The glowing outer atmosphere of the Sun, visible to us as a brilliant halo only during total eclipse, is called the Corona. It has no boundary but merely thins out into space, possibly coming out at times almost to the Earth’s orbit. The speed at which the particles of the Corona vibrate is so great that its temperature is around three million degrees Centigrade. Since the corona’s temperature was first measured 55 years ago, scientists have lacked a satisfactory explanation for why that temperature is three million degrees while the visible surface of the Sun is only 11,000 degrees Fahrenheit or about 6,000 degrees Celsius. We now know that the upward transfer of magnetic energy from the Sun's surface toward the corona above has more than enough energy to heat the corona to its known temperature.

K Streamers, SOHO EIT, LASCO C2, courtesy NASA/ESA