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While it's easy to understand gas pressure (as gas is heated it expands, increasing pressure, and as it cools, it contracts, decreasing pressure), magnetic pressure may be a tougher concept to grasp. David Dearborn explains, "If you take those places where there are concentrations of magnetic field and put them together, they have pressure of their own. You can feel magnetic pressure when you take two magnets and take the ends of the same polarity and try to put them together. The just don't quite want to go together. That's magnetic pressure."
George FisherDave Dearborn
George Fisher and David Dearborn Real Media Clip/ Help mapanswer the question, "What is a sunspot?"
Think of a sunspot as a bubble of magnetic pressure, surrounded, by the gas pressure of the photosphere. For the sunspot to exist, the total pressure must be in balance between the region inside and the region outside of the sunspot. David Dearborn elaborates on how magnetic fields keep sunspots cooler: " Outside a sunspot, you have only gas pressure, which depends on the temperature. In the sunspot you have both gas pressure and magnetic field pressure combined." Since the pressure must be in balance, magnetic pressure inside the sunspot allows the gas pressure (and thus the temperature) to remain lower than the areas outside of the sunspot.

 

 More About the Sun
To better understand the process that creates sunspots we first need to learn more about the sun. The sun is by far the largest object in the solar system, containing more than 99.8% of the total mass of the solar system (Jupiter contains most of the rest). The sun is made of about 75% hydrogen and 25% helium by mass, with tiny trace amounts of metals and other compounds. Over time, the nuclear fusion reactions that fuel the sun are converting hydrogen into helium in its core, changing the ratio of the two elements.
X ray image of the sun

 


The energy produced by nuclear fusion in the core of the sun is carried outward by convective motions in the outer 20-30% of the sun, called the convection zone. Convection is the process by which hot gas from the center of the sun rises to the surface, and cooler gas, which comes to the surface and radiates its heat away, sinks back towards the center.

The scale of the sun is hard to fathom. The sun is so large and so dense that it takes about 50,000,000 years for energy produced at its core to make its way to the sun's surface!
The sun has been radiating light and heat for the past four or five billion years. The sunspots to which this site is devoted appear as tiny spots on the sun--but an average-sized sunspot is as large as the earth.

Magnetic Fields and the Solar Dynamo
The sun, like the earth, generates a magnetic field that permeates the surface and extends out into space. The sun's magnetic field moves and changes over time, fluctuating in intensity in different areas of the sun's surface. The sun's magnetic field is thought to be produced by fluid motions within and just below the convection zone, something Dearborn refers to as the "solar dynamo," but the ultimate source of the sun's magnetic field, and the reasons for its fluctuations, is not well understood.


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Observatory  1998 The Exploratorium.