Corona
The Magnetic reconnection theory relies on the solar magnetic field to induce electric currents in the solar corona. The currents then collapse suddenly, releasing energy as heat and wave energy in the corona. This process is called "reconnection" because of the peculiar way that magnetic fields behave in a plasma (or any electrically conductive fluid such as mercury or seawater). In a plasma, magnetic field lines are normally tied to individual pieces of matter, so that the topology of the magnetic field remains the same: if a particular north and south magnetic pole are connected by a single field line, then even if the plasma is stirred or if the magnets are moved around, that field line will continue to connect those particular poles. The connection is maintained by electric currents that are induced in the plasma. Under certain conditions, the electric currents can collapse, allowing the magnetic field to "reconnect" to other magnetic poles and release heat and wave energy in the process.
Magnetic reconnection is hypothesized to be the mechanism behind solar flares, the largest explosions in our solar system. Furthermore, the surface of Sun is covered with million of small magnetized regions 50–1,000 km across. These small magnetic poles are buffeted and churned by the constant granulation. The magnetic field in the solar corona must undergo nearly constant reconnection to match the motion of this "magnetic carpet", so the energy released by the reconnection is a natural candidate for the coronal heat, perhaps as a series of "microflares" that individually provide very little energy but together account for the required energy.
The idea that micro flares might heat the corona was put forward by Eugene Parker in the 1980s but is still controversial. In particular, ultraviolet telescopes such as TRACE and SOHO/EIT can observe individual micro-flares as small brightenings in extreme ultraviolet light, but there seem to be too few of these small events to account for the energy released into the corona. The additional energy not accounted for could be made up by wave energy, or by gradual magnetic reconnection that releases energy more smoothly than micro-flares and therefore doesn't appear well in the TRACE data. Variations on the micro flare hypothesis use other mechanisms to stress the magnetic field or to release the energy, and are a subject of active research in 2005.
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References
- ^ Aschwanden, M. J. (2004). Physics of the Solar Corona. An Introduction. Praxis Publishing Ltd.. ISBN 3-540-22321-5.
- ^ Corfield, Richard (2007). Lives of the Planets. Basic Books. ISBN 978-0-465-01403-3.
- ^ Alfvén, Hannes (1947). "Magneto hydrodynamic waves, and the heating of the solar corona". MNRAS 107: 211–219.
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External links
- Coronal heating problem at Innovation Reports
- NASA/GSFC description of the coronal heating problem
- FAQ about coronal heating
- Solar and Heliospheric Observatory, including near-real-time images of the solar corona
- Sun|trek website An educational resource for teachers and students about the Sun and its effect on the Earth
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