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Coronal Seismology
[edit]Coronal seismology is a novel technique for the diagnostics of the plasma of the solar corona by means of magnetohydrodynamic (MHD) waves (see magnetohydrodynamics) and oscillations. Observed parameters of the waves (e.g. periods and their ratios, spectra, characteristic signature) are linked with the results of MHD wave theory and full scale numerical modelling, revealing the physical parameters which are not open to direct observations. In particular, the potential of coronal seismology in the estimation of the coronal magnetic field, density scale height, fine structuring and transport coefficient has been demonstrated by different research groups. Originally, the method of MHD coronal seismology was theoretically suggested by Uchida (1970) for global and Roberts et al. (1984) for local diagnostics, but was not practically applied until late 90th because of lack of necessary observational resolution.
Philosophically, Coronal Seismology is similar to the Earth's seismology, helioseismology, and MHD spectroscopy of laboratory plasma devices. In all these approaches, waves of various kind are used to probe a medium.
Wave and oscillatory phenomena are observed in the hot plasma of the corona mainly in EUV, optical and microwave bands with a number of spaceborne and ground-based instruments, e.g. the Solar and Heliospheric Observatory (SoHO), the Transition Region and Coronal Explorer (TRACE), the Nobeyama Radioheliograph (NoRH, see the Nobeyama radio observatory). Phenomenologically, the researchers distinguish between compressible waves in polar plumes and in legs of large coronal loops, flare-generated transverse oscillations of loops, acoustic oscillations of loops, propagating kink waves in loops and in supra-arcade structures, sausage oscillations of flaring loops, and oscillations of prominences and fibrils (see solar prominence), and this list is continuously updated.
Theoretical foundation of coronal seismology is the dispersion relation of MHD modes of a plasma cylinder: a plasma structure nonuniform in the transverse direction and extended along the magnetic field. This model suits well for the description of a number of plasma structures observed in the solar corona: e.g. coronal loops, prominence fibrils, plumes. Such a structure acts as a waveguide of MHD waves. There are several distinct kinds of MHD modes which have quite different dispersive, polarisation, and propagation properties:
- Kink (or transverse) modes which are oblique fast magnetoacoustic waves guided by the plasma structure; these modes are weakly compressible, but could be observed with imaging instruments as periodic standing or propagating displacements of coronal structures, e.g. coronal loops.
- Sausage (or peristaltic) modes which also are oblique fast magnetoacoustic waves guided by the plasma structure; these modes are compressible and cause significant variation of the absolute value of the magnetic field in the oscillating structure.
- Longitudinal (or slow, or acoustic) modes which are slow magnetoacoustic waves propagating mainly along the magnetic field in the plasma structure; these mode are compressible. The magnetic field perturbation is these modes is negligible.
- Torsional (Alfvén or twist) modes which are incompressible transverse perturbations of the magnetic field along certain individual magnetic surfaces. In contrast with kink modes, torsional modes cannot be observed with imaging instruments, as they do not cause the displacement of the structure axis and boundary.
References
1. Roberts, B., Nakariakov, V.M., "Coronal seismology – a new science", Frontiers 15, 2003. [2]
2. Nakariakov, V.M., Verwichte, E., "Seismology of the corona of the Sun", Astronomy & Geophysics 45, 5.32-5.33, 2004.
3. Nakariakov, V.M., Verwichte, E., "Coronal Waves and Oscillations", Living Rev. Solar Phys. 2, (2005), 3. URL (cited on <date>): [3]