Solar Wind

Beautiful aurorae are caused when charged particles, like protons and electrons, stream into the earth's atmosphere and excite the nitrogen and oxygen atoms in the upper atmosphere. When these atoms return to their normal, nonexcited state, they emit the shimmering, green or red curtains of light so familiar to individuals living in parts of Canada or the northern United States.

If the solar wind is continuous, why don't we see aurorae all the time? Earth is surrounded by a magnetic field, generated by its rotation and the presence of molten, conducting iron deep in its interior. This magnetic field extends far into space and deflects most particles that encounter it. Most of the solar wind therefore streams around the earth before continuing on its way into space. Some particles get through, however, and they eventually find their way into two great rings of charged particles that surround the entire Earth. These are called the Van Allen belts, and they lie well outside the atmosphere, several thousand kilometers up.

Besides the gentle, continuous generation of the solar wind, however, the Sun also periodically injects large quantities of protons and electrons into the solar wind. This happens after a flare, a violent eruption in the Sun's atmosphere. When the burst of particles reaches the earth, the magnetic field is not sufficient to deflect all the particles, and the Van Allen belts are not sufficient to trap them all above the atmosphere. Like water overflowing a bucket, the excess particles stream along the earth's magnetic field lines and flow into the upper atmosphere near the poles. This is why aurorae typically appear in extreme northern or southern latitudes, though after particularly intense solar flares, aurorae may be seen in middle latitudes as well.