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The Photosphere

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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Click on image for larger version.

The photosphere is the visible surface of the Sun that we are most familiar with. Since the Sun is a ball of gas, this is not a solid surface but is actually a layer about 100 km thick (very, very, thin compared to the 700,000 km radius of the Sun). When we look at the center of the disk of the Sun we look straight in and see somewhat hotter and brighter regions. When we look at the limb, or edge, of the solar disk we see light that has taken a slanting path through this layer and we only see through the upper, cooler and dimmer regions. This explains the "limb darkening" that appears as a darkening of the solar disk near the limb.

A number of features can be observed in the photosphere with a simple telescope (along with a good filter to reduce the intensity of sunlight to safely observable levels). These features include the dark sunspots, the bright faculae, and granules. We can also measure the flow of material in the photosphere using the Doppler effect. These measurements reveal additional features such as supergranules as well as large scale flows and a pattern of waves and oscillations.

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Click on image for animation.

The Sun rotates on its axis once in about 27 days. This rotation was first detected by observing the motion of sunspots in the photosphere. The Sun's rotation axis is tilted by about 7.25 degrees from the axis fo the Earth's orbit so we see more of the Sun's north pole in September of each year and more of its south pole in March.

Since the Sun is a ball of gas it does not have to rotate rigidly like the solid planets and moons do. In fact, the Sun's equatorial regions rotate faster (taking about 24 days) than the polar regions (which rotate once in more than 30 days). The source of this "differential rotation" is an area of current research in solar astronomy.

A 4.3 Mb MPEG movie showing magnetic features in the photosphere over a 36 day period is available here. These data were obtained with the GONG solar telescope network. The movie illustrates the rotation of the Sun as well as the evolution of the Sun's magnetic features - including sunspots.

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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