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Astron. Astrophys. 327, 377-387

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Magnetoacoustic wave propagation in current sheets

J.M. Smith1, B. Roberts1, and R. Oliver2

1School of Mathematical and Computational Sciences, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK
2Departament de Física, Universitat de les Illes Balears, E-07071 Palma de Mallorca, Spain

Received 13 February 1997 / Accepted 28 March 1997


Magnetoacoustic waves in a Harris neutral current sheet are investigated. The magnetic field strength, Alfvén speed, sound speed and plasma density vary continuously across the structure. The sheet supports body, surface and hybrid modes. Two surface modes exist (a kink and a sausage). In addition, fundamental and harmonic body waves are found when their phase speeds approach the maximum Alfvén speed in the sheet. As the phase speeds of the modes decrease, the waves have properties of both body and surface modes - a hybrid wave. In the long wavelength limit the phase speed of the fundamental kink body mode and sausage surface mode approach the maximum Alfvén and tube speeds respectively. At short wavelengths, the phase speeds tend to constant values, each pair of kink and sausage modes merging after passing through the maximum value of the sound speed, csmax. For all modes, in the inner part of the current sheet, the gas pressure is out of phase with the magnetic pressure, with the gas pressure dominating (a property of a slow mode). In the outer part of the sheet the magnetic pressure is in phase with the total pressure (a fast mode characteristic). No purely fast or slow mode exists. For uniform density a minimum in the group velocity occurs only for the fundamental kink mode. For the Epstein density profile, minima in the group velocity are found for the other modes also. Impulsively generated waves are expected to show temporal variations that are similar to observations of oscillations of radio and X-ray emission of the solar corona and neutral sheet oscillations in the Earth's magnetotail. Calculated periods of oscillation are in good agreement with observations.

Key words: MHD - Sun: corona - Sun: magnetic field - Sun: oscillations - solar system: Earth

Send offprint requests to: J.M. Smith

© European Southern Observatory (ESO) 1997

Online publication: October 1, 1997
Last change: April 8, 1998