Active Regions
During a sunspot cycle, the general level of all activity in the solar atmosphere follows the number of sunspots. Thus a sunspot group seen in white-light photospheric pictures is just the most visible indicator of a large disturbed region in the solar atmosphere called an active region. Such regions can be up to several hundred thousand kilometers in extent. The various transient phenomena that are part of the active region are summarized in the table below. The common bond among these visible features is magnetic fields. The field appears first, followed by a facula in the photosphere and a plage in the chromosphere. This in turn can be followed by a sunspot group, flare activity, and prominences. The precise behavior is somewhat different for each active region, but there is little doubt that the phenomena in different layers of the solar atmosphere are related. The solar cycle of activity (a succession of active regions over a 22-year period) is fundamentally a magnetic cycle. It is a many-year variation in the quantity of magnetic field that emerges in the solar atmosphere.
Active Regions The Sun's transient activity produces a variety of transient effects here on Earth, which follows a rough cycle in unison with the Sun's cycle. An example of this relation is the increase in auroral activity in the Earth's atmosphere during sunspot maxima and its decrease during minima. Changes in solar activity also affect Earth's weather and climate, but how and over what time scales these effects occur we do not clearly understand. In recent years some interesting discoveries have been made about the constancy of the Sun's activity and its relation to the Earth.
How Constant is the Solar Constant?
Approximately 1.4 million erg of radiant energy fall on each square centimeter of the Earth every second. The theory of stellar evolution suggests that the Sun's luminosity must have increased by perhaps 30 percent since it began its existence some 4.6 billion years ago. However, as recently as 1975, paleoclimatic evidence concerning long-term temperature variations on Earth showed that any change has been less than 3 percent in the last 1 million years. One can argue that if the solar luminosity had been as much as 25 percent less than the present value, the oceans would have frozen, preventing biological evolution. This suggests that if the solar luminosity has increased significantly since Earth's formation, something about the Earth has compensated for such a change.
Solar Max measurements provide evidence that sunspots and faculae, granulation, and solar oscillations are responsible for as much as 0.4 percent changes in the solar constant over a time scale of a week. And since 1980, the solar constant has steadily decreased by 0.02 percent per year. What longer-term changes exist in the solar constant, if any, are important to our knowledge of the Sun and the Earth.
Is Solar Activity Constant?
Another fundamental question is the degree to which the 22-year cycle of solar transient activity repeats itself over long stretches of time. In 1893, E. Walter Maunder (1851-1928), a British astronomer, found in European historical records that very few sunspots were seen in the period from 1645 to 1715, now known as the Maunder Minimum. Within the last several years, Maunder's work has been confirmed and extended by astronomers worldwide. In addition to the absence of sunspots during the Maunder Minimum, very few aurora were observed in Europe, and during eclipses, the corona was also absent or very weak. Virtually no sunspots were reported in Asia during the Maunder Minimum, even though naked-eye sunspot-sighting reports exist there from as early as 28 B.C. Finally, measurements of the amount of carbon 14 (14C6), a radioactive isotope of carbon, in tree rings show that during the Maunder Minimum there was an excess of this isotope in the Earth's atmosphere. We believe that the high-energy subatomic particles called cosmic rays>, moving randomly through the Galaxy, collide with the nucleus of nitrogen atoms (14N7) in our atmosphere, converting it to carbon 14. When the Sun is very active, the interplanetary magnetic field is strong, and Galactic cosmic rays are deflected away from the Earth. Thus high levels of carbon-14 in the Earth's atmosphere correspond to low levels of solar activity.
Historical research has shown a correlation among carbon 14 abundance in the tree rings, winter severity, Galactic cosmic-ray activity, and solar activity. Periods of colder climate appear to coincide with low levels of solar activity. Evidence exists that at least a dozen similar periods of minimal solar activity, lasting from 50 to 200 years, have occurred over the last 8000 years. Answers as to why the Sun should experienced decreases in transient activity are to be found in changes in magnetic activity which are probably caused by changes in the pattern of convection underneath the photosphere. Consequently the interior of the Sun may not be as constant in its behavior as once thought.