Damping of Slow Surface Sausage Modes in Photospheric Waveguides

Abstract

There has been considerable interest in sausage modes in photospheric waveguides like pores and sunspots, and slow surface sausage modes (SSSMs) have been suggested to damp sufficiently rapidly to account for chromospheric heating. Working in the framework of linear resistive magnetohydrodynamics, we examine how efficient electric resistivity and resonant absorption in the cusp continuum can be for damping SSSMs in a photospheric waveguide with equilibrium parameters compatible with recent measurements of a photospheric pore. For SSSMs with the measured wavelength, we find that the damping rate due to the cusp resonance is substantially less strong than theoretically expected with the thin-boundary approximation. The damping-time-to-period ratio (τ /P ) we derive for standing modes, equivalent to the damping-length-to-wavelength ratio for propagating modes given the extremely weak dispersion, can reach only ∼ 180. However, the accepted values for electric resistivity (η) correspond to a regime where both the cusp resonance and resistivity play a role. The values for τ /P attained at the largest allowed η may reach ∼ 30. We conclude that electric resistivity can be considerably more efficient than the cusp resonance for damping SSSMs in the pore in question, and it needs to be incorporated into future studies on the damping of SSSMs in photospheric waveguides in general.

Finding the mechanism of wave energy flux damping in solar pores using numerical simulations

“…Although they find that Article number, page 1 of 12 arXiv:2102.12420v1 [astro-ph.SR] 24 Feb 2021 A&A proofs: manuscript no. main this damping mechanism is stronger than previously expected, the numerical studies of Chen et al (2018), validated by analytic calculations of Geeraerts et al (2020), show that damping due to electrical resisitvity is much more potent than that due to resonant absorption. However, this alone is not enough to account for the damping.…”

Magnetoacoustic wave energy dissipation in the atmosphere of solar pores

“…Such energy structuring can be attributed to the presence of surface (pores 2, 3 and 4) and body (pores 1 and 5) mode waves, which have only recently been conclusively observed in photospheric imaging data [21]. Previous identifications of surface mode waves have come from theoretical estimations [17,18,58,59]. However, here we clearly show the existence of heightened wave energy towards the edges of the pore boundaries, which has only been made possible by the high spectral, spatial and temporal resolutions of our dataset coupled with modern spectropolarimetric inversion routines.…”

“…Recent theoretical work has shown that the degree of magnetic twist in the waveguide can drastically affect the damping rate of surface sausage mode waves [68]. Furthermore, the interplay between electric resistivity and resonant absorption may also contribute to the differing damping lengths found between the surface- and body-mode waves [59].…”

“…Both of these values are on the same order of magnitude as the photospheric scale height, which we estimate to be H ≈ 170 km. We hypothesize that the surface mode waves may be able to dissipate their energy more readily, through a combination of magnetic twist, electric resistivity and/or resonant absorption mechanisms [59,68]. We appreciate that the measured difference in damping lengths is small and based around a limited number of pores.…”

Mixed properties of magnetohydrodynamic waves undergoing resonant absorption in the cusp continuum

“…Yu et al (2017b) analytically calculated the damping rate of these modes by resonant absorption in the slow continuum in the thin boundary limit. In a numerical study, Chen et al (2018) compared the damping rates of SSSMs due to resonant absorption in the slow continuum with the damping rate due to Ohmic resistivity. Their numerical results were later confirmed by the analytical derivations in Geeraerts et al (2020).…”