To the casual observer, the relationship between the Sun and Earth may look simple, even sedate. The two are bound by gravitation to a one-beat, 365-day tango. The Sun showers Earth with light, day after day. To scientists and informed enthusiasts, however, the solar—terrestrial relationship is far from simple. Changes are taking place, some of which can have far-reaching consequences for humanity.
The Sun's variability is best seen through its 11-year sunspot cycle. More violent episodes occur for shorter periods. Often, what happens is that quantities of magnetic flux greater than can be contained locally create an intensified magnetic field that rushes upward to the surface of the Sun—similar to the way a wooden log rises to the surface of a river—carrying with it a large amount of interior mass. Once exposed, the solar particles will either emit intense electromagnetic radiation (solar flares) or be thrown off into space (coronal mass ejection).
Electromagnetic radiation, travelling at the speed of light, hits Earth first. A solar flare typically affects Earth by heating and swelling the whole atmosphere, and possibly by changing the chemistry of the middle atmosphere. During a large solar flare, the ultraviolet radiation causes an increase in ozone on Earth, and may have some long-term effects.
After a delay of one to several days, the ejected solar mass also reaches Earth. Unlike free-travelling photons, the solar wind has an arduous journey. Earth's magnetic field acts to repel foreign materials from its magnetosphere. Once this barrier is breached by the combined effect of the solar wind and Earth's magnetic field, the particles, which have spent a while sweeping along in the Earth's wake, move Earthward. They stagnate at an altitude of about 40,000 km, slowly building energy, and they eventually cause the magnetic field to explode. In the aftermath, the solar particles rain down on the atmosphere and can be seen in remarkable displays of aurora borealis, or northern lights.
This short description of the Sun—Earth connection hides many unsolved scientific problems of great importance and complexity: How is a magnetic field generated in an astrophysical object? How do the magnetic field and the charged particles interact over the great range and scales of the Sun—Earth System (suggested, for example, by the stunning structures in auroras)? These are inquiries of universal significance, and must be studied on or
Beyond its scientific significance, understanding the Sun—Earth connection is also of immediate concern to humanity. Like hurricanes in Gulf Coast areas in the south, solar storms in northern-latitude regions such as Canada can wreak havoc. High-energy particles create beautiful aurora, but they have proven to be capable of causing great destruction to communications satellites and long-distance land systems, such as power grids and undersea cables. Therefore, this research has a direct bearing on our ability to predict the space-weather events and keep the systems that people rely on from harm.
William Liu, Ph.D.
Program Scientist, Space Environment