Kink Waves in Twisted and Expanding Magnetic Tubes

Solar Physics. 2023. Vol. 298. Article 125.

Ruderman M. S., N. S. Petrukhin

We study kink and fluting waves in expanding and twisted magnetic flux tubes. We use the
thin-tube and zero-beta plasma approximations. The equilibrium magnetic field is force free
with a constant proportionality coefficient between the electrical current and the magnetic
field. We derive the equation governing the kink and fluting waves in a tube. Using this
equation we study the propagation of kink waves in a particular case of a magnetic tube
homogeneous in the axial direction. We show that while there is only one propagating kink
wave with the phase speed equal to the kink speed in an untwisted tube, in a twisted tube
there are two wave modes, accelerated and decelerated. The phase speed of the accelerated
wave exceeds the kink speed, while the phase speed of the decelerated wave is less than
the kink speed. We also show that the standing modes are defined by the same eigenvalue
problem as that in the case of an untwisted tube. Hence, the frequencies of the standingwave
modes are not affected by the twist. This implies that the seismological results based
on the observation of the standing-wave mode frequencies remain valid when the twist is
taken into account. The only effect of twist is the variation of the direction of polarisation
of the coronal magnetic-loop displacement along the loop. As a result, an apparent node can
be detected near the loop apex if only one component of the loop displacement is observed.
This can lead to an incorrect conclusion that the observed coronal loop kink oscillation was
the first overtone, while in fact it was the fundamental mode

Keywords: колебания, волны, waves, oscillations, coronal magnetic loops, корональные магнитные петли, корона, corona