picture src: Universe (Fifth Edition)

    The corona is just above the chromosphere and is the outermost part of the Sun. It extends thousands of kilometers into space and is in continual change. It is unlike the atmosphere of Earth, which is spherical. Corona is not a continuous sphere around the sun. It changes with the sunspot cycle.

    The temperature and density of the corona is not uniform. The region of the corona that appears to be brighter is hotter and denser as compared to regions that is darker (cooler and less dense). This region consist of super-heated gases (about 1,000,000 C). At this temperature, Hydrogen atoms and Helium Atoms are stripped of their electrons, only trace elements such as Iron and Calcium are able to hold some of their electrons. Emission from this highly ionised elements produce the spectral emission lines.

    There are parts of the corona where it was observed that there are no luminous gas. These regions are the coronal holes. These holes have been found to have particles escaping at very high speed from the sun into space. This is the solar wind. It was observed that the solar wind from the coronal holes at the poles (permanent coronal holes) travel at 800km/s, twice the speed of the solar wind from the holes at the Equator. Why is there a difference in the speed of the solar wind from the poles and the equator is still unknown.

Resource book: "Universe (Fifth Edition)"   
Picture of "Universe (Fifth Edition)"   

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