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The Big Questions


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


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


Photospheric Features 
Chromospheric Features 
Coronal Features 
Solar Wind Features 


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


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

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The Coronal Heating Process

The Sun's outer atmosphere (the Corona) is hotter than 1,000,000ºC (1,800,000ºF) while the visible surface has a temperature of only about 6000ºC (10,000ºF). The nature of the processes that heat the corona, maintain it at these high temperatures, and accelerate the solar wind is a third great solar mystery. Usually temperatures fall as you move away from a heat source. This is true in the Sun's interior right up to the visible surface. Then, over a relatively small distance, the temperature suddenly rises to extremely high values. Several mechanisms have been suggested as the source of this heating but there is no consensus on which one, or combination, is actually responsible.

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The Nature of Solar Flares

Areas on the Sun near sunspots often flare up, heating material to millions of degrees in just seconds and blasting billions of tons of material into space. The precise causes of solar flares and coronal mass ejections is another one of the great solar mysteries. Here again, we now know many details about these explosive events and we understand the basic mechanisms, but many details are missing. We still cannot reliably predict when and where a flare will occur or how big it will be. This problem is a little like trying to predict tornadoes.

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The Origin of the Sunspot Cycle

Over about 11 years the number of sunspots seen on the Sun increases from nearly zero to over 100 and then decreases to near zero again as the next cycle starts. The nature and causes of the sunspot cycle constitute one of the great mysteries of solar astronomy. While we now know many details about the sunspot cycle, (and also about some of the dynamo processes that must play key roles in producing it), we are still unable to produce a model that will allow us to reliably predict future sunspot numbers using basic physical principles. This problem is a little like trying to predict the severity of next year's winter or summer weather.

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The Missing Neutrinos

The Sun should produce more than twice as many neutrinos than are observed. These ghostly subatomic particles are released by nuclear reactions in the Sun's core. They then pass directly through the Sun and out into space. Detecting neutrinos is difficult, but the results from several independent experiments now confirm that only about a third of the expected numbers are counted here on Earth. Solar astronomers have attempted to alter their models of the Sun and its evolution over the last 4.5 billion years to make a model of the Sun that produces fewer neutrinos. These attempts have proven to be unsuccessful. This has led many scientists to question our understanding of neutrinos themselves. This final mystery may thus shake some of the foundations of Physics itself.

June 5, 1998 - Neutrinos have mass! see:

For more on neutrinos see:


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


Responsible Official: John M. Davis,, (256) 544-7600
Mail Code SD50, NASA/Marshall Space Flight Center, Huntsville, AL 35812


Last revised 2000 July 17 - D. H. Hathaway

Reproduced from