快猫短视频

IT IS A SIGHT that burns itself into your memory. During a total solar
eclipse, the Sun鈥檚 outer atmosphere, the corona, appears as a glorious radiance
encircling the black disc of the Moon. But it is a tissue-paper crown: the
corona is made of incredibly tenuous ionised gas, only visible because it is
also incredibly hot.

And this is the great puzzle. Why is the corona so very hot? Sizzling at
around 2 million degrees Celsius, it lies between the relatively cool solar
surface at 5500 掳C and the bitter cold of space. It must be constantly
losing heat to the Sun鈥檚 surface and space. So why doesn鈥檛 it cool down? After
all, you wouldn鈥檛 expect an espresso to stay hot for long at the South Pole.
Several expensive and sophisticated spacecraft are trying to find out just how
the corona does it. But during the total eclipse on 11 August, astronomers will
also have the help of something less fancy: the Moon.

For up to 2 minutes and 23 seconds, the Moon will reveal more of the corona
than any artificial device. A shielding disc of metal, as used in the satellites
and in solar telescopes on Earth, has to be made extra large to stop unwanted
light from scattering off the inside of the telescope tube. That cuts out the
lowest regions of the corona. 鈥淓clipse observations still give us the clearest
view of the corona,鈥� says Craig DeForest, a solar physicist at NASA鈥檚 Goddard
Space Flight Center near Washington, DC.

Total eclipses happen somewhere on Earth once every eighteen months, on
average, but the 11 August eclipse could be especially useful. For one thing, it
should expose a more active corona than astronomers have seen in an eclipse
since 1990. That is because the Sun is nearing the peak of its 11-year cycle of
activity, when magnetic churnings beneath its surface will spawn a growing
number of sunspots and flares, as well as giant outbursts of gas called coronal
mass ejections (鈥淥ur tortured star鈥�, 快猫短视频, 1 May, p 44). Even
in between these explosive events the corona will look more dynamic than usual
and the solar wind will stream outwards with particular vigour.

And this time several spacecraft will be studying the Sun before, during, and
after the eclipse鈥攏otably the Solar and Heliospheric Observatory (SOHO),
the Japanese satellite Yohkoh, and a relatively new mission, the Transition
Region and Coronal Explorer (TRACE).

SOHO has already unveiled one possible way of heating the corona: a seething
鈥渕agnetic carpet鈥�, woven from small bundles of magnetic field. These loops
continually emerge from the Sun鈥檚 surface and collide with each other,
triggering tens of thousands of Earth-sized explosions at a time. Such magnetic
clashes certainly release enough energy to heat the corona, but they all happen
close to the Sun鈥檚 surface. No one yet knows how that energy could climb
hundreds of thousands of kilometres up.

Some theorists believe that something else could be broiling the
corona鈥攔apidly oscillating magnetic waves, generated by turbulence in the
upper layers of the Sun. Unlike the magnetic carpet these wigglings are not
confined to the surface. Instead they zip up along magnetic loops hundreds of
thousands of kilometres long. By violently stirring the coronal plasma they
might heat it to millions of degrees.

TRACE, which began watching the Sun in April 1998, is the best equipped
satellite to monitor these loops in ultraviolet light, where they stand out most
clearly. 鈥淭RACE has spectacular spatial resolution of the same region of the
lower corona that we鈥檒l be looking at,鈥� says Jay Pasachoff of Williams College
in Massachusetts, who observes eclipses from the ground. But like the other
satellites, TRACE has one drawback: it sends data back to Earth so slowly that
it can鈥檛 take detailed pictures in rapid succession.

Split-second timing

Rapid observations are crucial, because the magnetic loops must vibrate at
least once or twice per second to heat the corona effectively. Having no
data-transmission bottleneck, ground-based observations can be this fast.
Although no one has seen convincing evidence of these oscillations yet,
Pasachoff saw what he calls 鈥減reliminary evidence鈥� for 1-second coronal
oscillations last year in the Caribbean (see 鈥淭otal eclipse鈥�, New
快猫短视频, 4 April 1998, p 26). In August, Pasachoff鈥檚 team will snap 10
pictures per second of the corona with an improved camera near Bucharest,
Romania, where the eclipse will reach its maximum duration.

A team led by solar physicist Ken Phillips of the Rutherford Appleton
Laboratory in Oxfordshire hopes to do even better with an experiment called
SECIS鈥攖he Solar Eclipse Coronal Imaging System. Phillips will set up an
electronic camera at a site near Varna, Bulgaria, which can take up to 50 frames
a second. If fast coronal wigglings do exist, the thousands of images from SECIS
should expose them. 鈥淟ooking at these higher frequencies is a new way of
observing the Sun,鈥� Phillips says. 鈥淚t should reveal new things, just as
observing at higher radio frequencies revealed pulsars for the first time.鈥�

The eclipse will also let astronomers learn more about movement in the corona
on a larger scale. One ambitious ground-based effort aims to record changes in
the corona over the whole 90 minutes that it will take the Moon鈥檚 shadow to dash
across Europe. It is called TECONet, for Trans-European Coronal Observing
Network. TECONet may succeed thanks to one aspect of the 1999 eclipse that sets
it apart: its long trek over dry land. 鈥淲e don鈥檛 often see a land track as
extended as this,鈥� says veteran eclipse scientist Fred Espanek of NASA Goddard.
鈥淰ery often, most of the path is over water.鈥�

Frederic Clette of the Royal Observatory of Belgium is TECONet鈥檚 coordinator.
He has recruited teams of professional and amateur astronomers to gather similar
data at vantage points from England to Turkey. Every team will take repeated
pictures of the corona through a polarising filter that rotates through a
60-degree angle after each image. That鈥檚 the only way to cut out unwanted light
scattered off dust between the Sun and Earth. 鈥淚n past eclipses, every team has
used different equipment and experimental designs,鈥� Clette notes. 鈥淚f everyone
has the same field of view, the same spatial resolution, and the same
polarisation, the data sets will be easy to compare.鈥�

Particle blast

Clette hopes his network will catch a coronal mass ejection at it happens.
CMEs are huge balls of writhing plasma, hundreds of thousands of kilometres
across. Some unknown process flings them out from the corona into deep space at
speeds of more than a million kilometres per hour. They can confuse satellites
and damage electrical power grids if they hit the Earth, and sometimes they are
preceded by a blast of charged particles which could kill unprotected
astronauts.

Astronomers did spy a blob of ejected material in the corona during an
eclipse in 1980, a feature dubbed the 鈥渢ennis racquet鈥� because of its
distinctive shape. But it was seen only from Kenya and India, and the two images
didn鈥檛 reveal much about how the blob evolved.

This time, with the Sun in a high state of activity, CMEs should be happening
half a dozen times per day. The long track over land could give us a better
understanding of how CMEs are launched. 鈥淲e can get exceptionally clear
snapshots of the corona from the ground,鈥� says DeForest. 鈥淭hen, satellites allow
us to trace the origins and evolution of features we may see.鈥� Furthermore, the
eclipse passes over Bucharest Observatory. According to Magda Stavinschi,
director of the observatory, the pair of 50-centimetre refracting telescopes
should yield exquisite details of a CME, if they catch one.

Even if astronomers don鈥檛 hit the jackpot by capturing a CME or the rapid
magnetic oscillations, the eclipse will open a rare two-minute window onto part
of the Sun鈥檚 atmosphere that is normally hidden. So like the millions of us who
look forward to seeing the corona hang brightly in the brief night, scientists
are keeping their fingers crossed for good weather.

ECLIPSES once made kings cower and put ancient armies to flight. But modern
science makes a mockery of such extreme reactions: how can a mere shadow have
any effect here on Earth?

That is what makes the curious case of Dr Saxl and his twisting pendulum so
intriguing. In 1964, Erwin Saxl, the head of a precision instrument company
called Tensitron, had been doing experiments with an electrically charged weight
twisting on wires inside a metal cage鈥攁 torsion pendulum. In Nature (vol
203 p 136), he reported that the twist-rate of the pendulum slowed very
slightly, by just 0.35 per cent, when a lunar eclipse took place over his
laboratory.

Saxl could not explain his effect, but over the years a host of ideas have
been put forward, ranging from the effect of subtle changes in atmospheric
pressure during an eclipse to esoteric interactions between the forces of
gravity and electromagnetism. But most scientists remain convinced that the Saxl
effect is just an experimental artefact.

One physicist tried to recreate the effect鈥擝ruce Maccabee, then a PhD
student at the American University in Washington DC. Maccabee performed his
experiment during another lunar eclipse in October 1968, but found no change in
the pendulum鈥檚 period. Whether that was because the Saxl effect does not exist,
Maccabee can鈥檛 say. 鈥淚 wasn鈥檛 able to duplicate Saxl鈥檚 pendulum, so I can鈥檛 rule
out the possibility that mine wasn鈥檛 big enough to produce a measurable effect,鈥�
he says. But Saxl claimed to find the effect again, this time during a total
solar eclipse in March 1970. It was reported in another prestigious journal,
Physical Review D (vol 3 p 823).

Just last year a team of Chinese scientists claimed to have found faults in
Saxl鈥檚 experimental design鈥攖hough they didn鈥檛 attempt to repeat his
eclipse experiments. But with this summer鈥檚 eclipse passing right over the
physics departments of some major European universities, perhaps someone will
see fit to solve the mystery of the Saxl Effect once and for all.

Robert Matthews is science correspondent of The Sunday Telegraph