Our Sun, an average star in a backwater of the Milky Way galaxy, has excited both awe and curiosity since the dawn of humankind. Scientists have been studying the Sun intensively since around the mid 1800s, when the study of sunspots led the English amateur astronomer Richard Carrington to observe a solar flare, and suspect a connection with the geomagnetic storm which followed it.
The immense quantity of radiation continually pouring our way from the solar surface is mostly in the visible band, so it appears yellow in the sky (or orange or red, depending on atmospheric distortions). Naturally most of the knowledge we have gained about the Sun in the past – such as its size and chemical composition – is based on visual observations. In recent decades, however, technology has developed to provide us with further sources of information which have greatly added to our understanding of our Sun, and of stars generally.
One of the most interesting and mysterious discoveries to have been made in recent decades is that the Sun’s corona, or atmosphere, is massively hotter than its surface temperature: a fact that seems to fly in the face of the laws of physics.
The sun radiates at many wavelengths other than those of the very narrow visible part of the electromagnetic spectrum. The Earth’s atmosphere and magnetic field prevent much of this radiation from reaching the Earth’s surface and being observed. Only visible light, some infrared and ultraviolet, and certain radio wavelengths make it through. Luckily for life on earth, the high-energy x-rays and gamma rays don’t make it at all.
With the advent of the space age, however, we have been able to send telescopes and cameras into space, such as the Soft X-Ray Telescope on the Yohkoh satellite which orbited the Earth during the nineties and transmitted stunning images for over nine years. These images have provided answers to some questions, but have raised new ones in their place. The discovery, first made in the 1940s, that our sun produces x-rays came as something of a surprise; images from the Yohkoh telescope clearly confirm x-ray emissions from various points on the Sun’s surface. Although the temperature at its core is around 15 million Kelvin, the surface measures a mere 5770K (NASA’s figure) or around 5500 degrees Celsius – nowhere near hot enough to produce such high energy radiation. In fact the source of the x-rays is the corona, the Sun’s tenuous atmosphere.
The Sun’s corona extends to around 1 million km above the photosphere (surface), and consists of a plasma, a hot ionised gas, defined as the fourth state of matter. A plasma will only produce x-rays when heated to above 1 million K. The corona is estimated to have a temperature of 1-2 million K! Why the corona is over 170 times hotter than the sun’s surface, when it should be cooler according to the laws of physics, is a mystery which has not yet been fully solved, although the solution seems to be near.
Part of the puzzle lies in the fact that without a consistent and uniform method of heating, the plasma in the corona would cool down in an hour. Until recently scientists were at a loss for a reasonable hypothesis that could explain the phenomenon. Then small patches of magnetic field were found to cover the Sun’s entire surface – thanks to images produced by instruments devoted to studying the Sun aboard the spacecraft SOHO and TRACE. Unlike the large magnetic loops associated with active regions during solar maxima (the peak of the 11-year sunspot cycle), these small patches come and go randomly in periods of very roughly 40 hours. It is now generally believed that magnetic field lines radiating from these patches interact by coming close, snapping and ‘rearranging’ themselves, according to the laws of electromagnetism which prevent their crossing, and that these small interactions, which are nevertheless vast in number, are the cause of the heating of the corona. This theory has not yet been proven; in fact, no direct observation of such ‘magnetic reconnection’ has yet been made.
However, x-ray and other observations of solar surface activity do seem to support the theory so far. The Japanese Hinode (‘Sunrise’) Observatory, launched into Earth orbit in September 2006 and carrying three instruments for recording optical, extreme ultraviolet and x-ray data, has given the world some magnificent images which support the magnetic reconnection theory.
Never before has the solar surface been seen in such fine detail! The Hinode instruments are able to resolve an area of the Sun the size of Wales – nearly six times smaller than previously. The x-ray telescope/camera is able to capture images very quickly (one per second) and has a resolution three times that of its forerunner, Yohkoh. When it was first brought online in 2006 it was pointing at a ‘dark’ area of the sun where little activity was expected, only to reveal an abundance of activity in the form of ‘microflares’, or plasma jets, twinkling all over the visible surface.
The large number and the frequency of these plasma jets, visible in the chromosphere and lower corona, have at last provided the scientific community with a possible explanation of how the Sun’s corona might be heated. The magnetic interactions produce large amounts of energy which heat up the plasma and cause the jets. These occur at a rate of up to 240 per day and are uniformly distributed around the Sun’s surface, including at the poles and in sunspots. The jets in turn produce what are known as ‘Alfven’ waves which propagate and in theory carry enough energy into the corona to heat it to millions of degrees Kelvin. They have also been estimated to be responsible for up to 25% of the solar wind, the stream of highly charged particles which escapes the Sun at a rate of over 1.5 million kph.
The solution of the 50 year old mystery seems close, and it appears likely that in the next few years complete answers will be found as Hinode continues its mission to explore the link between the Sun’s magnetic field and the mechanisms powering the solar atmosphere. In the meantime the world can look forward to many more pretty images of our mysterious Sun.
