We investigate the conditions at which high-mode magnetohydrodynamic (MHD) waves propagating in a spinning solar macrospicule can become unstable with respect to the Kelvin-Helmholtz instability (KHI). We consider the macrospicule as a weakly twisted cylindrical magnetic flux tube moving along and rotating around its axis. Our study is based on the dispersion relation (in complex variable) of MHD waves obtained from the linearized MHD equations of incompressible plasma for the macrospicule and cool (zero beta) plasma for its environment. That dispersion equation is solved numerically at appropriate input parameters to find out an instability region/window that accommodates suitable unstable wavelengths of the order of macro\-spicule's width. It is established that a m = 52 MHD mode propagating in a macro\-spicule with width of 6~Mm, axial velocity of 75 km s-1 and rotating one of 40 km s-1 can become unstable against KHI with instability growth times of 2.2 and 0.57~min at 3 and 5~Mm unstable wavelengths, respectively. These growth times are much shorter than the macrospicule lifetime of around 15~min. An increase/decease in the width of the jet would change the KHI growth times remaining more or less of the same order when are evaluated at wavelengths equal to the width/radius of the macrospicule. It is worth noticing that the excited MHD modes are supper-Alfvénic waves. A change in the background magnetic field can lead to another MHD mode number m that ensures the required instability window.
Authors: Ivan Zhelyazkov and Ramesh Chandra
Publication Status: accepted for publication in Solar Physics
Last Modified: 2018-12-26 12:06