Toward detailed prominence seismology - II. Charting the continuous magnetohydrodynamic spectrum

Abstract

Starting from accurate MHD flux rope equilibria containing prominence condensations, we initiate a systematic survey of their linear eigenoscillations. To quantify the full spectrum of linear MHD eigenmodes, we require knowledge of all flux-surface localized modes, charting out the continuous parts of the MHD spectrum. We combine analytical and numerical findings for the continuous spectrum for realistic prominence configurations. The equations governing all eigenmodes for translationally symmetric, gravitating equilibria containing an axial shear flow, are analyzed, along with their flux-surface localized limit. The analysis is valid for general 2.5D equilibria, where either density, entropy, or temperature vary from one flux surface to another. We analyze the mode couplings caused by the poloidal variation in the flux rope equilibria, by performing a small gravity parameter expansion. We contrast the analytical results with continuous spectra obtained numerically. For equilibria where the density is a flux function, we show that continuum modes can be overstable, and we present the stability criterion for these convective continuum instabilities. Furthermore, for all equilibria, a four-mode coupling scheme between an Alfvenic mode of poloidal mode number m and three neighboring (m-1, m, m+1) slow modes is identified, occurring in the vicinity of rational flux surfaces. For realistically prominence equilibria, this coupling is shown to play an important role, from weak to stronger gravity parameter g values. The analytic predictions for small g are compared with numerical spectra, and progressive deviations for larger g are identified. The unstable continuum modes could be relevant for short-lived prominence configurations. The gaps created by poloidal mode coupling in the continuous spectrum need further analysis, as they form preferred frequency ranges for global eigenoscillations.

Accepted by Astronmy & Astrophysics, 21 pages, 15 figures

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• Solar prominences embedded in flux ropes: morphological features and dynamics from 3D MHD simulations
• MHD waves in two-dimensional prominences embedded in coronal arcades
• The influence of flux rope heating models on solar prominence formation
• Toward detailed prominence seismology - I. Computing accurate 2.5D magnetohydrodynamic equilibria
• Three-dimensional MHD wave propagation near a coronal null point: a new wave mode decomposition approach
• Quasi-periodic oscillations in superfluid, relativistic magnetars with nuclear pasta phases
• Toward detailed prominence seismology - II. Charting the continuous magnetohydrodynamic spectrum
• The role of Rayleigh-Taylor instabilities in filament threads