Sun's Corona

Corona is the outermost part of the Sun’s atmosphere. Depends on where the Sun is at in its solar cycle, corona can extend millions of kilometers to outer space of the Sun. However, given the massive size of the Sun, it is still a thin layer compared to Earth’s atmosphere to the size of the Earth. Corona is about 1 millions times less bright than the bright photosphere. As a result, we can only observe corona during a solar eclipse or through the use of coronagraph. During eclipse time, corona appears whitish cloud or white crown and the overall shape of corona changes with the sun cycle.

Unlike Earth’s cool atmosphere, corona has temperature of about one million degrees Celsius, much more than the visible part of the Sun, which is only 5800 Kelvin. This is very strange because the outer layer is much hotter than the surface temperature, about 200 times hotter. This can be explained because of the complex magnetic field of the Sun. At this temperature, it emits light at X-ray wavelengths. The spectral lines of corona shows that it contains highly ionized elements, such as iron or calcium. Compare to Earth’s atmosphere, corona is millions less dense.

The shape of corona depends on the where the Sun is in its 11 years solar cycle. At the solar minimum, corona is confined around the equator but at the active time during the solar maximum, corona is evenly distributed at around the Sun. The main features of corona are plumes, helmet streamers, coronal loops, coronal holes.

            We often see helmet streamers during the quiet cycle of the sun. Helmet streamers have loop-like shapes that usually surround the equator of the Sun. Because of the high temperature, corona contains charged particles in plasma state. Sun’s magnetic field thread through the corona and trap these particles in plasma state and make them accumulate over the equator where the magnetic lines are strongest.

Solar plumes are long, thin streamer that extend from the north and the south poles of the Sun. Plume are formed the same way as helmet streamer but at the poles, the magnetic filed are weaker or open which cause the streamer to be thin and less accumulated.

Coronal holes are the dark region of the Sun atmosphere. This region is where there is no charged particle in plasma states accumulate in the Sun atmosphere as a result of open magnetic lines. Coronal holes are often found near the poles of the Sun and this is also the region where solar wind forms.

Coronal loops are the most active of the corona. Coronal loops are usually formed over the active region of the sun where magnetic fields between magnetic regions are strongest.  

References:

http://www.michielb.nl/sun/corona.htm

http://solarscience.msfc.nasa.gov/corona.shtml

http://www.windows2universe.org/sun/atmosphere/streamers.html

http://solarscience.msfc.nasa.gov/feature3.shtml

http://en.wikipedia.org/wiki/Corona