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Cutoff period for slow magnetoacoustic waves in coronal plasma structures 

Andrey Afanasyev Submitted: 20150927 23:26
Context. There is abundant observational evidence of longitudinal compressive waves in plasma structures of the solar corona, which are confidently interpreted in terms of slow magnetoacoustic waves. The uses of coronal slow waves in plasma diagnostics, as well as analysis of their possible contribution to coronal heating and the solar wind acceleration, require detailed theoretical modelling.
Aims. We investigate the effects of obliqueness, magnetic field, and nonuniformity of the medium on the evolution of longwavelength slow magnetoacoustic waves guided by fieldaligned plasma nonuniformities, also called tube waves. Special attention is paid to the cutoff effect due to the gravity stratification of the coronal plasma.
Methods. We study the behaviour of linear tube waves in a vertical untwisted straight fieldaligned isothermal plasma cylinder. We apply the thin flux tube approximation, taking into account effects of stratification caused by gravity. The dispersion due to the finite radius of the flux tube is neglected. We analyse the behaviour of the cutoff period for an exponentially divergent magnetic flux tube filled in with a stratified plasma. The results obtained are compared with the known cases of the constant Alfvén speed and the pure acoustic wave.
Results. We derive the wave equation for tube waves and reduce it to the form of the Klein?Gordon equation with varying coefficients, which explicitly contains the cutoff frequency. The cutoff period is found to vary with height, decreasing significantly in the lowbeta plasma and in the plasma with the beta of the order of unity. The depressions in the cutoff period profiles can affect the propagation of longitudinal waves along coronal plasma structures towards the higher corona and can form coronal resonators.
Authors: A.N. Afanasyev, V. M. Nakariakov
Projects: None

Publication Status: Accepted for publication in A&A
Last Modified: 20150928 16:18




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