspots are approximately 1500 degrees (K) cooler. This temperature difference gives the appearance of dark spots (cooler) on the sun's surface. Another characteristic of sunspots is their highly magnetized nature. When sunspots occur in pairs, each has an opposite magnetic pole and the north-south relationship developed by any given pair will be identical to all other sunspots located within the same hemisphere. Conversely, all sunspots in the opposite hemisphere will have the same magnetic relationship to each other, but opposite the relationship of those in the other hemisphere. (Tarbuck and Lutgens 2003).

Image 1.5: Grouping of sunspots. Image courtesy of SOHO@NASA

History of Sunspot Observations

Different cultures throughout human history have observed the sun in one form or another and for various lengths of time. The first known evidence for sunspot observations (naked-eye) date back to China in 28 BCE. Some literature suggests, however, the Greek philosopher Anaxagoras observed a spot in 467 BCE. Records for the West are often considered problematic, however, as the Aristotelian cosmology which dominated for centuries declared the heavens (universe) as perfect and unchanging. The Aristotelian cosmology greatly hindered solar studies and record keeping of a celestial body deemed to be perfect (Rice 1995). Not until the early 17th century, with the development of the telescope, did the "discovery" and acceptance of sunspots finally occur.

The first widely understood and more complete records date back to the early 1600s in Europe where astronomers Galileo Galilei, Thomas Harriot and later Johannes and David Fabricius and Christoph Scheiner used telescopes to make observations of sunspots in 1610 and 1611 respectively. Furthermore, observations made by astronomers Pierre Gassendi (France), Johannes Hevelius (Poland) and Giovanni Battista Riccioli (Italy), provide a record of sunspot activity through the years 1610-1645. From approximately 1645-1710 sunspot activity was thought to be nearly (if not completely) non-existent with one exception occurring in 1671 when a prominent spot was observed by many scientists. Following this period, known as the "Maunder Minimum" aptly named for one of the first modern astronomers Edward W. Maunder (1851 -1928), observations of sunspots continued with various record keeping systems until 1849 when the Zurich Observatory in Switzerland began taking continuous daily records of solar activity. The Royal Greenwich Observatory in England began similar observations of solar and sunspot activity in 1874 (NASASP 2011).

The two most prominent American astronomers to study the sunspot phenomenon included Edward W Maunder (mentioned above) and George Ellery Hale (1868- 1938), inventor and builder of the first spectro-heliograph and the Yerkes and Mount Wilson observatories (Rice 1995). Contemporary "official" record keeping of sunspots is performed by two at least two groups, the Solar Influences Data Analysis Center in Belgium and the National Oceanic and Atmospheric Administration in the United States, although this is not considered a comprehensive list (NASASP 2011).

The Solar Cycle and Sunspots

The Sun’s magnetic poles reverse approximately every 22 years. This polar reversal is known as the solar cycle and two sunspot cycles occur at 11 year intervals within each solar cycle. Sunspot cycles are marked by rhythmic fluctuations in the number of sunspots occurring on the Sun’s photosphere and approximately 11 years pass between the greatest (maximum) and smallest (minimum) numbers of sunspot occurrences. Coronal loops are thought to arc above sunspots and directly influence solar and corona winds, flares and coronal mass ejections (CMEs) due to stresses placed on the Sun’s magnetic fields (SDO 2011).

The number of sunspots occurring on the Sun is determined by calculating the number of sunspot groups, followed by the number of individual sunspots. A calculation used by the National Aeronautics and Space Administration (NASA) and National Oceanic and Atmospheric Administration (NOAA) is the sum of the number of individual sunspots and ten times the number of groups. Multiplying the number of sunspot groups by ten is a reliable means of calculating total sunspot numbers during unfavorable viewing conditions since it is known that, on average, ten spots occur within each group (NASASP 2011). Furthermore, detailed data stretching back to 1874 and obtained from the Royal Greenwich Observatory, has helped rule-out random appearance of sunspots on the photosphere. Research has also shown sunspots concentrate in mid-latitude regions of the sun, and as they widen as the cycle progresses, migrate towards the equator and form butterfly wing-like latitudinal bands (one “wing” in each hemisphere). During a sunspot minimum, there may be no visible spots. Conversely, a sunspot maximum may produce several hundred sunspots (NASASP 2011).

Image 2: 11 year sunspot cycle. Courtesy of NASA

Image 3: Butterfly wing-like development of sunspot groups near solar equator. Courtesy of NASA

Sunspots and Earth Changes

Between 1645 and 1715, the sun went through a period of extremely low sunspot activity. This period of solar inactivity is labeled the “Maunder Minimum” and closely corresponds to a climatic event referred to as the “Little Ice Age.” During the Little Ice Age, rivers that had been historically ice free, (I.e. Thames) froze over and lower latitudes experienced snow fields which remained year round. Much debate surrounds the scale of the Little Ice Age, most research, however, suggests it was not a globally-synchronous event but was instead a phenomenon experienced predominately in the North Atlantic and Europe. Research into solar activity and the terrestrial environment suggests events similar to the Maunder Minimum have happened in the past and have initiated climatic events (by altering Earth's electromagnetic field and directly influencing the amount of ultraviolent radiation moving through upper atmosphere) similar to that of the Little Ice Age, leading some scientists to suggest sunspots significantly influence climate on Earth. Solar cycle data does reveal a correlation between sunspots and the Earth’s climate, however, the field is relatively new, somewhat speculative and research is on-going. In other words, correlation does not necessarily reveal causation (NASASP 2011).

Image 4: Maunder Minimum. Courtesy of NASA

Predictions

The Sun is currently in Sunspot Cycle 24 which began on January 4th 2008. Cycle 24 is expected to reach a sunspot maximum of 89 in May of 2013. Original predictions for cycle 24 had placed the number of spots below the exceptionally low 1907 number of 64.2 (cycle 14). Despite the recent modification (increase) of the projected sunspot number due to increased solar activity, 89 would still be the smallest number of spots in over 80 years.

Several methods exist for predicting the behavior of the sunspot cycle. Typically, behavior and numbers of spots are easier to predict once the new cycle is underway. Predictions have proven most accurate approximately 3 years following the sunspot minimum (Hathaway et al. 1994). Techniques for predicting sunspot activity are normally designed around the time near and before sunspot minimums and emphasize the relationships between: 1) the length of the previous cycle; 2) level of activity at sunspot minimum; and 3) size of the previous cycle, all in relation to size of the next cycle maximum. Building and extrapolating on years of this type of modeling has allowed scientists and researchers to make extremely reliable predictions.

The time near and before sunspot minimums are crucial for developing the most reliable prediction methods. During this time, changes in the Earth’s electromagnetic field due to solar storms are recorded and compared against sunspot activity. Currently, a strong connection is suspected between solar storms, the Earth’s electromagnetic field and subsequent sunspot activity but the exact relationship is unknown. Various techniques surrounding the geomagnetic fluctuations caused by solar storms have been developed to predict subsequent sunspot activity. Among these techniques, three are widely used and include: 1) developing an index to determine the value of geomagnetic fields at sunspot minimum which correlates to the geomagnetic field during the ensuing maximum; 2) developing a geomagnetic index which has one component in phase and proportion to the sunspot number and another component which remains as the signal and occurs as a magnetic maximum near the sunspot minimum; 3) comparing the relationship between the number of days the Earth experiences geomagnetic disturbances from the sunspot cycle and the amplitude of the next sunspot maximum. Other researcher groups like NASA, have used averages obtained from combining some or all of the methods listed above (NASASP 2011).

Image 5: Sunspot cycle 24 predictions. Courtesy of NASA

Conclusions

Sunspots are dark, highly magnetized features which appear, develop and then disappear, near the mid-latitude and equatorial regions of the Sun’s surface or photosphere. Sunspot cycles occur in nearly regular intervals of 11 years and appear dark in relation to the rest of the sun because they are nearly 1500 degrees (K) cooler than surrounding regions. Rare and incomplete records of sunspot observations date as far back as 467 BCE, while regular record keeping began in the 17th century and has developed exponentially ever since. Sunspot cycles, which occur approximately every 11 years, are found within a larger solar cycle which refers to a 22-year solar magnetic polar reversal. Thus, two 11-year sunspot cycles occur within one solar cycle.

Disruptions in the regular 11-year sunspot cycle have occurred during Earth’s history with the best example being that of the Maunder Minimum. During this time, which lasted from approximately 1645 to 1715, extremely low sunspot activity occurred. This event coincided with what scientists refer to as the Little Ice Age, which led to ice age-like conditions for much of the North Atlantic and large portions of Europe. The Maunder Minimum has been a topic of much research and debate in various academic disciplines. While a correlation has been shown to exist between a decrease in sunspot activity and a resurgence of ice-age conditions for portions of the Earth in the past, research is on-going and in many cases speculative. In 2008 the 24th sunspot cycle began and with it came the usual predictions for spot numbers. Predictive methods have become increasingly complex, varied and more reliable with each passing cycle. The current cycle is predicted to peak with approximately 89 sunspots in May of 2013, though changes in solar activity may change this number as the cycle progresses.

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References