Instability of twisted magnetic tubes with axial mass flows

摘要

Context. Recent observations of various kinds of jets in thesolar atmosphere motivate studying the influence of mass flow on thestability of solar magnetic structures. Aims. We study the influence of axial mass flows on the stability of twisted magnetic flux tubes. Methods. We use the incompressible magnetohydrodynamic equationsto get the dispersion relation governing the behaviour of normal modesin uniformly twisted magnetic tubes with sub-Alfvénic flows. Thedispersion relation is then solved analytically and numerically to findstability criteria for twisted tubes with flow. Results. Two main important results are found. First, the axialmass flow reduces the threshold of kink instability in twisted magnetictubes. Second, the twist of magnetic tubes leads to theKelvin-Helmholtz instability of sub-Alfvénic flows for the harmonicswith a large enough azimuthal wave number -m. Conclusions. The observed mass flow may trigger the kinkinstability in magnetic configurations that are near their stabilitythreshold, leading to solar flares and coronal mass ejections. Theeffect is more significant for photospheric magnetic tubes than forcoronal ones. Sub-Alfvénic flows undergo the Kelvin-Helmholtzinstability in slightly twisted magnetic tubes if the azimuthalwavenumber is big enough. Key words: magnetohydrodynamics (MHD) - instabilities - Sun: atmosphere