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:
These mechanisms were tested both theoretically and observationally
and, currently, only acoustic waves were ruled out so far.
- acoustic waves
- fast and slow magneto-acoustic body waves
- Alfven body waves
- slow and fast magneto-acoustic surface waves
- current (or magnetic field) dissipation
- 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
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.)