Chromospheric evaporation and phasemixing of Alfvén waves in coronal loops

Abstract

Context. Phasemixing of Alfvén waves has been studied intensively as a possible coronal heating mechanism but without the full thermodynamic consequences considered self-consistently. Cargill et al. (2016) argued conceptually that in some cases, the thermodynamic feedback of the heating could substantially affect the transverse density gradient and even inhibit the phasemixing process. Aims. In this paper, for the first time, we use MHD simulations with the appropriate thermodynamical terms included to quantify the evaporation following heating by phasemixing of Alfvén waves in a coronal loop and the effect of this evaporation on the transverse density profile. Methods. The numerical simulations are performed using the Lagrangian Remap code Lare2D. We set up a 2D loop model consisting of a field-aligned thermodynamic equilibrium and a cross-field (background) heating profile. A continuous, sinusoidal, high-frequency Alfvén wave driver is implemented. As the Alfvén waves propagate along the field, they undergo phasemixing due to the cross-field density gradient in the coronal part of the loop. We investigate the presence of field aligned flows, heating from the dissipation of the phasemixed Alfvén waves and the subsequent evaporation from the lower atmosphere. Results. We find that phasemixing of Alfvén waves leads to modest heating in the shell regions of the loop and evaporation of chromospheric material into the corona with upflows of the order of only 5-20 m/s. Although the evaporation leads to a mass increase in the shell regions of the loop, the effect on the density gradient, and hence on the phasemixing process, is insignificant. Conclusions. This paper investigates self-consistently the effect of chromospheric evaporation on the cross-field density gradient and the phasemixing process in a coronal loop. We found that the effects in our particular setup (small amplitude, high frequency waves) are too small to significantly change the density gradient.