With the sun near its maximum activity this year, many people have been able to witness the beautiful light show called aurora borealis (northern auroras) or aurora australis (southern auroras). These curtains, streamers, and flickering patterns of light are caused when charged particles from the sun (mostly electrons and protons) hit atoms in the upper atmosphere. The most common color is pale yellow-green emitted by oxygen atoms below 250 miles altitude (400 km). The classic curtain-shaped auroras are almost always oxygen-green. Sometimes the green curtains show a tinge of red near the tops, also from oxygen atoms. Nitrogen atoms can also emit red light, and rarely it is possible to see a deep blue-violet light from charged molecules of nitrogen. When an electron or proton from the sun hits an atom, it can knock one of the electrons into a high-energy orbit. When the electron falls back to its normal orbit, it releases a burst of light. aurora colors: 極光 aurora borealis: 北極光 aurora australis: 南極光 charged particles: 帶電的粒子 electrons and protons: 電子和質(zhì)子 Nitrogen atoms: 氮原子