The properties of magnetohydrodynamic (MHD) waves are governed by the conditions of the plasma through which they travel. On the other hand, by measuring the properties of the wave the local plasma conditions can be inferred. This technique is general in physics and is called seismology. In plasma physics, there are two interesting applications of seismology: MHD coronal seismology and MHD spectroscopy.
Important physical coronal parameters such as the magnetic field strength and dissipation coefficients, essential to understanding the coronal heating problem, are not well-known. In 1970 Yutaka Uchida recognised that Moreton waves may be used as a mechanical diagnosing tool of coronal structures. In 1984 Bernard Roberts and colleagues (Roberts, Edwin & Benz, 1984) demonstrated that magnetacoustic oscillations in solar coronal loops provides a potentially useful diagnostic tool for determining physical conditions in the inhomogeneous corona. This diagnostic tool is often known as MHD coronal seismology.
With the advent of space-born observatories in the last decade, i.e. Yohkoh, SoHO and TRACE, the spatial and temporal signatures of (the predicted) coronal MHD waves have been identified, and also now found in ground-based observations (recent and in the past):
From the MHD wave signatures it has been possible to derive physical parameters of the solar corona: Alfvén speed and magnetic field strength from transverse loop oscillations (Nakariakov & Ofman, 2001), transport coefficients from dissipative models of rapid damping of transverse loop oscillations (Nakariakov et al., 1999) and propagating slow magnetoacoustic waves in loops (Verwichte et al., 2001), transverse loop structuring using curved loop models on transverse loop oscillations (Verwichte et al., 2006).
Goedbloed et al. (1993) applied the same principles to magnetically controlled fusion plasmas where the technique is called MHD spectroscopy. There, in frequency gaps between successive Alfvén wave continua, which are indiced by geometrical factors (toroidicity, ellipticity,...), discrete Alfvén Eigenmodes exist. Such modes provide information about the safety factor q in the plasma centre (Holties et al., 1997), rotation speed of bulk plasma and impurities (Testa et al., 2002), role of fast particle drive,...
Compared with MHD coronal seismology, MHD spectroscopy has the added advantage that the MHD modes can be actively excited and intensively studied. Therefore, we distinguish active and passive MHD spectroscopy.
Research review article:
Nakariakov, V.M. and Verwichte, E.: 2005, Coronal Waves and Oscillations, Living Rev. Solar Phys. 3, 1-63.
Research review article:
Nakariakov, V.M. and Verwichte, E.: 2004, Seismology of the corona of the Sun, Astron. Geophys. 45 (4), 26-27.