快猫短视频

THE mystery of why the corona that surrounds the Sun is millions of degrees
hotter than the surface just beneath it may have been solved. Theorists already
suspected that the reason may be magnetic field lines snapping and reconnecting.
Now an experiment has reproduced this phenomenon in the lab and confirmed that
it produces huge amounts of heat.

The corona reaches temperatures of millions of kelvin, while the Sun鈥檚
surface is only 10,000 kelvin. Last year, scientists saw strange magnetic loops
on the surface of the Sun, which hinted strongly that magnetism is at work in
the corona (快猫短视频, 7 October 2000, p 17).

The favoured explanation is that different regions of magnetic field collide,
causing the field lines to snap and reconnect, and converting large amounts of
magnetic energy into heat. Several teams around the world have tried to study
reconnection in the lab. But the resulting heat is confined to a small area
surrounding the reconnection and measuring it has proved difficult.

Now Masaaki Yamada, Scott Hsu and colleagues at Princeton University have
cracked the problem. They positioned two flux cores鈥攎agnetic field
sources鈥攊n a plasma of helium ions at a temperature of 40,000 degrees
kelvin (see Diagram).
The field generated by the flux cores encircles both of
them, but when the researchers suddenly decrease the field from the cores, the
field lines contract until they eventually snap and reconnect around each
core.

Yamada and his colleagues measured the localised heating with a specially
designed probe. It detects the speed of the ions in the plasma at its tip by
measuring the tiny Doppler shift in the frequency of the light they emit. This
speed corresponds to the ions鈥� temperature. The team found that reconnection
produced an enormous burst of energy, causing the temperature at the point of
reconnection to soar to 100,000 kelvin. This is ten times the energy expected
from the classical theory of magnetism, says Hsu, now at Caltech in
Pasadena.

No one is yet sure where this burst of energy comes from, but Yamada thinks
the colliding fields may be setting up turbulent waves in the plasma that cause
particles to vibrate extremely fast. However, Ambrogio Fasoli, who is studying
reconnection at the Massachusetts Institute of Technology, says there may be no
need to invoke such waves. The combined effect of the plasma and the magnetic
field may cause individual ions to cross field lines, fracturing them as they do
so. 鈥淲e need a new kinetic theory,鈥� he says.

Fasoli has designed an experiment that may show which of these explanations
is correct. He passes a laser beam through the plasma to measure the number of
ions moving at different speeds. If Yamada is right, the speeds of the ions
should cluster around certain fixed values. But Fasoli thinks the distribution
will be flatter, supporting the kinetic theory. 鈥淭hat鈥檚 what my preliminary
results show,鈥� he says.