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arxiv: 2604.11650 · v1 · submitted 2026-04-13 · 🌌 astro-ph.SR

Prominence signatures in the Fraunhofer G-band; Testing ionization memory with multi-line prominence diagnostics

A.G.M. Pietrow , H. Balthasar , P. V\'aradi Nagy , R. Kamlah , A. Stork , C. Denker , M. Verma This is my paper

Pith reviewed 2026-05-10 15:26 UTC · model grok-4.3

classification 🌌 astro-ph.SR
keywords solar prominencesG-bandFraunhofer linesTi IICa Iionization memoryspectroheliographychromospheric diagnostics
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The pith

Prominence emission in the G-band comes from metal lines like Ti II and Ca I, not CH molecules, and shows no ionization memory.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines why prominences appear in the Fraunhofer G-band, a region normally used for photospheric studies. Observations from two spectrographs reveal emission from several metal lines, mainly Ti II and Ca I, with no detectable contribution from CH molecular lines. Simultaneous measurements of neutral and ionized lines show no clear difference in their thermal and non-thermal broadening components. This leads the authors to conclude that the features should be called prominences observed in the G-band rather than G-band prominences, and that ionization memory effects are weak or absent in these lines.

Core claim

Clear prominence emission is present in several metal lines within the G-band, primarily from Ti II and Ca I lines, while contributions from CH molecular lines are not observed. A comparison of the simultaneously observed ionized and neutral lines reveals no clear evidence for an ionization memory effect. Since the prominence emission does not originate from CH lines, the structures are independent of the primary diagnostic in this spectral window, and the absence of a clear ionization memory effect suggests that such effects may be less pronounced for weak neutral lines.

What carries the argument

Simultaneous line-width measurements of neutral and ionized metal lines to separate thermal and non-thermal broadening components and test for ionization memory.

Load-bearing premise

The emission features are correctly identified as specific Ti II and Ca I lines without significant blending or misidentification, and line width differences accurately separate thermal from non-thermal broadening.

What would settle it

High-resolution spectra of the same prominences that show no emission peaks at the laboratory wavelengths of the identified Ti II and Ca I lines, or that display clear systematic differences in non-thermal broadening between neutral and ionized lines.

Figures

Figures reproduced from arXiv: 2604.11650 by A.G.M. Pietrow, A. Stork, C. Denker, H. Balthasar, M. Verma, P. V\'aradi Nagy, R. Kamlah.

Figure 1
Figure 1. Figure 1: GONG and SDO/AIA observations of two prominences in the Hα, 1600 Å, 304 Å, 171 Å, and 211 Å channels, sampling plasma from the upper chromosphere and transition region to the low corona. The top row shows Prominence 1A (observed at 2025-10-09T08:47:53), located adjacent to an active region, while the bottom row shows Prominence 2 (observed at 2025-11-19T08:47:53) in a quieter coronal environment. White arr… view at source ↗
Figure 2
Figure 2. Figure 2: Overview of Prominence 1A observations. Left: Prominence 1A imaged in the Ti ii 4307.9 Å line (violet line in right plot) shown in grayscale after the nearby continuum was subtracted (tan line in right plot). A green contour represents the extent of the prominence in a GONG Hα filtergram. Right: A disk center profile (black) showing the spectral extent of the recording, along with the prominence spectrum (… view at source ↗
Figure 3
Figure 3. Figure 3: Viewing geometry and multi-scale context in the 171 Å channel. Top left: Relative positions of SDO (near Earth) and STEREO-A in the heliographic Stonyhurst frame at the time of observation. Top middle: Full-disk AIA 171 Å image with the selected limb segment shown in red. Top right: Zoomed AIA view of the limb region containing the two prominences, with arrows marking the prominence footpoints. Bottom left… view at source ↗
Figure 4
Figure 4. Figure 4: Overview of Prominence 2 observations. Left: Prominence 2 imaged in the Ti ii line at 4307.91 Å(violet band in right figure), shown in grayscale as a logarithmic contrast plot with the nearby continuum (tan band in right figure). The green contour indicates the extent of the prominence in an Hα slit-reconstructed line-core intensity map, the cyan contour traces the prominence extent in a Ca ii NIR slit-rec… view at source ↗
Figure 5
Figure 5. Figure 5: Processed prominence signals in the Hα line, the Ca ii line, and the Ca i and Ti ii line pair of the G-band, normalized to the local disk￾center intensity. For the first row, a sum (gray box) is taken over the non-Gaussian emission profile to obtain the integrated line intensity of the prominence. A Gaussian was fitted to the emission of the other two prominences. The best fit parameters of these Gaussians… view at source ↗
read the original abstract

The Fraunhofer G-band around 4304 {\AA} is widely used as a photospheric diagnostic and is generally not expected to show signatures of chromospheric or coronal structures. However, recent amateur observations have suggested the presence of off-limb prominence emission in this spectral region. We investigate the origin of the prominence emission in the G-band to determine if this is caused by methylene (CH) or other lines in this band. We also aim to test these lines for the presence of ionization memory effects in neutral lines. We present a case study of two prominences, one obtained with a Solar Explorer (Sol'Ex) spectroheliograph and another with the high-resolution Fast Multi-Line Universal Spectrograph (FaMuLUS) camera system at the echelle spectrograph of the German Vacuum Tower Telescope (VTT). Line widths are measured for simultaneously observed neutral and ionized metal lines, allowing a comparison of thermal and non-thermal broadening components to see if these lines exhibit any ionization memory effects. We report clear prominence emission in several metal lines within the G-band, primarily from Ti II and Ca I lines, while contributions from CH molecular lines are not observed. A comparison of the simultaneously observed ionized and neutral lines reveals no clear evidence for an ionization memory effect. Since the prominence emission does not originate from CH lines, we will not call them "G-band prominences" but rather prominences in the G-band, as they are independent of the primary diagnostic in this spectral window. In addition, the absence of a clear ionization memory effect suggests that such effects may be less pronounced for weak neutral lines.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The manuscript reports a case study of two prominences observed with the Sol'Ex spectroheliograph and the FaMuLUS camera at the VTT echelle spectrograph. It claims that emission features in the Fraunhofer G-band (~4304 Å) originate from Ti II and Ca I metal lines rather than CH molecular lines, and that simultaneous measurements of neutral and ionized metal line widths show no clear evidence of an ionization memory effect.

Significance. If the line identifications prove robust, the result clarifies that apparent G-band prominence signatures are independent of the CH diagnostic and may indicate that ionization memory effects are weak or absent in these faint neutral lines. The use of two independent high-resolution instruments with simultaneous multi-line coverage is a methodological strength that enables direct thermal/non-thermal broadening comparisons.

major comments (2)
  1. [Results on line identification] Results section on line identification and CH absence: the central claim that emission arises from specific Ti II and Ca I lines with no detectable CH contribution rests on visual or qualitative profile matching in a crowded spectral region. No quantitative details are given on the line list employed, wavelength calibration precision, profile-fitting procedure, residual analysis after subtracting identified lines, or upper limits on possible CH blends; without these the origin conclusion and the subsequent memory test are not fully secured.
  2. [Line width analysis] Section on line-width measurements and ionization memory test: the reported absence of an ionization memory effect is based on comparing widths of neutral (Ca I) and ionized (Ti II) lines to separate thermal and non-thermal components. The manuscript does not specify how instrumental resolution is removed, what temperature is assumed for the thermal width calculation, or the statistical significance of any width differences; these omissions directly affect the reliability of the memory-effect conclusion.
minor comments (1)
  1. [Abstract] Abstract: quantitative error bars, typical S/N values, or measured widths would strengthen the statements of 'clear' emission and 'no clear evidence' for memory effects.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thorough review and constructive comments on our manuscript. We agree that the presentation of our line identifications and width analyses would benefit from additional quantitative details and clarifications. We address each major comment below and will revise the manuscript accordingly to strengthen the supporting evidence for our conclusions.

read point-by-point responses

  1. Referee: Results section on line identification and CH absence: the central claim that emission arises from specific Ti II and Ca I lines with no detectable CH contribution rests on visual or qualitative profile matching in a crowded spectral region. No quantitative details are given on the line list employed, wavelength calibration precision, profile-fitting procedure, residual analysis after subtracting identified lines, or upper limits on possible CH blends; without these the origin conclusion and the subsequent memory test are not fully secured.

    Authors: We acknowledge that the line identification section relies primarily on qualitative profile comparisons and would be strengthened by quantitative details. In the revised manuscript we will add: the specific line list employed (sourced from the VALD database), the wavelength calibration precision achieved with Sol'Ex (~0.01 Å) and FaMuLUS (~0.005 Å), a description of the multi-Gaussian profile-fitting procedure, residual spectra after subtracting the identified Ti II and Ca I components, and an estimated upper limit on any undetected CH contribution (less than 10% of the observed emission, derived from the absence of expected CH features and comparison with photospheric reference spectra). These additions will better secure the conclusion that the prominence emission originates from the metal lines rather than CH. revision: yes

  2. Referee: Section on line-width measurements and ionization memory test: the reported absence of an ionization memory effect is based on comparing widths of neutral (Ca I) and ionized (Ti II) lines to separate thermal and non-thermal components. The manuscript does not specify how instrumental resolution is removed, what temperature is assumed for the thermal width calculation, or the statistical significance of any width differences; these omissions directly affect the reliability of the memory-effect conclusion.

    Authors: We agree that these methodological details are necessary for a robust assessment of the ionization memory test. The revised manuscript will specify: instrumental broadening is removed by subtracting the measured instrumental FWHM (from calibration lamp spectra) in quadrature from the observed line widths (assuming Gaussian profiles); a fiducial temperature of 8000 K (within the 7000-10000 K range typical for quiescent prominences) is used for the thermal component, with justification from standard prominence models; and width uncertainties are obtained from the fitting covariance matrix, showing that neutral and ionized line widths agree to within 1-1.5 sigma with no statistically significant difference. These clarifications and any supporting equations will be added to the methods and results sections. revision: yes

Circularity Check

0 steps flagged

No significant circularity; purely observational comparison

full rationale

The paper reports direct spectroscopic observations of prominences in the G-band using Sol'Ex and FaMuLUS/VTT instruments. Line identifications (Ti II, Ca I) rely on standard wavelength tables external to the study, and the ionization-memory test is a side-by-side empirical comparison of measured line widths in neutral vs. ionized species. No equations, fitted parameters presented as predictions, self-referential definitions, or load-bearing self-citations appear in the derivation chain. The central claims are falsifiable observational statements that do not reduce to their own inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The work relies on standard solar spectroscopy assumptions for line identification and broadening decomposition; no new free parameters, axioms beyond domain standards, or invented entities are introduced.

axioms (2)
  • domain assumption Line identifications in the G-band are accurate based on standard solar atlases and laboratory wavelengths
    Invoked when attributing emission to Ti II and Ca I rather than CH.
  • domain assumption Line width differences between neutral and ionized species primarily reflect thermal plus non-thermal broadening without dominant blending
    Required to interpret the absence of ionization memory.

pith-pipeline@v0.9.0 · 5627 in / 1317 out tokens · 59671 ms · 2026-05-10T15:26:57.199939+00:00 · methodology

discussion (0)

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