Cutoff of transverse waves through the solar transition region

Abstract

Context. Transverse oscillations are ubiquitously observed in the solar corona, both in coronal loops and in open magnetic flux tubes. Numerical simulations suggest that their dissipation could heat coronal loops, thus counterbalancing radiative losses. These models rely on a continuous driver at the footpoint of the loops. However, analytical works predict that transverse waves are subject to a cutoff in the transition region. It is thus unclear whether they can reach the corona and indeed heat coronal loops. Aims. Our aims are to determine how the cutoff of kink waves affects their propagation into the corona and to characterize the variation of the cutoff frequency with altitude. Methods. Using 3D magnetohydrodynamic simulations, we modelled the propagation of kink waves in a magnetic flux tube, embedded in a realistic atmosphere with thermal conduction, which starts in the chromosphere and extends into the corona. We drove kink waves at four different frequencies and determined whether they experienced a cutoff. We then calculated the altitude at which the waves were cut off and compared it to the prediction of several analytical models. Results. We show that kink waves indeed experience a cutoff in the transition region, and we identified the analytical model that gives the best predictions. In addition, we show that waves with periods shorter than approximately 500 s can still reach the corona by tunnelling through the transition region with little to no attenuation of their amplitude. This means that such waves can still propagate from the footpoints of loop and result in heating in the corona.