Corona

:For other meanings, see Corona (disambiguation)

Coronal heating problem

The coronal heating problem is the title of a problem in astronomy and astrophysics as to why the temperature of the Sun's corona is millions of kelvins higher than that of the surface. The high temperatures require energy to be carried from the solar interior to the corona by non-thermal processes, because the second law of thermodynamics prevents heat from flowing directly from the solar photosphere, or surface, at about 5800 kelvins, to the much hotter corona at about 1 to 3 MK (parts of the corona can even reach 10 MK). The amount of power required to heat the solar corona can easily be calculated. It is about 1 kilowatt for every square metre of surface area on the Sun, or 1/40,000 of the amount of light energy that escapes the Sun.

Related Topics:
Astronomy - Astrophysics - Second law of thermodynamics - MK

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Two separate theories have emerged to explain why the corona is so hot, wave heating and magnetic reconnection. Through most of the past 50 years, neither theory has been able to account for the coronal heat. Most solar physicists now believe that some combination of the two theories can probably explain coronal heating, although the details are not yet complete.

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The wave heating theory, proposed in 1949 by Evry Schatzman, uses waves to carry energy from the solar interior to the solar chromosphere and corona. The Sun is made of plasma rather than ordinary gas, so it supports several types of waves analogous to sound waves in air. The most important types of wave are magneto-acoustic waves and Alfvén waves. Magneto-acoustic waves are sound waves that have been modified by the presence of a magnetic field, and Alfvén waves, are similar to ULF radio waves that have been modified by interaction with matter in the plasma. Both types of waves can be launched by the turbulence of granulation and supergranulation at the solar photosphere, and both types of waves can carry energy for some distance through the solar atmosphere before turning into shock waves that dissipate their energy as heat.

Related Topics:
1949 - Evry Schatzman - Plasma - Sound waves - Magneto-acoustic wave - Alfvén wave - ULF - Radio waves - Matter - Granulation - Supergranulation - Shock waves

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One problem with wave heating is delivery of the heat to the appropriate place. Magneto-acoustic waves cannot carry sufficient energy upward through the chromosphere to the corona, both because of the low pressure present in the chromosphere and because they tend to be reflected back to the photosphere. Alfvén waves can carry enough energy, but do not dissipate that energy rapidly enough once they enter the corona. Waves in plasmas are notoriously difficult to understand and describe analytically, but computer simulations, carried out by Thomas Bogdan and colleagues in 2003, seem to show that Alfvén waves can transmute into other wave modes at the base of the corona, providing a pathway that can carry large amounts of energy from the photosphere into the corona and then dissipate it as heat.

Related Topics:
Reflected - Thomas Bogdan - 2003

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Another problem with wave heating has been the complete absence, until the late 1990s, of any direct evidence of waves propagating through the solar corona. The first direct observation of waves propagating into and through the solar corona was made in 1997 with the SOHO space-borne solar observatory, the first platform capable of observing the Sun in the extreme ultraviolet for long periods of time with stable photometry. Those were magneto-acoustic waves with a frequency of about 1 millihertz (mHz, corresponding to a 1,000 second wave period), that carry only about 10% of the energy required to heat the corona. Many observations exist of localized wave phenomena, such as Alfvén waves launched by solar flares, but those events are transient and cannot explain the uniform coronal heat.

Related Topics:
1990s - 1997 - SOHO - Extreme ultraviolet - Photometry - Millihertz

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It is not yet known exactly how much wave energy is available to heat the corona. Results published in 2004 using data from the TRACE spacecraft seem to indicate that there are waves in the solar atmosphere at frequencies as high as 100 mHz (10 second period). Measurements of the temperature of different ions in the solar wind with the UVCS instrument aboard SOHO give strong indirect evidence that there are waves at frequencies as high as 200 Hz, well into the range of human hearing, but they cannot (yet) be detected or measured directly.

Related Topics:
2004 - TRACE - Ions - UVCS

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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.

Related Topics:
Magnetic reconnection - Mercury - Seawater - Magnetic field lines - Topology - Magnetic pole

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Magnetic reconnection is known to be the mechanism behind solar flares, the largest explosions in our solar system. Furthermore, the surface of the Sun is covered with literally millions 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.

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The idea that microflares 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 microflare hypothesis use other mechanisms to stress the magnetic field or to release the energy, and are a subject of active research in 2005.

Related Topics:
Eugene Parker - 1980s - Ultraviolet - SOHO/EIT - 2005

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