I. Why We Study The Sun
up
1.The Climate Connection
The Sun is a source of light and heat for life on Earth. Our ancestors realized that their lives depended upon the Sun and they held the Sun in reverent awe. We still recognize the importance of the Sun and find the Sun to be awe inspiring. In addition we seek to understand how it works, why it changes, and how these changes influence us here on planet Earth. The Sun was much dimmer in its youth and yet the Earth was not frozen. The quantity and quality of light from the Sun varies on time scales from milli-seconds to billions of years. Some of these variations most certainly affect our climate but in uncertain ways.
2. Space Weather
The Sun is the source of the solar wind; a flow of gases from the Sun that streams past the Earth at speeds of more than 500 km per second (a million miles per hour). Disturbances in the solar wind shake the Earth's magnetic field and pump energy into the radiation belts. Regions on the surface of the Sun often flare and give off ultraviolet light and x-rays that heat up the Earth's upper atmosphere. This "Space Weather" can change the orbits of satellites and shorten mission lifetimes. The excess radiation can physically damage satellites and pose a threat to astronauts. Shaking the Earth's magnetic field can also cause current surges in power lines that destroy equipment and knock out power over large areas. As we become more dependent upon satellites in space we will increasingly feel the effects of space weather and need to predict it.
3.The Sun as a Star
The Sun also serves an important role in helping us to understand the rest of the astronomical universe. It is the only star close enough to us to reveal details about its surface. Without the Sun we would not have easily guessed that other stars also have spots and hot outer atmospheres. The Sun is the key to understanding other stars. We know the Sun's age, radius, mass, and luminosity (brightness) and we have also learned detailed information about its interior and atmosphere. This information is crucial for our understanding of other stars and how they evolve. Many physical processes that occur elsewhere in the universe can be examined in detail on the Sun. In this way solar astronomy teaches us much about stars, planetary systems, galaxies, and the universe itself.
4.The Sun as a Physical Laboratory
The Sun produces its energy by nuclear fusion - four hydrogen nuclei are fused to form single helium nuclei deep within the Sun's core. We have worked for decades to reproduce this process (in a controlled manner) here on Earth. Most of these efforts involve extremely hot plasmas in strong magnetic fields. (This plasma is not the blood product but rather a mixture of ions and electrons produced at high temperatures.) Much of solar astronomy involves observing and understanding plasmas under similar conditions. There continues to be much interaction between solar astronomers and scientific researchers in this and many other areas.
II.The Big Questions
up
1.The Coronal Heating Process
The Sun's outer atmosphere (the Corona) is hotter than 1,000,000ºC (1,800,000ºF) while the visible surface has a temperature of only about 6000ºC (10,000ºF). The nature of the processes that heat the corona, maintain it at these high temperatures, and accelerate the solar wind is a third great solar mystery. Usually temperatures fall as you move away from a heat source. This is true in the Sun's interior right up to the visible surface. Then, over a relatively small distance, the temperature suddenly rises to extremely high values. Several mechanisms have been suggested as the source of this heating but there is no consensus on which one, or combination, is actually responsible.
2.The Nature of Solar Flares
Areas on the Sun near sunspots often flare up, heating material to millions of degrees in just seconds and blasting billions of tons of material into space. The precise causes of solar flares and coronal mass ejections is another one of the great solar mysteries. Here again, we now know many details about these explosive events and we understand the basic mechanisms, but many details are missing. We still cannot reliably predict when and where a flare will occur or how big it will be. This problem is a little like trying to predict tornadoes.
3.The Origin of the Sunspot Cycle
Over about 11 years the number of sunspots seen on the Sun increases from nearly zero to over 100 and then decreases to near zero again as the next cycle starts. The nature and causes of the sunspot cycle constitute one of the great mysteries of solar astronomy. While we now know many details about the sunspot cycle, (and also about some of the dynamo processes that must play key roles in producing it), we are still unable to produce a model that will allow us to reliably predict future sunspot numbers using basic physical principles. This problem is a little like trying to predict the severity of next year's winter or summer weather.
