The sound of the Sun

by observing its resonant modes of oscillation. This field has been associated, historically, with internal modes that are acoustic in nature (p modes) – including radial oscillations of the whole interior – and attempts to uncover their gravitational counterparts. However, in the past half-decade or so, observations of resonant phenomena have been extended into the Sun’s tenuous outer atmosphere, where oscillations of coronal structures have been uncovered. Helioseismology should therefore now be regarded as a field that encompasses a “resonant choir” from the core all the way out to the corona. Intriguingly, observations of flares in some very active late-type stars have raised the possibility that coronal oscillations may turn out to be a ubiquitous feature (Mullan and Johnson 1995, Mathioudakis et al. 2003, Mitra-Kraev et al. 2005); internal p modes have already been detected, with impressive precision, in over halfa-dozen late-type, Sun-like stars (e.g. see Bedding and Kjeldsen 2003). The RAS discussion meeting “Seismology of the Sun: from the inner core to corona”, held earlier this year, focused on aspects of helioseismology in the interior and atmosphere (see Ballai and Chaplin 2005 for a full report). In this article we consider some of the important issues raised at the meeting that face both areas of the field. The Sun is an elastic, and therefore ideal, medium for wave propagation. Waves propagating in various solar regions have the unique ability to carry information about their environment, and therefore serve as a perfect tool for diagnostics. Measurement of the properties of waves and resulting resonant oscillations, combined with theoretical modelling of the wave phenomena, can lead to a determination of the mean plasma and field parameters. Internal seismology – that could be called “esoseismology”, with “eso” from the Greek meaning inside – is now well established. The first observations of the five-minute oscillations of the photosphere, the visible manifestation of standing acoustic waves in the interior, were made in 1960 (Leighton et al. 1962). It took a further decade before a theoretical basis for the phenomenon, internal standing waves, was proposed (offered independently by Ulrich [1970] and Leibacher and Stein [1971]); and half a decade after that before observational confirmation of the rich spectrum of overtones was forthcoming (Deubner 1975, Rhodes et al. 1977). These initial observations were of modes with small horizontal wavelengths (high angular degree) that penetrated only a shallow distance into the convection zone. That the supported range of modes extended all the way to those for which either a substantial, or the entire, interior volume was engaged in pulsation became apparent only at the end of the 1970s (Claverie et al. 1979) thanks to observations of low-degree overtones (although the possibility had already been discussed, e.g. Wolff 1972, 1973). The development of esoseismology, therefore, followed a path in which observations of new phenomena were made, competing theories were proposed to explain them, and improved observations allowed the theories to be tested and discriminated. In contrast the new field of coronal seismology – or, to maintain consistency in nomenclature, “coronoseismology” – and that of the solar atmosphere, saw theory come before observation. The essential theory of coronal oscillations in magnetic structures was first published in the early 1980s (e.g. Roberts et al. 1984). At the time, available data were of insufficient temporal and spatial quality to reveal, observationally, modes in individual structures. Only with the advent of high-resolution, space-borne observations in the latter part of the 1990s did this breakthrough become possible. Observations have now revealed evidence for wave propagation in almost every layer of the solar atmosphere. Since the associated wavelengths and periods are larger than the ion Larmor radius and gyration period of particles, respectively, these waves can be studied within the framework of magnetohydrodynamics (MHD). The role and importance of the magnetic field will determine a different approach inside the Sun and in its atmosphere. Below or near the photosphere the plasma β parameter (the ratio