Although the internal structure of the solar core is hidden from direct observations, one may conclude, from using various models, that the maximum temperature inside of our star is about 16 million degrees (Celsius). The photosphere - the visible surface of the Sun - has a temperature of about 6000 degrees C. However, the temperature increases very steeply from 6000 degrees to a few million degrees in the corona, in the region 500 kilometers above the photosphere.

Thus, the Sun is hotter on the inside than it is on the outside. However, the outer atmosphere of the Sun, the corona, is indeed hotter than the underlying photosphere.

In late thirties, Grotrian (1939) and Edlen discovered that the strange spectral lines observed in a spectrum of solar corona are emitted by elements such as iron (Fe), calcium (Ca), and nickel (Ni) in very high stages of ionization. Hence, they concluded that the coronal gas is extremely hot with temperature more than 1 million degrees.

The question of why the solar corona is so hot remains one of most exciting astronomy puzzles for the last 60 years. There is no definite answer to that question yet.

Although the solar corona is very hot, it also has very low density. Therefore, only a small fraction of the total solar radiation is required to power the corona. The total power emitted in X-rays is only about one-millionth of the Sun's total luminosity, so there is enough energy in the Sun to heat the corona. An important question, however, is how the energy is transported up to the corona, and what mechanism is responsible for the transport.

Over the years, several different mechanisms of powering the corona have been proposed:

- acoustic waves
- fast and slow magneto-acoustic body waves
- Alfven body waves 
- slow and fast magneto-acoustic surface waves
- current (or magnetic field) dissipation
- microflares/transients
- mass/particle flows and magnetic flux emergence.

Recently, the "magnetic carpet" was related to the coronal heating puzzle.

Observations with high spatial resolution show that the surface of the Sun is covered by the weak magnetic fields concentrated in small patches of opposite polarity (magnet carpet). These magnetic concentrations are believed to be a footpoints of individual magnetic flux tubes carrying electric currents.

Recent observations of this "magnetic carpet" show a very dynamical evolution: photospheric magnetic fields constantly move around, interact with each other, dissipate and emerge on very short period of time. Magnetic reconnection between magnetic field of opposite polarity may change topology of the field and release magnetic energy. The reconnection process will also result the dissipation of electric currents which will transform electric energy into the heat.

This is general idea of how magnetic carpet may be involved in the coronal heating. The idea is based on high resolution observations of magnetic fields and X-ray corona and existing theoretical mechanisms of coronal heating via electric current dissipation, microflares and magnetic flux emergence. It is too early, however, to state that the "magnetic carpet" ultimately solves the problem of the coronal heating, since a quantitative model of the process has not been proposed yet.

References

Grotrian, W. 1939, Naturwissen, 27, 214

L. Golub, J.M. Pasachoff, "The Solar Corona", 1997, Cambridge Univ. Press (This is an excellent source of information on solar corona.)