Eclipse sheds light on sizzling sun
Nature has bestowed a treat on solar scientists. About an hour after sunrise in north Queensland, the moon passed directly in front of the sun.
Those fortunate enough to be in Cairns enjoyed an early morning total eclipse lasting two minutes with the sun roughly 14 degrees above the eastern horizon.
For astrophysicists, it meant an opportunity to glimpse the outermost workings of the sizzling sun.
Here comes the sun: Total eclipse of the Sun
Solar Eclipse 2012 Palm Cove, Cairns Australia. Photo: Peter Rae
Eclipses give scientists their best chance to study the corona, the immensely hot outer atmosphere of the sun that boils off into space and wafts past the planets.
Even with modern Earth- and space-based telescopes, scientists can only observe parts of the corona normally because of the sun's overwhelming brightness.
"At eclipse, we can observe the whole thing!" says Monash University solar physicist Paul Cally. "The striking thing about the corona at total eclipse is its beautiful fine structure dominated by the magnetic fields associated with active regions - called sunspots - and coronal holes."
These phenomena change over days or weeks, and so the corona appears to be quite different at every solar eclipse.
"During the seconds or minutes of totality, we could make observations that yield an amazing quantity of scientific information," Professor Cally says. "By observing spectral lines associated with highly ionised iron and other elements, we can determine the composition, temperature (well over 1 million degrees) and density of the various structures in the corona."
This provides scientists with a better understanding of the corona as well as the solar wind, the charged particles that stream from the sun.
The eclipse itself did not "directly" reveal anything about the sun's activity because there is always large day-to-day variability; just like today's weather does not reveal anything about a decade's climate trends, says Curtin Univeristy astrophysicist Rob Soria.
"This eclipse occurs near the peak of what is known as Cycle 24, when coronal activity, mass ejections, solar flares are expected to be more energetic," Dr Soria says.
This cycle is the weakest it has been in more than 100 years. The current cycle follows a series of three strong solar cycles that occurred between 1980 and 2000.
"Weaker cycles generally lead to a cooling climate - as was the case in the 1810s and 1880s, with similarly weak cycles – and vice versa," Dr Soria says.
If scientists get a better understanding of temperature, structure and energy processes in the solar corona and the solar wind, they can then make better predictions about the long-term solar variability and the effect it has on the Earth's climate and magnetic field.
For one thing, it would help to make solar flares - the mightiest of explosions in the whole solar system - more predictable.
A solar flare occurs when energy from the core moves slowly outwards, sometimes leaping forth in the lower gaseous layer of the sun's atmosphere - the pinkish-red region known as the chromosphere or colour-sphere. It belches out in an immense jet of fiery flame, sometimes extending tens of thousands of kilometres into space.
Big flares cause power blackouts on Earth. They also affect communications satellites. The sun has an 11-year cycle, with huge flares occurring during peak activity, which we are approaching now. The peak is expected by the end of next year.
Call back the past
Before the dawn of the space age, a solar eclipse was the only way that scientists could study the solar corona.
"It was common for observatories around the world to mount large expeditions to remote parts of the world, carting heavy equipment to record the fleeting details of the corona in the few minutes of totality offered by an eclipse," says John Kennewell, director of the Australian Space Academy.
When it became possible to launch into space solar telescopes that could create artificial eclipses using an instrument called a coronagraph, the need for ground-based eclipse observations decreased.
"The reason for the continuing interest in ground-based eclipse observations was that the images from space, even up until the late 1980s were not all that good," Dr Kennewell explains.
The situation changed with the launch of the solar and heliospheric observatory (SOHO) in 1995. This spacecraft carried a coronagraph called LASCO (Large Scale Coronagraph), which has provided high-resolution images and scientific data on the solar corona.
"Not only has it answered many question about the sun's atmosphere, it revolutionised our knowledge of coronal mass ejections, which are clouds of plasma containing around 5 billion tons of matter that are flung from the sun and which can cause magnetic storms on the Earth," Dr Kennewell says.
Some scientists still study eclipses for the atmospheric effects observed, while some biologists monitor the behaviour of animals during eclipses.
"So while it is true that space based instruments have significantly reduced the interest in total solar eclipses, it has not totally disappeared, and with significantly reduced funding for replacement space observations, it may well be that future interest in ground-based eclipse studies will increase," Dr Kennewell says.
"Apart from the scientific interest, a total solar eclipse is an awe-inspiring event. Everyone should aspire to observe at least one such eclipse in their lifetime."
- The Milky Way — a fairly typical galaxy, as the sun is a typical star — has up to 400 billion stars, about 10 billion of them like our sun.
- The sun, which is 109 times the width of Earth, has a mass 745 times greater than that of all the solar system's planets combined. It consists of 73 per cent hydrogen, 24 per cent helium and a sprinkling of heavier elements.
- All the planets and their satellites orbit the sun, which travels around the middle of the Milky Way, obeying the laws that Isaac Newton uncovered.
- The sun completes one rotation in 27 days.
- The solar system, cavorting around the Milky Way at about 220 kilometres per second, has so far completed 20 galactic orbits in its 4.5-billion-year lifetime.
- Each galactic orbit takes more than 220 million years. That may seem a long time for such a fast mover, but the galaxy, after all, is a big place. Shaped like a disc, with flat, spiralling arms sprouting from a central, spherical bulge, the Milky Way measures 100,000 light years from one side to the other. Our solar system is about 30,000 light years from the galactic core. (A light year is the distance light travels in a year, almost 9.5 trillion kilometres.)
• Learn more about solar physics at solarscience.msfc.nasa.gov
• Check out the work of the Swinburne Centre for Astrophysics and Supercomputing at astronomy.swin.edu.au
• Join Roberto Soria as he studies the sky at the International Centre for Radio Astronomy Research at www.icrar.org
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