The Sun provides energy to all life on Earth and drives the climate system and is therefore very important to all of us. It powers photosynthesis in plants and is the ultimate source of all food and fossil fuel. However, storms on the Sun can also interfere with systems on Earth that our society depends upon.
Every 11 years the Sun undergoes a period of activity called the "solar maximum," followed about 5 years later by a period of quiet called the "solar minimum." During solar maximum there are many sunspots, and during solar minimum there are few. Thus, one way of tracking solar activity is by observing the number of sunspots. Sunspots are dark patches like freckles on the solar surface formed when magnetic field lines just below the Sun’s surface are twisted and poke through the solar surface.
However, the Vortex-like sunspots are only one element of solar activity. Sunspots are surrounded by areas with enhanced brightness called active regions. Thus, the Sun emits a great deal more energy when it is active. Increased solar activity also means stronger and more frequent solar flares, a dramatic release of energy equivalent to a million hundred-megaton nuclear explosions. There is also an increase in eruptions called Coronal Mass Ejections or CMEs. CMEs are responsible for some of the most dramatic effects on Earth including severe problems for power distribution, satellite control, navigation and global telecommunication systems. CMEs can carry up to 10 billion tons of electrified gas traveling at speed as high as 4.4 million miles per hour in a spectacular explosion. The solar material streaks out through the interplanetary medium, impacting any planets or spacecraft in its path.
The response of the space environment particularly around the Earth to the constantly changing Sun is known as Space Weather. Space weather is a hot topic today because of the increasing awareness that many modern technological systems are potentially vulnerable to the effects from solar storms.
The current solar cycle, the 23rd to have been measured by modern scientists, is called Solar Cycle #23. It was expected to be at least as strong as the two previous cycles with a potential to cause serious problems for our technology based society and to even be a hazard to humans in space. After a fairly slow start, the solar activity picked up dramatically. During spring 2000 we experienced some of the most powerful eruptions from the Sun since the last solar maximum around 1990. The Sun is expected to reach maximum activity sometime in early 2001. Based on previous solar cycles, the most dramatic effects on Earth from solar activity are expected to occur after solar maximum on the declining part of the solar cycle. While the solar activity can be disruptive, it also increases the strength of the auroras, making them more likely to be seen outside the polar regions. Thus, within the next few years the possibility to experience the beautiful northern lights should be good in the US.
Scientists increasingly suspect that solar activity affects more than just satellites and power grids. Although sunspots and active regions themselves produce only minor variations in the energy output from the Sun, the magnetic activity that accompanies these regions can produce dramatic changes in the ultraviolet and soft X-ray emission levels. These changes over the solar cycle have important consequences for the Earth’s upper atmosphere and are known to alter the dynamics, temperature and chemistry (e.g., ozone) in these layers. This may have implications on the Earth’s climate. The long term increase in the Sun's level of activity (both variations in the emitted energy and its magnetic fields) may have played a significant role in the measured global warming the last 150 years. It is important to quantify this effect before one can determine any human influences on our climate. Thus, it is of great importance to understand how the Sun works and how it varies over time so that we can better understand how it will affect us in the future.
Recently there has been a revolution in understanding the Sun due to two major advances. The first is in theoretical modeling of the way the Sun’s magnetic field interacts with solar matter. The second advance is a series of observational discoveries from three space born solar satellites:
Yohkoh (a Japan-USA-UK Satellite), SOHO (the Solar and Heliospheric Observatory, a joint ESA-NASA project), and TRACE (NASA). These missions are providing high resolution observations of the Sun using sophisticated imaging telescopes and spectrographs that separates out observed light into the colors it is made of. This coordinated attack on solar physics has provided breathtaking new views of the Sun and a wealth of information.
SOHO has been leading the way into a new era in the field of helioseismology – a study of the solar interior through the analysis of vibrations on the surface.
Scientists measure these movements, the result of many “notes” resonating within the Sun. Because the notes represent vibrations at different depths, one can derive the structure (e.g., temperature and density) of the solar interior.
SOHO has revealed that the interior of the Sun is rotating in a completely unexpected way. We knew that at the surface the equator rotates more rapidly than the poles. Moving deeper, this rotation is roughly that of a solid body. The intense shear at this internal boundary, located some 30% of the way into the core, is likely to induce turbulence. This is where the dynamo that generates the solar magnetic field probably originates. Furthermore, observations of the Sun’s structure just below the surface have made it possible to detect sunspots before they can be seen on the surface.
Spectroscopic observations, made in concert with the high resolution imaging, are able to provide details of conditions in the solar atmosphere. As with weather maps, we are able to determine temperatures, densities, flow speeds and even what the gases are made of. For instance the velocity of gases in the solar atmosphere are made by the Doppler effect – the same principles as that used by police radars to detect speeding motorists or by meteorology Doppler radars to track the movements of clouds and rain. These results enable theoreticians to develop models to aid our understanding of the complex solar atmosphere. In particular, we want to explain the outstanding mysteries of the Sun, such as (1) why is the solar extended atmosphere so hot (millions of degrees) when the ‘surface’ is at 6000 degrees, and (2) how does the solar atmosphere generate the solar wind, a constant stream of charged particles filling the entire solar system. The SOHO spacecraft has started to give answers to some of these questions and is continuously transmitting images back to Earth. These spectacular images are made available immediately on Internet so that also you can view the Sun as seen from space (see http://sohowww.nascom.nasa.gov/).
The fact that the Sun is affecting us in so many ways makes it very important to learn more about our own star. We need to monitor it continuously to better understand the solar cycle and any long term changes in the Sun’s activity level. 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, we are still unable to produce a model that will allow us to reliably predict how strong the solar activity will be for the next solar cycle or beyond. This problem is a little like trying to predict the severity of the next year’s winter or summer weather. Several new initiatives have been suggested to answer some of these questions. The most ambitious program of these is called “Living with a Star”. This is a NASA proposal which includes a number of new solar missions designed to monitor the Sun. The Sun is about middle age now, some 5 billion years old. We are still learning more about it, and how it effects everything on Earth, every day.
