The physical nature of internetwork H-alpha chromospheric fibrils and their MHD kink waves: magnetic connectivity, visibility cycles, wave damping, and magneto-seismology

Internetwork (IN) chromospheric fibrils are transient, elongated H-alpha absorption features that form inside supergranular cells, away from the kilogauss network flux that anchors most well-studied chromospheric structures. The first synoptic study of these features (Mooroogen, Morton and Henriques 2017) found that they repeatedly appear, disappear and re-appear on timescales of minutes to tens of minutes, that most lack identifiable photospheric footpoints, and that the majority support propagating magnetohydrodynamic (MHD) kink waves whose measured phase speeds — some exceeding 500 km/s — would imply surprisingly strong internetwork magnetic fields (~25–200 G). That study also showed that current magneto-seismology cannot uniquely infer density and magnetic-field stratification along the fibrils, because internal and external density variations and wave damping are entangled in the observed amplitude profiles. Follow-up work has characterized kink waves in adjacent structures (network, plage, superpenumbral and quiet-Sun fibrils) and advanced waveguide seismology theory, but no study has resolved the internetwork-fibril questions themselves. The composite problem — establishing what IN fibrils are magnetically, why they cycle in visibility, what damping acts on their waves, and whether those waves can seismologically probe the weak-field chromosphere — remains open.

Background

The Sun's chromosphere is the roughly 2000 km thick layer of the solar atmosphere lying between the visible surface (photosphere) and the multi-million-degree corona; in the line core of H-alpha (the n=3 to n=2 transition of hydrogen, the strongest chromospheric absorption line) it appears filled with thin, dark, elongated features called fibrils. Fibrils are widely believed to outline magnetic fields that fan out from photospheric flux concentrations. Most fibril studies concern the network — the lanes of strong (kilogauss) magnetic field bounding supergranular convection cells — and active-region plage. Far less is known about fibrils in the internetwork, the cell interiors where the magnetic field is weak and fragmented and which cover most of the solar surface.

A magnetic flux tube embedded in plasma supports a kink mode: a transverse oscillation in which the whole tube sways side-to-side, with magnetic tension as the restoring force. In the long-wavelength (thin-tube) limit its phase speed is the kink speed $c_k$, given by $c_k^2 = 2B^2/(\mu_0(\rho_i+\rho_e)) = 2v_A^2/(1+\zeta)$, where $B$ is the field strength, $\mu_0$ the magnetic permeability of free space, $\rho_i$ and $\rho_e$ the plasma densities inside and outside the tube, $v_A$ the internal Alfvén speed and $\zeta = \rho_e/\rho_i$. Because $c_k$ depends on density and field, observing kink waves allows magneto-seismology: using measured wave properties (phase speeds, amplitudes and their variation along the guide) to infer plasma parameters that cannot be measured directly. Ubiquitous transverse/kink waves had previously been detected in spicules (jets at the network) — including high apparent propagation speeds that remain unexplained (Okamoto and De Pontieu 2011) — and in chromospheric features rooted in network and plage fields (Morton et al. 2012). Radiation-MHD simulations indicate that H-alpha fibrils largely follow magnetic field lines (Leenaarts, Carlsson and Rouppe van der Voort 2015), although the field-tracing fidelity degrades in weakly magnetized regions and polarimetric inference of fibril field orientation only became possible with Bayesian analyses of Ca II 8542 Å data (Asensio Ramos et al. 2017).

Mooroogen, Morton and Henriques (2017) provided the first investigation of internetwork fibrils, using H-alpha imaging spectroscopy from the Swedish 1-m Solar Telescope. They found that IN fibrils appear, disappear and re-appear over minutes to tens of minutes (suggesting repeated, episodic heating), that no clear photospheric footpoints could be identified in simultaneous magnetograms or H-alpha wing images, and that most fibrils support quasi-periodic propagating transverse displacements with a modal period near 120 s, interpreted as kink waves. The measured phase speeds ranged up to and beyond 500 km/s. Interpreting them as kink speeds, and adopting chromospheric densities of $10^{-9}$–$10^{-10}$ kg m$^{-3}$ from model chromosphere simulations, implies magnetic field strengths of roughly 3–200 G along the fibrils, with the fastest events requiring $B \approx 25$–200 G — values hard to reconcile with the weak line-of-sight fields observed near the fibrils' apparent footpoints, and in excess of typical spicule field estimates (~50 G). The wave amplitudes varied with distance along the fibrils (both amplification and damping), which the authors analyzed with solar magneto-seismology under conservative assumptions: no wave damping, a WKB approximation (wavelength short compared with the longitudinal variation scale), a thin-tube approximation, and hydrostatic-like stratification. Those assumptions leave the internal and external density profiles — and any damping or amplification of the wave — entangled in the amplitude profile, so no unique stratification could be extracted. Candidate damping mechanisms for kink waves include resonant absorption (coupling of the kink mode to Alfvén waves on the tube boundary, when the wave is spatially resolved) and the Kelvin-Helmholtz instability (shear-flow instability at the oscillating tube boundary), both extensively studied in coronal and spicule contexts but unquantified for IN fibrils. The visibility cycling was tentatively attributed to thermal non-equilibrium — cyclic heating and cooling, as modeled for coronal loops — whose H-alpha signature depends on non-equilibrium hydrogen ionization, in which the ionization state of hydrogen lags the instantaneous temperature so that the line's opacity depends on the plasma's heating history.

Problem Statement

Establish the physical nature of internetwork H-alpha chromospheric fibrils and of the transverse (kink) MHD waves they support. As posed by the first study of these features (Mooroogen, Morton and Henriques 2017), this comprises five linked sub-questions about the same structures and observations: (1) Magnetic connectivity — how do photospheric internetwork flux elements map to IN fibrils, given that no clear footpoints are identifiable in current magnetograms and H-alpha wing images; what magnetic topology underlies features that form away from network flux? (2) Visibility cycles — what mechanism produces the repeated appearance, disappearance and re-appearance of IN fibrils on timescales of minutes to tens of minutes; is it episodic heating and cooling in thermal non-equilibrium, mass loading/unloading, or radiative-transfer and ionization non-equilibrium effects, as would be demonstrated by coordinated multi-wavelength observations and modeling with non-equilibrium hydrogen ionization and realistic heating histories? (3) Wave properties and the high-speed puzzle — the observed kink-wave phase speeds range up to and beyond 500 km/s; are these real (implying internetwork chromospheric field strengths of order 25–200 G, in tension with photospheric measurements), or artifacts of large measurement uncertainties, or contamination by partial standing modes rather than purely propagating modes? (4) Damping — what damping mechanisms (e.g. resonant absorption, Kelvin-Helmholtz instability) act on the observed waves? (5) Seismological invertibility — can an inversion framework (beyond the no-damping, WKB/thin-tube, hydrostatic-like assumptions of the 2017 analysis) disentangle internal from external density stratification and field-strength gradients along IN fibrils, with quantified uncertainties, given that wave amplification/damping is entangled with stratification in the observed amplitude profiles? A complete solution must address all five linked sub-questions; a partial solution must state which of the five sub-questions it resolves and which it leaves open.

References

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