Skip Navigation Links


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


The People  
Their Papers  
Their Presentations  


Flare Mechanisms  
3D Magnetic Fields  
The Solar Dynamo  
Solar Cycle Prediction  
Sunspot Database  
Coronal Heating  
Solar Wind Dynamics  


GOES SXI Instrument  
MSFC Magnetograph  
Orbiting Solar Obs.  
Solar Maximum Mission  
SpaceLab 2  
The Sun in Time (EPO)  




Solar Probe  
Interstellar Probe  


Solar Probe Plus  

The Big Questions

Ecl1991a_sm.jpg (10500 bytes)

Click on image for larger version.

The Coronal Heating Process

The Sun's outer atmosphere (the Corona) is hotter than 1,000,000°C (1,800,000°F) while the visible surface has a temperature of only about 6000°C (10,000°F). 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.

flare_sm.jpg (13100 bytes)

Click on image for larger version.

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.

ssn_recent.gif (2310 bytes)

Click on image for larger version.

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.

neutrino.jpg (3980 bytes)

Click on image for larger version.

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:


Web Links
NOAA's Space Weather Prediction Center - Today's Space Weather Updated Every 5-minutes - Latest Images from the Solar Dynamics Observatory
National Space Weather Program - The U.S. Government and Space Weather
First Gov Image + NASA Privacy Policy and Important Notices
+ Visit Solar Terrestrial Probes Program
+ Visit Living With a Star Program
NASA Logo Image Author: Dr. David H. Hathaway, david.hathaway @
Curator: Mitzi Adams, mitzi.adams @

Last Updated: August 11, 2014