Observations of Flare Induced Doppler Shifts in the Si~\textsc{iii} $1206\,\textrm{{\AA}}$ line

Luke H. Majury; Ryan O. Milligan

REVIEW 3 major objections 4 minor 52 references

Observations of Flare Induced Doppler Shifts in the Si~\textsc{iii} $1206\,\textrm{{\AA}}$ line

T0 review · 3 major / 4 minor · reviewed 2026-08-05 · deepseek-v4-flash

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Pith's one-line read 201 km/s flare downflow seen in a line never studied for shifts

desk verdict C3.4 redshift is real and interesting; the M5.3 blueshift is a detrending artifact, so the 'two events' claim overstates the evidence. read the letter →

arxiv 2509.05223 v1 pith:ZJGEQFYX submitted 2025-09-05 astro-ph.SR

Luke H. Majury , Ryan O. Milligan This is my paper

classification astro-ph.SR

keywords solarflaresDopplershiftsSiIII1206ÅchromosphericcondensationevaporationtransitionregionSORCE/SOLSTICEdisk-integratedspectroscopy

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper claims that the Si III 1206 Å line, formed near 120,000 K where the chromosphere meets the transition region, carries usable Doppler information about flare-driven mass motions, and that this is the first study to extract flare-induced Doppler shifts from it. Using one-minute disk-integrated profiles from SORCE/SOLSTICE wavelength calibration scans of 11 X-, M-, and C-class flares, the authors subtract a quiet-Sun profile and fit a Gaussian to isolate flare emission. The raw shifts follow a strong linear trend with solar longitude, which the authors remove as a systematic; two events then pass a 3-sigma threshold: a 201.36 ± 21.94 km/s redshift in a C3.4 flare and a −39.75 ± 11.00 km/s blueshift in an M5.3 flare. A bright eruption visible in SDO/AIA images at the time of the blueshifted event suggests erupting material, not evaporation alone, may drive that motion. If correct, the results open a new temperature window on chromospheric condensation and evaporation at a regime between lines already observed by IRIS.

What carries the argument

The analysis rests on two procedures. First, quiet-Sun subtraction: an average of three or more pre- or post-flare scans is spline-interpolated, rebinned to the flare scan's wavelength grid, and subtracted from the flaring profile, so the residual is treated as pure flare emission; a Gaussian plus a second-order polynomial is fit to the Si III line and the Ly α wing, and the centroid shift becomes a Doppler velocity through the classical formula v = c·Δλ/λ₀. Second, the longitude correction: raw velocities plotted against helioprojective longitude show a linear trend (Pearson r = 0.95), which is subtracted; the residuals are the reported detrended velocities. The first procedure isolates fla

What would settle it

A ray-trace simulation of the SOLSTICE entrance-slit geometry—future work the paper itself proposes—would predict wavelength shift versus pointing and show whether the longitude trend is instrumental; if it is, the detrended velocities shrink and the M5.3 blueshift likely falls below significance. Refitting the trend while including the C3.4 point would directly test how much of the 201 km/s redshift is an artifact of excluding that point. Spatially resolved spectroscopy of an eruptive M-class flare (IRIS or Hinode/EIS) could test whether erupting material contributes enough disk-integrated fl

Watch

Extended reading notes

Core claim

On its own terms, the paper establishes that disk-integrated SOLSTICE calibration scans can measure flare-induced Doppler shifts in the Si III 1206 Å line, and that after a linear correction for a longitude-dependent systematic, two of eleven events shift above the 3σ threshold: a 201.36 ± 21.94 km/s redshift in a C3.4 flare, interpreted as possible chromospheric condensation, and a −39.75 ± 11.00 km/s blueshift in an M5.3 flare. SDO/AIA images show a bright eruption nearly simultaneous with the blue-shifted Si III peak, so erupting material may drive that shift rather than evaporation alone. The authors keep alternatives open—uncatalogued line blends, raster-time effects, or an incomplete l

Load-bearing premise

The shifted velocities are real only if the measured Doppler velocities follow a straight-line trend with solar longitude that can be subtracted from all 11 events; the trend is fitted without the C3.4 point that produces the biggest shift, and the paper notes ray-trace evidence that SOLSTICE's entrance-slit misalignment can systematically shift wavelengths with pointing, which could mimic a longitude dependence.

Editorial extensions

If this is right

Si III 1206 Å becomes a usable Doppler diagnostic for flare-driven motions near 10^4.6 K, bridging the C II and Si IV lines routinely observed by IRIS.
A ~200 km/s downflow in a modest C3.4 flare implies an unusually high energy flux or an under-dense pre-flare chromosphere, or a mechanism other than condensation.
Blueshifts in disk-integrated flare spectra cannot be assumed to be evaporation: the M5.3 event ties its blueshift to a bright eruption, so erupting material must be included in interpretations.
The strong longitude-dependent trend in SOLSTICE Doppler velocities warns other disk-integrated flare studies to correct for or rule out an analogous systematic.
High-cadence imaging spectroscopy of Si III by upcoming instruments (SOLAR-C/EUVST, MUSE, SNIFS) should separate condensation, evaporation, and eruption contributions to the shifts.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

The 201 km/s redshift magnitude depends on the choice to exclude the C3.4 event when fitting the longitude trend; refitting with that point included, or with a leave-one-out scheme, would bound how much of the speed is an artifact of the fit—an exercise the published table makes possible.
Because these are disk-integrated, quiet-Sun-subtracted profiles, the shift could be carried by a small bright kernel; in that case the true line-of-sight motion of that kernel would be faster still, while the total emitting mass would be small—a degeneracy only spatially resolved spectra can break.
The same pipeline could be run on the Ly α line in these scans, for which the paper measured only enhancements, to test whether eruption-driven blueshifts appear consistently across lines formed at different heights.
Should the longitude trend turn out to be instrumental, the detrending recipe here would transfer directly to other disk-integrated spectrometers that lack an absolute wavelength reference, making the procedure as useful as the two detections.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

First thing you should know: this is the first paper to measure Doppler shifts in the Si III 1206 Å line during flares, and it finds one solid detection. The C3.4 flare on 2009-12-10 shows a redshift of 113±13 km/s even before applying the longitude correction, so that result does not depend on the detrending. The corrected value of ~201 km/s is striking but its size hinges on the same trend removal. The second claimed detection, the M5.3 blueshift, is not solid: the raw velocity is -9±8 km/s, and it only becomes -39.75±11.00 km/s after subtracting a linear fit of Doppler velocity vs. solar longitude. That fit is made to the same 11-event sample with the C3.4 point excluded, and the uncertainty in the fitted slope is not propagated into the detrended values. So the M5.3 result is manufactured by the analysis, unless the longitude trend can be independently calibrated.

The paper is transparent about this. It tells you the detrending may overcorrect, that ray-tracing could yield more modest velocities, and that the slit alignment issue is communicated personally. The quiet-Sun subtraction and Gaussian fitting are standard and well described. The longitude trend itself (Pearson r=0.95) is worthwhile, even if its interpretation is open. The AIA images for the M5.3 flare showing an eruption coincide with the blueshift is suggestive context, and the discussion of blends and raster timing is careful.

The main soft spot is the circularity of the detrending. The paper effectively fits a systematic effect and then subtracts it from the same data to produce its headline results. That is fine if the trend is calibrated independently or if its uncertainty is small, but neither is true here. The slope is 104.1 km/s per x/Rsun, estimated from ten points, with the most extreme point removed; its error is not quoted. At the M5.3 longitude, a plausible slope uncertainty of tens of km/s per unit x would push the blueshift below 3σ. The C3.4 redshift survives this critique but its magnitude is overstated by the same correction.

Who is this for? Solar physicists working on flare dynamics and transition-region diagnostics. It is a useful pilot study that identifies Si III as a line worth watching, especially with upcoming EUVST and MUSE. The paper deserves a serious referee — I would send it to review — but I would not accept the two-event claim as stated. The abstract should be revised to present the C3.4 redshift as the robust detection and the M5.3 blueshift as tentative.

Recommendation: engage with the work, cite it for the C3.4 detection and the longitude-trend caveat, but treat the M5.3 blueshift as unconfirmed. A revision that propagates the fit uncertainty or calibrates the trend independently would strengthen it considerably.

Referee Report

3 major / 4 minor

Summary. The paper analyzes disk-integrated SORCE/SOLSTICE wavelength-calibration scans to measure Doppler shifts in the transition-region Si III 1206 Å line during 11 solar flares. For each event, quiet-Sun profiles are subtracted and Gaussian fits yield raw Doppler velocities. A linear trend between raw velocity and solar longitude is fitted to 10 of the 11 events (excluding the C3.4 point), subtracted, and the resulting detrended velocities are searched for significant shifts. Two events are reported as significant: a C3.4 flare with a 201.36 ± 21.94 km/s redshift and an M5.3 flare with a -39.75 ± 11.00 km/s blueshift. The M5.3 blueshift is associated in SDO/AIA imaging with a bright eruption, and the paper proposes Si III as a new diagnostic of flare-driven chromospheric and transition-region motions.

Significance. If the reported detections are robust, this is a novel observational result: it is, to the authors' knowledge, the first flare Doppler-shift study of Si III 1206 Å, a line that bridges the temperature range between C II and Si IV. The paper also makes a useful methodological point about longitude-dependent systematic shifts in disk-integrated flare spectroscopy, and it provides a quantitative comparison of Si III enhancements with GOES, RHESSI, and AIA data. The work is careful in its event selection, use of FSS pointing information to exclude spacecraft-maneuver-affected intervals, quiet-Sun subtraction, and reduced-χ² checks. However, the central claim of 'two events with significant Doppler shifts' rests on an empirical longitude detrending whose statistical uncertainty is not propagated; the M5.3 blueshift in particular is only significant after that correction. The C3.4 redshift is significant even before detrending (113 ± 13 km/s), but the headline 201 km/s value is almost entirely a product of the same correction.

major comments (3)

[§2, Figure 4; Table 1] The quoted uncertainties for the detrended velocities are only the Gaussian centroid-fit uncertainties. The linear trend fit (slope 104.1 km/s per x/R_sun, intercept not stated) has its own uncertainty, which is not propagated into the detrended values. For the M5.3 event at x ≈ +0.31, the longitude correction is about +32 km/s; a slope uncertainty of only 30 km/s per x/R_sun would add ~9 km/s to the error, dropping the blueshift from 3.6σ to about 2.7σ. Please report the covariance of the trend fit, propagate it through Eq. (1), and test sensitivity to excluding individual events. Without this, the 'two significant detections' claim is not quantitatively supported.
[§2 and §4, detrending paragraph] The detrending is applied to the same sample used to estimate the trend, and the most extreme point (C3.4) is excluded from the fit but then receives the largest correction (+88 km/s), which changes its velocity from 113 ± 13 km/s to 201 ± 22 km/s. The paper acknowledges in §4 that the first-order polynomial 'may not be comprehensive' and that ray-tracing 'may reveal more modest velocities.' This caveat is load-bearing: both headline values depend on the functional form and validity of the longitude trend. Please provide a systematic uncertainty estimate for the correction (e.g., a range of velocities under alternative trend models, or the ray-trace result when available) or explicitly present the raw C3.4 redshift as the primary detection and the detrended values as model-dependent.
[§3, Figure 5/6 and §4] For the M5.3 event, the AIA images show an eruption with a projected plane-of-sky motion of ~10 arcsec in 72 s, which the authors use to infer a possible line-of-sight velocity of ~90 km/s. This is plausible supporting context, but the contribution of the eruption to the disk-integrated Si III signal is not quantified, and the paper correctly notes this. The interpretation of the blueshift as eruption-related rather than evaporation is therefore suggestive rather than established. This is not an error, but the abstract's phrasing ('suggesting the shift may have resulted from the eruption rather than evaporation alone') should be softened to reflect the lack of spatial resolution.

minor comments (4)

[Figures 2, 3, 4, 5, 7] Axis labels and captions contain numerous garbled characters (e.g., 'P oi2ing', 'i-e', 'W avelength', 'Irr d( nce', 'Ce.20/id'). These appear to be PDF rendering artifacts, but they should be corrected in the final version for readability.
[Eq. (1)] The 'classical Doppler formula for a stationary observer' is fine for v/c ~ 10^-4, but the text should define the sign convention explicitly (positive = redshift) and state whether the observed helioprojective x-coordinate follows the standard solar-west-positive convention; the current Figure 4 discussion is easy to misread.
[Table 1 and §2] The text says reduced chi-squared values were below unity for 10 of 11 events, but Table 1 lists values such as 11.415 for the X3.6 event. The lone exception is clear, but consider explicitly identifying which event it is in the main text. Also, for the X3.6 event, the poor χ²_ν suggests the single-Gaussian fit is inadequate; the authors mention this in §4, but the uncertainty estimate for that event should be treated with caution.
[References] The paper uses personal communications for the FSS pointing shifts and the SOLSTICE slit-alignment issue. These are relevant, but consider adding any publicly available calibration documentation or at least explicitly stating the date and context of the communication.

Circularity Check

1 steps flagged · score 6.0 of 10

M5.3 blueshift detection is a residual of a longitude-trend fit to the same sample; fit uncertainty is not propagated.

fitted input called prediction [Section 2 (Figure 4, detrending paragraph); Section 3 (M5.3 result); Table 1]
"To retrieve signatures of flare motions from these data, a linear fit to the data was subtracted, providing the detrended velocities displayed in the bottom panel of Figure 4."

The subtracted linear trend is v = 104.1 (x/Rsun) km/s, fit to the same 11-event SOLSTICE sample with the C3.4 outlier explicitly excluded. The 'detrended velocities' are therefore residuals of that fit. For M5.3 (x ≈ +0.295), the raw shift Δλ = −3.78 ± 3.15 pm gives v = −9 ± 8 km/s; subtracting the fitted trend turns it into −39.75 ± 11.00 km/s. The M5.3 detection is thus produced by removing a trend estimated from the same data point—it is the residual of the fit, not an independent measurement. The quoted ±11 km/s uncertainty comes from the Gaussian centroid fit only; the uncertainty in the fitted slope is not propagated, so the 3.6σ significance is not robust to detrending uncertainty. The C3.4 value is obtained by extrapolating the same fitted slope to the excluded point, making its m

full rationale

The paper is largely self-contained in its Gaussian fitting and quiet-Sun subtraction, and its self-citations to earlier SOLSTICE studies are background context, not load-bearing. The central claim of two significant Doppler shifts, however, depends directly on the longitude detrending. The M5.3 blueshift is not significant in the raw data (−9 ± 8 km/s) and becomes significant (−39.75 ± 11 km/s) only after subtracting a linear function of longitude that is fitted to the same 11-event sample. This fits the pattern of a fitted input being used to produce the claimed detection: the detrended velocity is, by construction, the residual from a model trained on the same event. The uncertainty of the fitted slope is not propagated into the final significance, and the paper's own caveat that a first-order polynomial may not be comprehensive and that ray-tracing could reveal more modest velocities acknowledges the fragility. The C3.4 redshift retains independent significance even before detrending (113 ± 13 km/s), so the paper is not entirely circular, but the M5.3 blueshift—one of the two headline detections—reduces to the detrending procedure. Score 6 reflects partial circularity: one central 'detection' is an artifact of subtracting a same-sample fit.

Assumptions & free parameters 2 free parameters · 4 assumptions · 0 invented entities

This is an observational paper, so no new physical entities are introduced. The central measurement is a fitted line centroid, not a derivation, and the load-bearing fitted quantities are the longitude-trend correction and the per-event quiet-Sun reference wavelength. Both are fitted to the same data that produce the claimed Doppler velocities, creating the main circularity burden.

free parameters (2)

Linear longitude-trend slope and intercept = slope = 104.1 km/s per x/R_sun (Figure 4); intercept not explicitly given
A linear fit to the Doppler velocities versus longitude, computed after excluding the C3.4 outlier, is subtracted from all 11 events to produce the claimed detrended velocities. The M5.3 blueshift significance and the inflated C3.4 redshift magnitude both depend on this fit. Section 2, Figure 4.
Per-event quiet-Sun rest wavelength lambda_0 = Varies per event, ~1206.5 to 1206.7 Å
The Doppler velocity is computed relative to a Gaussian fit to the quiet-Sun profile, making lambda_0 an empirically fitted reference. Any drift or bias in the quiet-Sun centroid shifts the zero point of all measured velocities. Section 2, Equation 1.

assumptions (4)

domain assumption Flare excess emission can be represented by a single Gaussian component after quiet-Sun subtraction
Single Gaussians are fit to the quiet-Sun-subtracted flare profiles; the X3.6 event has chi2_nu=11.4 and the authors note a two-Gaussian fit might be better. Section 2 and Section 4.
ad hoc to paper The longitude-dependent trend is a systematic effect that is linear and can be removed by a first-order polynomial
The paper detrends the Doppler velocities by subtracting a linear fit to the same sample, and later states the first-order polynomial assumption 'may not be comprehensive.' This is the load-bearing correction for the M5.3 blueshift. Section 2 and Section 4.
domain assumption The Neupert effect holds for the C3.4 event, allowing the GOES 1-8 Å flux derivative to proxy for HXR timing
RHESSI data were unavailable for the C3.4 flare, so the impulsive driving of the Si III enhancement is inferred from the derivative of GOES 1-8 Å flux under the Neupert effect. Section 3.
domain assumption The CHIANTI line list is complete within ±4 Å of the Si III centroid, so no blending lines affect the line center
The paper argues that no CHIANTI lines exist within ±4 Å and therefore blends would produce shifts larger than observed, but notes uncatalogued blends closer than 4 Å could be missed at SOLSTICE resolution. Section 4.

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Cite this review

Pith. "Pith review of Observations of Flare Induced Doppler Shifts in the Si~\textsc{iii} $1206\,\textrm{{\AA}}$ line." pith.science (2026). https://pith.science/paper/ZJGEQFYX

@misc{pith2026250905223,
  author       = {Pith},
  title        = {Pith review of: Observations of Flare Induced Doppler Shifts in the Si~\textsciii $1206\,\textrm\AA$ line},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/ZJGEQFYX}},
  note         = {Machine review of arXiv:2509.05223}
}

abstract

Doppler shifts in chromospheric and transition-region lines during solar flares are often interpreted as chromospheric condensation or evaporation. However, alternative sources of Doppler-shifted emission have been suggested, such as filament eruptions, jets or chromospheric bubbles. We analyse high-cadence scans from SORCE/SOLSTICE, which provide one-minute resolution profiles of the transition-region Si~\textsc{iii} ($1206\,\textrm{{\AA}}$, $\textrm{T} = 10^{4.6}\,\textrm{K}$) line. 11 X-, M-, and C-class events observed during these scans with clear impulsive phase Si~\textsc{iii} enhancements were identified. By subtracting a quiet-Sun profile and fitting Gaussian profiles to the Si~\textsc{iii} line, measurements of flare-induced Doppler shifts were made. After correcting for a systematic trend in these shifts with solar longitude, two of the 11 events were found to exhibit a significant Doppler shift, \comment{}one with a $201.36\pm21.94\;\textrm{km\,s}^{-1}$ redshift and the other with a $-39.75\pm11.00\;\textrm{km\,s}^{-1}$ blueshift\commentend{}. Intriguingly, SDO/AIA $304\,\textrm{{\AA}}$ and $1600\,\textrm{{\AA}}$ imaging revealed a bright eruption coincident with the event that exhibited a blueshift, suggesting the shift may have resulted from the eruption rather than evaporation alone. Our results highlight Si~\textsc{iii} as a useful diagnostic of flaring dynamics at a temperature that has received limited attention to date. Future comparisons of these observations with radiative hydrodynamic simulations, along with new observations from state-of-the-art spectrometers such as SOLAR-C/EUVST and MUSE, should clarify the mechanisms behind the observed shifts in this study.

Figures

Figures reproduced from arXiv: 2509.05223 by the authors.

**Figure 1.** Plot of Si iii and Lyα line profiles from SORCE/SOLSTICE during the impulsive phase (flare; red) and postflare (quiet-Sun; blue) of M8.3 flare on 7 Jan 2004. Irradiance is plotted on a log scale. To support Doppler measurements derived from SORCE/SOLSTICE observations, comparisons are drawn with photometric observations from the E-channel of the Extreme Ultraviolet Sensor on the Geostationary Operational Environme… view at source ↗

**Figure 2.** FSS pointing information for an M5.3 and C3.4 flares, shown in panels a.) and b.), respectively. Panels c.) and d.) show the evolution of disk-integrated Si iii centroid position during the respective events. The GOES flare start, peak, and end times are marked by black dashed lines, for the respective events. scan in the observation, was selected. For each of the 37 events, the centroid position of the disk-integra… view at source ↗

**Figure 3.** Observed and best-fit profiles to the Si iii line for M5.3 (left) and C3.4 (right) flares. Panels a1.) & a2.) show flaring (maroon) and quiet-Sun (blue) profiles and their best fits (red and cyan, respectively). A dashed cyan line illustrates the polynomial fit to Lyα wing emission for the quiet-Sun profiles. Panels b1.) and b2.) show the associated quiet-Sun-subtracted flare profiles (dark green) and their respecti… view at source ↗

Figures from the paper (4 more)

**Figure 4.** Figure 4: Top panel: Doppler velocities, from fit Si iii profiles, against longitude (in fraction of solar radius), data points shown as black dots, best fit line shown as dashed red line. Bottom panel: detrended Doppler velocities against longitude, data points shown as black d…

**Figure 5.** Figure 5: Lightcurves of M5.3 flare. Panel a.) shows X-ray observations in 1 − 8 Å (black), 6 − 12 keV (magenta) and 25 − 50 keV (cyan). Panel b.) shows chromospheric emission in a variety of lines, including the 1600 Å (orange) and 304 Å (red) channels of SDO/AIA, the Lyα line …

**Figure 6.** Figure 6: SDO/AIA images of M5.3 flare in the 1600 Å (top) and 304 Å (bottom) channels. Panels a1.) and a2.) show the active region (NOAA 11515) before the flare. b1.) and b2.) show flare footpoints during the impulsive phase. A blue arrow points to a bright eruption in panels c…

**Figure 7.** Figure 7: Lightcurves of C3.4 flare. Panel a.) displays 1 − 8 Å observations (black). Panel b.) shows Si iii (blue) and Lyα (green) observations from SORCE/SOLSTICE and GOES/EUVS-E, respectively. Panel c.) shows the centroid position of the Si iii line during the event in blue. …

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