MHD Oscillations

Abstract

Just as a music orchestra contains many instruments with different sounds, it was recently discovered that the solar corona also contains an impressively large ensemble of plasma structures that are capable of producing sound waves and harmonic oscillations. Global oscillations from the solar interior were discovered four decades ago, and are mainly pressure-driven (p-mode) oscillations at fundamental periods of 5 minutes, which display spatial nodes on the solar surface according to the spherical harmonics functions, similar to the nodes on the skin of a vibrating drum. The systematic study of these global oscillations created the discipline of helioseismology. Thanks to the high spatial resolution, image contrast, and time cadence capabilities of the SoHO and TRACE spacecraft, oscillating loops, prominences, or sunspots, and propagating waves have been identified and localized in the corona and transition region, and studied in detail since 1999. These new discoveries established a new discipline that became known as coronal seismology. The theory of coronal MHD oscillations and waves was developed two decades ago and was ready for applications, but had to await the high-resolution EUV imaging capabilities that can only be obtained from space. One of the most exciting benefits of coronal seismology is the probing of physical parameters such as Alfvén velocities and magnetic field strengths, which are very difficult to measure in the solar corona by other means. Initial research in the new field of coronal seismology concentrates on measurements of oscillation periods, spatial displacements, damping times, and temperature and density diagnos-tics of oscillating structures. Future exploration is likely to reveal additional aspects on the fundamental physics of wave excitation, wave propagation, waveguides, wave damping, phase mixing, and resonant absorption. Theoretical reviews on coronal oscillations can be found in