A rare red aurora as seen from Fairbanks, Alaska. Photograph ©Jan Curtis

The most spectacular manifestation of the connection between the Sun and the Earth is the Aurora Borealis (Northern Lights) and the Aurora Australis (Southern Lights). For millennia, people have watched them and worried about what ill portents they were heralding. It wasn't until the mid-1800s that scientific studies began to uncover many of their mysteries.

Magnetic Storms
Scientists learned that aurora often accompanied magnetic 'storms' and an unsettled magnetosphere; they were produced by flows of charged particles entering the atmosphere; they came and went with the sunspot cycle; and their colors were the product of excited oxygen and nitrogen atoms about a hundred miles above the surface of the Earth.

This NASA animation shows the Sun interacting with the Earth's magnetosphere causing auroras. (Click to launch movie.)

By the turn of the 20th century, scientists actually created artificial aurora in their laboratories, and once television and the fluorescent lamp were invented, it became pretty clear just how auroras could be created by electrons hitting the gas in the atmosphere. These collisions would cause atoms of oxygen and nitrogen to fluoresce and produce the characteristic colors of red, blue and green. What scientists still didn't understand was where the electrons came from. Some thought it was direct currents of particles from the Sun itself. Others felt it was more complicated than that.

What we have learned from direct satellite studies in the 1970s is that these flows of particles definitely do not come from the Sun. Despite what many simplified accounts might suggest, auroras are not caused by the direct flow of particles from the Sun into the polar regions, guided by Earth's magnetic fields. Instead, these currents are generated in the distant, comet-like tail of Earth's magnetosphere, whenever solar activity and severe solar storms are in progress. During a solar storm, some of the energy stored in the tail of the Earth's magnetic field is transformed into high-speed currents of charged particles.

Charged Particles
These accelerated particles flow into the equatorial regions of near-Earth space and become trapped as the ring current. Positively charged particles drift westward while negatively charged particles drift eastward. In a process scientists don't fully understand, some of these particles also flow along the magnetic field into the polar regions. As they enter the upper atmosphere they are accelerated to even higher energies before colliding with atoms of oxygen and nitrogen to produce the aurora's colors. These million-ampere currents not only cause the spectacular displays we see as aurora, but they also heat the upper atmosphere and ionosphere.

Ultraviolet images of the aurora from space. Taken by the Far Ultraviolet Imager (FUV). (Click to launch movie.)

Find out more about the Sun-Earth Connection at the Sun-Earth Connection Education Forum Web site.

Text adapted from the Sun-Earth Connection Tutorial courtesy of NASA, originally written by Dr. Sten Odenwald. Images and videos courtesy of NASA unless otherwised noted.