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We investigate the conditions at which high-mode magnetohydrodynamic (MHD)waves propagating in a spinning solar macrospicule can become unstable withrespect to the Kelvin--Helmholtz instability (KHI). We consider themacrospicule as a weakly twisted cylindrical magnetic flux tube moving alongand rotating around its axis. Our study is based on the dispersion relation (incomplex variables) of MHD waves obtained from the linearized MHD equations ofan incompressible plasma for the macrospicule and cool ($\beta = 0$, rate ofthe plasma to the magnetic pressure) plasma for its environment. Thisdispersion equation is solved numerically at appropriate input parameters tofind out an instability region or window that accommodates suitable unstablewavelengths on the order of the macro\-spicule width. It is established that an$m = 52$ MHD mode propagating in a macro\-spicule with width of $6$~Mm, axialvelocity of $75$~km\,s$^{-1}$, and rotating one of $40$~km\,s$^{-1}$ can becomeunstable against the KHI with instability growth times of $2.2$ and $0.57$~minat $3$ and $5$~Mm unstable wavelengths, respectively. These growth times aremuch shorter than the macrospicule lifetime, which lasts about $15$~min. Anincrease or decease in the width of the jet would change the KHI growth times,which remain more or less on the same order when they are evaluated atwavelengths equal to the width or radius of the macrospicule. It is worthnoting that the excited MHD modes are super-Alfv\'enic waves. A change in thebackground magnetic field can lead to another MHD mode number $m$ that ensuresthe required instability window.

Can High-Mode Magnetohydrodynamic Waves Propagating in a Spinning Macrospicule Be Unstable due to the Kelvin--Helmholtz Instability?