High-resolution Ca II 854.2 nm observations of the quiet Sun (Wedemeyer-Böhm 2009) showed that the low chromosphere is divided into a magnetic 'canopy' of fibrils spanning supergranular cells and an underlying 'sub-canopy' domain of acoustic shock waves, and that where the canopy lies low, shock formation is suppressed — but the observations were intensity-only and could yield only qualitative constraints. The open problem is to replace those qualitative constraints with quantitative determinations: (1) calibrated spectropolarimetric mapping of the strength, geometry, connectivity, and lower-boundary height of the canopy and sub-canopy fields in quiet (including internetwork) regions; (2) the physics and energy budget of the interaction between upward-propagating acoustic waves and the canopy — the relative roles of dissipation, partial mode conversion, wave guiding, scattering at the canopy boundary, and suppression of downflows; and (3) the identification, energetics, and heating contribution of the associated small-scale transients: bright blobs moving along fibrils, fast diffuse propagating features, and chromospheric swirls. More than sixteen years of follow-up work has advanced each thread — swirls were reinterpreted as Alfvénic wave pulses, mode conversion at the canopy is observed, and Sunrise III/SCIP has delivered the first precision quiet-Sun chromospheric line-of-sight field maps — but no component is closed: internetwork chromospheric vector fields, the canopy-height validation, the wave–canopy energy budget, and the transient energy contribution all remain unresolved.

Background

The solar chromosphere is the roughly 1000–2000 km thick, partially ionized layer between the photosphere (the visible solar surface) and the transition region to the million-degree corona; maintaining it against radiative losses requires a mechanical energy flux whose origin and partitioning remain among the classic open problems of solar physics. In the photosphere, magnetic flux is concentrated in the network lanes that bound supergranular convection cells and in sparse, weak internetwork elements inside the cells. As network flux expands with height it fans out horizontally over the internetwork, forming a dome-like overlay called the magnetic canopy; the volume beneath it, with weaker and more vertical fields where upward-propagating acoustic waves steepen into shock waves, is the sub-canopy domain. Three-dimensional radiation-hydrodynamics simulations of weakly magnetized convection predict that these shocks produce a mesh-like, rapidly evolving intensity pattern sometimes called the 'fluctosphere' (or 'clapotisphere'). The key diagnostics are spectral lines: the Ca II 854.2 nm line, whose core forms in the middle chromosphere and whose wings sample the photosphere, and Hα, whose elongated dark-and-bright structures (fibrils) trace the orientation of the horizontal chromospheric field. Measuring the chromospheric field itself requires spectropolarimetry — recording the four Stokes parameters (intensity plus two linear and one circular polarization states), from which Zeeman splitting yields the line-of-sight and transverse field — but the chromospheric signals are weak (polarization at the 10−310^{-3} level or below) and the lines form out of local thermodynamic equilibrium (non-LTE), so retrieving height-dependent fields requires non-LTE inversions, i.e. fitting model atmospheres to the observed line profiles; in 2009 this was beyond reach for the weak internetwork chromosphere. Wedemeyer-Böhm (2009) analysed Ca II 854.2 nm image sequences from the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope, comparing a coronal-hole quiet region (a coronal hole being a region where the magnetic field opens outward into interplanetary space) with a supergranular cell covered by fibrils in the remainder of decaying active region AR10998. Three findings defined the problem. First, in the fibril-covered cell the reversed granulation (the inverse-contrast remnant of photospheric granulation seen a few hundred kilometres above the visible surface) looked normal yet shock signatures were absent, implying that acoustic waves were excited but the horizontal canopy field suppressed shock formation — candidate mechanisms being wave-energy dissipation, partial mode conversion (transformation between acoustic and magnetic wave types), wave guiding along the field, scattering at the canopy's lower boundary, and suppression of the downflows that feed shock formation. Comparing with simulations placed the canopy's lower boundary between roughly 400 km and 700–1000 km above the photosphere. Second, individual bright blobs were seen to move along fibrils at 4–8 km s−1^{-1} with a tentative (unconfirmed) ~7 min modulation, and fast, more diffuse intensity features propagated across the field of view without following the fibrils — possibly wavefronts scattered at the canopy boundary or 'failed shocks' damped before steepening. Third, small-scale chromospheric swirls (discovered in the companion study of the same data by Wedemeyer-Böhm & Rouppe van der Voort 2009) appeared only in the coronal-hole data away from fibrils, hinting that inclined canopy fields hinder the vortical motions. The paper's conclusion was explicit: intensity-only diagnostics cannot determine the magnetic topology with height, and quantitative spectropolarimetric measurements of the chromospheric field — combined with matched observations and numerical experiments — are needed to turn this qualitative picture into a physical, quantitative model of the quiet-Sun low atmosphere.

Problem Statement

For quiet-Sun regions of the solar atmosphere, determine quantitatively — replacing the qualitative, intensity-only constraints available in 2009 — (1) the structure of the magnetic field above the photosphere: the strength, inclination, connectivity, and height of the magnetic canopy that spans supergranular cells and of the enclosed sub-canopy domain, including a validation of the canopy's lower-boundary height (estimated at ~400–1000 km from simulations), based on calibrated spectropolarimetric measurements sensitive to the weak internetwork fields; (2) the physics of the interaction between upward-propagating acoustic (magnetoacoustic) waves and this canopy field — establishing the relative contributions of wave-energy dissipation, partial mode conversion, wave guiding, scattering at the canopy boundary, and suppression of downflows to the observed suppression of shock formation beneath a low canopy, with the associated energy fluxes quantified; and (3) the nature, excitation mechanism, and energy budget of the associated small-scale transient phenomena as diagnostics of the same wave–field coupling: the bright blobs propagating along fibrils (observed at 4–8 km s−1^{-1} with a tentative ~7 min modulation), the fast diffuse propagating features, and the small-scale chromospheric swirls, including their prevalence, formation conditions, and quantitative contribution to chromospheric and coronal heating. These deliverables are components of one common objective: a quantitative, observationally validated model of the small-scale quiet-Sun chromosphere in which the magnetic topology (item 1), the wave and magnetic energy transport through it (item 2), and the transient signatures of that transport (item 3) are jointly determined and energetically closed, with the canopy-height validation tying the magnetic and wave-dynamical determinations together. A complete answer is expected to combine spectropolarimetry of sufficient polarimetric sensitivity for internetwork chromospheric fields with coordinated high-cadence, multi-line observations and numerical experiments matched to the observed topologies.

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