Convection

Consider a thin layer of fluid which is heated from below and cooled from above. At low temperature differences, the viscosity of the fluid keeps the fluid from moving and the heat is conducted from the bottom to the top.

However, there is an instability in the system due to the fact that the water on the bottom is expanding due to the higher temperature and becoming less dense, and hence experiencing an upward bouyancy force, while the fluid on the top is cooler and denser and experiencing a downward gravitational force.

There is some temperature difference above which this instability leads to a steady flow of fluid, which rises and falls in small rolls. This temperature depends on many physical characteristics of the system, such as fluid viscosity and density, cell geometry, etc. This fluid motion is called convection.

This convection is the same phenomenon which can be seen in a pot of water on the stove. On the surface of the sun, however, there is a highly charged plasma rather than water, and when convective flows begin, magnetic fields form. These magnetic fields interact with the charged particles which in turn change the magnetic fields. Regions where very high magnetic fields form create sunspots.

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Eigenfunction Analysis of Coherent Structures on the Solar Surface
Authoring NASA Official:Dr. Milton Halem, Chief, Earth and Space Data Computing Division
Contact:Marilyn Mack/Code 930 marilyn.j.mack.1@gsfc.nasa.gov