For other meanings, see corona (disambiguation)

The corona is the luminous "atmosphere" of the Sun extending millions of kilometres into space, most easily seen during a total solar eclipse, but also observable in a coronagraph.

An interesting feature of the corona is the fact that it is much hotter than the visible "surface" of the Sun: the photosphere is approximately 6000 °C compared to the corona at over one million degrees Celsius or kelvins (they're essentially the same thing in this temperature range; that 273 K offset between the scales is insignificant). The corona is much less dense than the photosphere, however, and so produces less light. The corona is separated from the photosphere by the relatively shallow chromosphere. The exact mechanism by which the corona is heated is still the subject of some debate, but likely possibilities include induction by the Sun's magnetic field and sonic pressure waves from below (the latter being less probable now that coronae are known to be present in early-type stars). The outer edges of the sun's corona are constantly being lost as solar wind.

During periods of quiet Sun the corona is more or less confined to the equatorial regions, with "coronal holes" covering the polar regions. During the Sun's active periods, the corona is evenly distributed over the equatorial and polar regions, though it is most prominent in areas with sunspot activity.

Generated by solar flares or large solar prominence s, "coronal transients" are sometimes released. These are enormous loops of coronal material travelling outward from the Sun at over a million kilometres per hour, containing roughly 10 times the energy of the solar flare or prominence that triggered them.

Stars other than the Sun have coronae, which can be detected using X-ray telescopes. Some stellar coronae, particularly in young stars, are much more luminous than the sun's.

The high temperature of the corona gives it unusual spectral features, which led some to suggest that it was composed of a previously unknown substance, "coronium"; however these spectral features have since been traced to known elements in high states of ionisation.

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 6,000 kelvins, to the much hotter corona at about 1 MK to 10 MK. The amount of power required to heat the solar corona can easily be calculated. It is about 1 kilowatt for every square meter of surface area on the Sun, or 1/40,000 of the amount of light energy that escapes the Sun.

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.

The wave heating theory, proposed in 1949 by Evry Schatzman , uses waves to carry energy from the solar interior to 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.

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 recent 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.

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.

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.

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 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.

The idea that microflares might heat the corona was forwarded 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.

External links

• Coronal heating problem at Innovation Reports http://www.innovations-report.com/html/reports/physics_astronomy/report-33153.ht
  ml