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The Solar Corona

THE SUN 

Why We Study the Sun 
The Big Questions 
Magnetism - The Key 

SOLAR STRUCTURE 

The Interior 
The Photosphere 
The Chromosphere 
The Transition Region 
The Corona 
The Solar Wind 
The Heliosphere 

SOLAR FEATURES 

Photospheric Features 
Chromospheric Features 
Coronal Features 
Solar Wind Features 

THE SUN IN ACTION 

The Sunspot Cycle 
Solar Flares 
Post Flare Loops 
Coronal Mass Ejections 
Surface and Interior Flows 
Waves and Helioseismology 

RESEARCH AREAS 

Flare Mechanisms 
3D Magnetic Fields 
The Solar Dynamo 
Sunspot Cycle Predictions 
Solar Wind Dynamics 

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The White-Light Corona

The Corona is the Sun's outer atmosphere. It is visible during total eclipses of the Sun as a pearly white crown surrounding the Sun. The corona displays a variety of features including streamers, plumes, and loops. These features change from eclipse to eclipse and the overall shape of the corona changes with the sunspot cycle. However, during the few fleeting minutes of totality few, if any, changes are seen in these coronal features.

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The Emission Line Corona

Early observations of the visible spectrum of the corona revealed bright emission lines at wavelengths that did not correspond to any known materials. This led astronomers to propose the existence of "coronium" as the principal gas in the corona. The true nature of the corona remained a mystery until it was determined that the coronal gases are super-heated to temperatures greater than 1,000,000ºC (1,800,000ºF). At these high temperatures both hydrogen and helium (the two dominant elements) are completely stripped of their electrons. Even minor elements like carbon, nitrogen, and oxygen are stripped down to bare nuclei. Only the heavier trace elements like iron and calcium are able to retain a few of their electrons in this intense heat. It is emission from these highly ionized elements that produces the spectral emission lines that were so mysterious to early astronomers. We can now produce artificial eclipses in coronagraphs that cover the disk of the Sun and filter out everything except the emission due to these coronal ions. These coronagraphs produce images of the "emission line corona." Examples of these observations can be seen at the National Solar Observatory's Coronal Data page.

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The X-Ray Corona

The corona shines brightly in x-rays because of its high temperature. On the other hand, the "cool" solar photosphere emits very few x-rays. This allows us to view the corona across the disk of the Sun when we observe the Sun in X-rays. To do this we must first design optics that can image x-rays and then we must get above the Earth's atmosphere. In the early 70's Skylab carried an x-ray telescope that revealed coronal holes and coronal bright points for the first time. Today we have the Yohkoh, SOHO, and TRACE satellites obtaining new and exciting observations of the Sun's corona, its features, and its dynamic character.

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Author: David H. Hathaway, david.hathaway@msfc.nasa.gov, (256) 544-7610
Mail Code SD50, NASA/Marshall Space Flight Center, Huntsville, AL 35812

 

Responsible Official: John M. Davis, john.m.davis@msfc.nasa.gov, (256) 544-7600
Mail Code SD50, NASA/Marshall Space Flight Center, Huntsville, AL 35812

 

Curator
Last revised 2000 July 17 - D. H. Hathaway


Reproduced from http://science.nasa.gov