The solar corona is the outer shell of the Sun's atmosphere and traditionally, has been observed at the time of solar eclipse. The plasma in this zone is so hot (1–2×106K) that even the Sun's enormous gravity cannot retain it, and a continuous outflow of mass, the “solar wind,” fills the interplanetary region. Electrons and protons are the major constituents of this fluid. A few centuries ago, some of the rare and unpredictable phenomena related to the solar coronal expansion, such as comet tails and aurorae, were considered to be forecasts of the world's destruction. Recent studies of the subject, however, have contributed to our knowledge in such diverse fields as plasma theory, astrophysics, and geomagnetism.