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Coronal activity captured Vibrating magnetic by the SOHO spacecraft structures in the solar Studies so far seem to atmosphere are indicate that the key to opening up a new way understanding the corona lies in its complex magnetic structure. to study the Sun The corona is dominated by a magnetic field. That field is rooted in the dense layers of the photosphere, where it is influenced by the churning and convective upwelling of material from the layers beneath, interacting with localised magnetic fields that are in the form of magnetic tubes. The tubes range in diameter from a few hundred kilometres up to tens of thousands of kilometres, the larger structures being visible as sunspots. High above the photosphere, in the corona, magnetic fields expand to fill the . There, they control much of what we see – be it the dramatic solar flares with loops of material ejected violently into space, or the quieter long-lived events that generate the numerous wispy structures seen in the CBernard Robertso and Valeryr Nakariakovonal corona, including the coronal loops. he Sun’s corona is remarkable for its beauty. First revealed in photographs of total solar eclipses,T the corona is now routinely studied seismology from spacecraft, which have exposed in dramatic detail a stunning pattern of – a new science streamers and loops in the thin ionised gas, or plasma, constituting it. This complexity is physicists. They are important not only for Magnetic vibrations nowhere more evident than in the our understanding of fundamental aspects Recently, observations from TRACE and remarkable data gathered daily by the of plasma astrophysics but also for the SOHO began revealing a new phenomenon NASA solar space mission – the Transition new branch of applied space physics known which may provide the answers we are Region And Coronal Explorer (TRACE), as space weather (which discusses how seeking. Magnetic fields embedded in a and the ESA-NASA mission – Solar and the solar wind affects our near-space plasma like the corona are elastic, and this Heliospheric Observatory (SOHO). environment). Plasmas are ubiquitous in the means they can oscillate, creating ‘magnetic Nevertheless, the corona presents an Universe and play significant roles in, for ’. We can describe the waves using the enigma. It is much hotter than the example, the generation of magnetic fields theory of (MHD) photosphere (the apparent surface of Sun within a star’s interior and the production – a subject that is a marriage of fluid as we see it in the sky) that lies below, of activity in its atmosphere, including its mechanics and electromagnetism. This reaching one million degrees or more ability to produce and emit X-rays. In any theoretical approach gives rise to three (compared with about 6000 degrees for the case, a full understanding of the physical kinds of MHD , which are in fact 12 surface). The corona also releases into space processes operating in the Sun’s corona – analogous to the more familiar sound waves a stream of accelerated charged particles, and by implication in the coronae of many in an ordinary gas such as air and the the solar wind. The mechanisms behind stars – is likely to involve a mixture of new vibrations of a guitar string (the magnetic these phenomena have long intrigued solar observations and theory. field playing the role of the elastic string). Researchers have long thought that MHD which tends to look at the rapidly! This is in marked contrast to sound waves would occur in the corona but there solar interior as a whole (though it is also waves in the solar interior. We do not know were few observations to support this idea. beginning to probe local features). what is responsible for this dramatic coronal Now, TRACE and SOHO have provided We have already made progress in this effect, though very recent theoretical work unequivocal evidence that the magnetic direction. One major uncertainty is the carried out at the Universities of Sheffield loop structures seen in the corona do in fact magnetic field strength in localised and St Andrews reveal a promising vibrate, the vibration perhaps following a possibility connected with the flare or similar event. In fact, the key to inhomogeneity of the corona. But they oscillate in distinctive whatever is responsible for the decay, this ways that may be interpreted understanding the corona too carries distinct information in terms of MHD wave theory about the coronal plasma. All this – the basis of which our research group at lies in its complex encourages us to believe that the University of St Andrews developed magnetic structure coronal seismology will develop almost two decades ago! fruitfully in the near future. ✦ The presence of these waves in the corona is perhaps not surprising, given its structures such as coronal loops. We can Professor B. Roberts is an applied magnetic nature. What is exciting is that determine magnetic field strengths at the mathematician and Professor of Solar such magnetic waves, long thought to play solar surface from spectra (a magnetic field Magnetohydrodynamics at the University of a part in explaining coronal heating, have splits the spectral lines, producing the St Andrews. Dr V. Nakariakov is a senior now been detected and their properties so-called Zeeman splittings). This reveals lecturer in physics at Warwick University. measured, even though their role in coronal fields of 3000 gauss in sunspots, to be E-mails: [email protected]; heating remains unclear. compared with the Earth’s magnetic field of [email protected] less than a gauss. But we cannot apply this A new source of information method in the almost vacuum-like medium The first observational example of ‘kink’ oscillations (waving from side to side) of coronal loops taken Waves have another role: they carry of the corona. However, by fitting the by the TRACE satellite in 1998. These were first information about the medium through observed oscillations with MHD wave predicted by Bernie Roberts 20 years ago. The which they pass. This means that detecting theory, we can determine that the field diagram shows a possible mechanism for excitation of kink oscillations of coronal loops by a solar flare magnetic waves in the corona offers the strength in a is about 13 gauss. possibility of obtaining information about These studies, which were carried out at the coronal plasma that is otherwise Warwick University and elsewhere, are unavailable. This is by no means a new still at an early stage, but they suggest approach. Just as geophysicists have used that coronal seismology using MHD waves elastic waves to find out about the Earth’s will be a useful tool in studying the outer interior (seismology), so solar physicists solar structure. have exploited sound waves to learn about There is another feature of the coronal the solar interior (helioseismology). Sound oscillations seen by TRACE and SOHO that waves trapped in the Sun are measured, in is worth mentioning: the oscillations decay the surface oscillations they produce, by ground-based observatories such as BISON (operated by Birmingham University) and spacecraft such as SOHO. The data can then Flare epicentre be used to determine, for example, how the various layers in the Sun rotate or how the Kink oscillations sound speed increases with depth beneath the surface of the Sun. Helioseismology has proved a great Loop success and we hope to repeat this success for the corona. Observing coronal oscillations, coupled with magnetic wave theory, offers the prospect of developing a new method for studying the Sun – coronal 13 seismology. Because magnetic fields control Blast wave Footpoint (fast magnetoacoustic) the structure of the corona it is likely that the technique will be able to investigate Footpoint localised phenomena – in contrast to