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Revealing the True Solar Corona

22 May 2010

This is an article from American Scientist May/June 2010 volume 98:3, p212-9, and is written by Richard Woo. It can be fully downloaded from www.americanscientist.org/issues/pub/2010/3/revealing-the-true-solar-corona/1 but when downloading set the printer to landscape mode as the text is wide.

In his 1996 book The Solar Corona, Donald Billings described the 19th century roots of coronal observation and science. The white light corona, a halo of light revealed during solar eclipses, was judged to be the atmosphere of the Sun – and it has taken another 150 years of investigation of the visible corona to come to some kind of resolution. Recent results have completely revised current tenets of the solar atmosphere – not least regarding the solar wind. Determining where this might emanate from on the Sun and how it evolves over distances between the Sun and the planets, has proved difficult. Although the magnetic field of the Sun is thought to have a role there are no remote sensing measurements to define the global distribution of the magnetic field or plasma flow in the corona. Faced by this obstacle, research into solar wind looked at coronal imaging. However, ultraviolet imaging of magnetic field lines in a loop between the earth and the Sun  revealed plasma trapped in such loops  but no such images were produced of the outward flow of the solar wind which does not involve such a closed loop structure. This was a failure on the part of science – what can't be seen is not there, unless it can be proved to be so in some other fashion. At the same time science failed to understand the white light images themselves and it was assumed that solar wind velocity measurements from the Ulysses spacecraft actually confirmed the current tenets – but they did not. In such a paradigm much of the solar wind originated from polar coronal holes during solar minumum before diverging into the solar system at large. Its expansion is described as super radial. Recent findings now suggest super radial expansion is a false impression and polar coronal holes are an illusion, if you like, artifacts produced by density differencing. The erroneous picture of the origin and evolution of the solar wind has dominated solar wind studies for the last 60 years or so, perpetuated by theoretical models that are inspired by and continue to replicate the same unconfirmed preconception and misinterpretation of processed white light pictures.These have been extrapolated into the heliosphere at large and are the basis of models – computer simulations of solar wind activity. Plasma density is the only solar wind parameter that has been extensively observed in both corona and the solar wind so one approach is to explore the expansion of the solar wind by following plasma density as wind flows outwards from the Sun – and density appears to spread out in all directions (rather than in an arc). It is now realised the solar wind emanates from the entirety of the Sun – and not just from its poles. Spacecraft radio signals have been used to compliment optical measurements and as such, radio signals have been instrumental in understanding the structure of white-light and radio data. Up to now part of the problem has been on relying on images. Pictures are instant and influential – and seeing is believing. Images and computers have made it easy to model and produce verisimilar simulations of the preconceived – but unverified – corona. Images can hinder, blur, or even obscure knowledge – in this instance ultraviolet imaging was unable to show the full radial outflow of the solar wind and therefore models incorporated a completely false picture of what is happening. How true is this of other sciences, I wonder, where models dominate and computer simulation, on occasion, replaces logic – and fails to join up the dots.  

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