High-resolution narrowband Ca II H images from the Sunrise balloon observatory reveal slender bright fibrils (SCFs) whose orientation overlaps magnetostatic extrapolations of the photospheric field at roughly 700–1000 km height, suggesting that they chart the low-chromospheric magnetic canopy. The underlying report, however, left three linked questions open: what localized heating mechanism makes these thin structures bright (wave damping, magnetic reconnection, spicule-II/RBE/RRE-related mass loading — RBEs and RREs being the rapid blue- and red-shifted excursions that mark type-II spicules in on-disk Hα images — or something else); whether the SCF-to-field correspondence holds when tested against actual chromospheric field measurements rather than photosphere-limited extrapolations and LTE-constrained inversions, including an explanation of the misaligned SCF minority in less-organized quiet areas; and at which heights SCFs really form, given that current formation-height estimates rest on 1D model atmospheres and a filter passband that may be offset from line center. No study to date identifies the SCF heating mechanism, tests SCF alignment against chromospheric spectropolarimetry, or recomputes SCF formation heights with 3D non-LTE forward modeling, so the problem remains open.

Background

The solar chromosphere is the weakly ionized atmospheric layer between the photosphere and the corona, threaded by magnetic fields that expand from photospheric concentrations in plage (strong-field active-region patches) and the magnetic network (supergranular cell boundaries). In high-resolution images taken in the core of the Ca II H resonance line at 396.8 nm, the chromosphere shows a dense forest of very thin (sub-arcsecond) bright elongated structures near plage and network, called slender Ca II H fibrils (SCFs). Because fibrils in chromospheric lines look like field lines, they have long been used as a visual proxy for magnetic topology, but this assumption had rarely been tested quantitatively at the heights where SCFs are thought to form.

Jafarzadeh et al. (2017, ApJS 229, 11) analyzed SCFs in images from SuFI, the narrowband UV imager of the Sunrise balloon-borne solar observatory (its second flight is described by Solanki et al. 2017), together with photospheric vector magnetograms from the on-board IMaX instrument (the Imaging Magnetograph eXperiment, a filter spectropolarimeter). They compared SCF orientations with (i) magnetostatic extrapolations — field models computed by assuming the atmosphere is close to magnetostatic equilibrium above the observed photospheric boundary — and (ii) field lines from MASI, an MHD-assisted Stokes inversion technique (Riethmüller et al. 2017) that reshuffles columns of a numerical magnetoconvection simulation (here MURaM, a widely used solar magnetoconvection code) until the synthetic polarization profiles best match the observed ones. They found that the majority of SCFs are aligned with extrapolated field lines at heights of roughly 700–1000 km, forming a low canopy-like dome over neighboring quiet areas, and that many SCFs are qualitatively consistent with MASI photospheric loops. A small fraction of SCFs, mostly in less magnetically organized quiet regions, did not map the extrapolated field lines.

Independent work on other chromospheric diagnostics supports the fibrils-as-field-tracers picture only statistically: de la Cruz Rodríguez & Socas-Navarro (2011) inverted Ca II 8542 Å spectropolarimetry (full-Stokes profiles of a chromospheric spectral line) and answered the question 'are fibrils tracing the magnetic field?' with 'mostly yes, but not always'; Schad et al. (2013) used He I 10830 Å vector magnetometry of superpenumbral fibrils and found alignment within about ±10°; Asensio Ramos et al. (2017) compared Ca II 8542 Å field azimuths with fibril directions and found a dispersion of about 16° in penumbrae increasing to about 34° in weakly magnetized regions. On the theoretical side, Leenaarts et al. (2015) showed, in a 3D radiation-MHD simulation computed with the Bifrost code, that Hα fibrils follow magnetic field lines only where the plasma beta (the ratio of gas pressure to magnetic pressure) is well below unity; where beta approaches one, fibril-shaped density structure and field direction can decouple — a plausible candidate explanation for misaligned fibrils that has not been tested for SCFs.

The physical origin of the fibrils' brightness is a separate, unsettled issue. Bright Ca II K fibrils were studied by Pietarila et al. (2009), and later non-LTE inversions (spectral inversions that abandon the local thermodynamic equilibrium assumption, needed for chromospheric lines) by Kianfar et al. (2020) found bright Ca II K fibrils to be roughly 100 K hotter than their surroundings — 'hot threads' — while explicitly stating that the mechanisms forming them and causing their brightness remain unknown. Formation-height estimates for SCFs themselves rest on line-depression contribution functions (quantities describing where in the atmosphere an observed line intensity originates) computed for the SuFI filter with 1D quiet-Sun, network, and plage model atmospheres, which may misrepresent the 3D dynamic chromosphere; the SuFI Ca II H filter transmission may also be slightly offset from line core (a caveat noted by Solanki et al. 2017).

Problem Statement

The overarching objective is to establish, on physical grounds, whether and how the observed texture of slender Ca II H fibrils maps onto the magnetic structure of the low solar chromosphere. This requires answering three linked questions left open by Jafarzadeh et al. (2017):

  1. Heating mechanism. Identify the physical mechanism(s) that produce the localized heating events near plage and network field concentrations that render SCFs as thin bright features in narrowband Ca II H (and, by extension, Ca II K) images — in particular, determine whether these events are related to type-II spicules (short-lived, rapidly upflowing chromospheric jets) and their on-disk Hα signatures (rapid blue-shifted and red-shifted excursions, RBEs and RREs), to wave heating, to magnetic reconnection, or to other mechanisms. This requires coordinated observations (simultaneous high-resolution Hα together with Ca II H/Ca II K) probing the dynamic and thermodynamic signatures of the events across heights.

  2. Field-mapping validation. Test the correspondence between SCFs and chromospheric magnetic fields using field determinations at chromospheric heights themselves — not only photosphere-limited magnetostatic extrapolations or MHD-assisted Stokes inversions, which assume LTE, are restricted to photospheric heights, and inherit noise-limited weak-field detection from the photospheric magnetograms — and quantitatively explain the minority of SCFs that do not align with modeled fields, which occur mainly in less-organized quiet areas (assessing, e.g., missing weak-field components, breakdown of the magnetostatic equilibrium assumption, transient dynamics, or non-magnetic origins).

  3. Formation heights. Determine the actual heights at which SCFs are formed in the 3D, dynamic solar atmosphere — going beyond 1D model-atmosphere contribution-function estimates that may over- or underestimate formation heights, and accounting for the possibility that the narrowband Ca II H filter passband is slightly offset from line center.

A complete answer requires all three sub-questions: identification of the heating mechanism(s) (question 1); validation of the SCF-to-chromospheric-field correspondence, including a quantitative explanation of the misaligned minority (question 2); and determination of the actual SCF formation heights (question 3). Together these establish a validated, physically interpreted mapping between the SCF texture and the low-chromospheric (roughly 700–1000 km) magnetic canopy, such that narrowband Ca II H imaging can be used (or refused) as a quantitative proxy for low-chromospheric field topology.

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