Magnetohydrodynamic Waves in Multi-Layered Asymmetric Waveguides: Solar Magneto-Seismology Theory and Application

Matthew Allcock,Robert Erdélyi,Daria Shukhobodskaia,Noémi Kinga Zsámberger

doi:10.3389/fspas.2019.00048

Abstract

Diagnosing the solar atmospheric plasma is one of the major challenges in solar physics. Magnetohydrodynamic (MHD) waves, by means of applying the powerful concept of solar magneto-seismology (SMS), provide a tool to obtain diagnostic insight into the magnetised solar plasma in MHD waveguides. This paper provides a road-map of simple but applicable models of solar atmospheric waveguides in the framework of Cartesian geometry. We focus on exploiting the diagnostic potential of waveguide asymmetry and consider the effects of steady flow. In particular, the dispersion relation describing linear MHD wave propagation along a multi-layered MHD waveguide is derived. Aiming at lower solar atmospheric applications of SMS, the special case of a single magnetic slab embedded in an asymmetric magnetised plasma environment is revisited. As a proof of concept, the Amplitude Ratio Method is used to make a seismological estimate of the local Alfvén speed in several chromospheric fibrils that exhibit asymmetric oscillations. Absolute ratios of boundary oscillations between 1.29 and 3.42 are detected and, despite the significant errors expected, the local Alfvén speed estimates agree with previously derived estimates from magnetic field extrapolations. Finally, the effects of asymmetric shear flows present in these slab MHD waveguides are considered as a suitable model of Kelvin-Helmholtz instability initiation that is applicable, for example, to coronal mass ejection flanks.

Highlights

Over the past few decades, vast improvements in solar telescope technology, with the likes of both space-borne instrumentation, e.g., Solar and Heliospheric Observatory (SoHO), Solar Dynamics Observatory (SDO), and Interface Region Imaging Spectograph (IRIS), and ground-based solar observing facilities, e.g., Dunn Solar Telescope (DST) and Swedish Solar Telescope (SST), have enabled us to resolve the fine sub-structure within many of the larger magnetic features that bejewel the solar atmosphere
MHD waves are a mechanism to transfer non-thermal energy between distant locations in the solar plasma, that is dissipated by physical processes that are yet to be fully understood, like resonant absorption (Goossens et al, 2011), phase mixing (Heyvaerts and Priest, 1983), non-linear shock damping (Ballai and Ruderman, 2011), rather, they are excellent tools to be exploited for plasma diagnostics by solar magneto-seismology (SMS)
In the absence of MHD wave theory in asymmetric cylindrical waveguides, we model each fibril as an isolated magnetic slab whose boundaries are parallel discontinuities between the uniform internal plasma and the asymmetric external plasma (e.g., Figure 2)

Summary

Introduction

Over the past few decades, vast improvements in solar telescope technology, with the likes of both space-borne instrumentation, e.g., Solar and Heliospheric Observatory (SoHO), Solar Dynamics Observatory (SDO), and Interface Region Imaging Spectograph (IRIS), and ground-based solar observing facilities, e.g., Dunn Solar Telescope (DST) and Swedish Solar Telescope (SST), have enabled us to resolve the fine sub-structure within many of the larger magnetic features that bejewel the solar atmosphere. Magnetohydrodynamic (MHD) waves are a key in our understanding of the physical processes in the hot solar plasma. MHD waves are a mechanism to transfer non-thermal energy between distant locations in the solar plasma, that is dissipated by physical processes that are yet to be fully understood, like resonant absorption (Goossens et al, 2011), phase mixing (Heyvaerts and Priest, 1983), non-linear shock damping (Ballai and Ruderman, 2011), rather, they are excellent tools to be exploited for plasma diagnostics by solar magneto-seismology (SMS) (see the reviews by Andries et al, 2009; Ruderman and Erdélyi, 2009; Mathioudakis et al, 2013). SMS employs the measured properties of MHD waves, e.g., amplitude, frequency, and phase speed, and, using a suitable inversion, yields information about the waveguide properties that are often very hard to measure (e.g., magnetic field, gravity, magnetic scale heights, or density)

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