Solar photospheric velocities measured in space: a comparison between SO/PHI-HRT and SDO/HMI

A. Feller; A. Gandorfer; A. Ulyanov; D. Calchetti; D. Orozco S\'uarez; F.J. Bail\'en; G. Valori; H. Strecker; J. Blanco Rodr\'iguez; J. Hirzberger

arxiv: 2510.25515 · v3 · submitted 2025-10-29 · 🌌 astro-ph.SR

Solar photospheric velocities measured in space: a comparison between SO/PHI-HRT and SDO/HMI

D. Calchetti , K. Albert , F.J. Bail\'en , J. Blanco Rodr\'iguez , J.S. Castellanos Dur\'an , A. Feller , A. Gandorfer , J. Hirzberger

show 10 more authors

J. Sinjan X. Li T. Oba D. Orozco S\'uarez T.L. Riethm\"uller J. Schou S.K. Solanki H. Strecker A. Ulyanov G. Valori

This is my paper

Pith reviewed 2026-05-18 03:16 UTC · model grok-4.3

classification 🌌 astro-ph.SR

keywords solar photosphereline-of-sight velocitySO/PHI-HRTSDO/HMIinstrument comparisonformation heightEvershed flow

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The pith

Line-of-sight velocity signals from SO/PHI-HRT and SDO/HMI agree with slope 0.96 and 92% correlation after corrections.

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

This paper compares line-of-sight velocity measurements of the solar photosphere taken simultaneously by the SO/PHI instrument on Solar Orbiter and the HMI instrument on SDO. The observations were made on 29 March 2023 when Solar Orbiter was nearly aligned with Earth at 0.39 au, allowing a direct side-by-side view. After aligning and remapping the images and removing large-scale velocity components, the authors find a strong linear relationship between the two datasets. The velocities correlate at 92 percent with a slope of 0.96, and the signals appear to form at heights differing by only 7 plus or minus 14 kilometers. Such agreement is essential for using SO/PHI to measure flows on the far side of the Sun and for combining views from two directions to map horizontal motions.

Core claim

The data are aligned and remapped to allow a pixel-by-pixel comparison of the whole time series of 4 hours length. Temporal and spatial variations are considered for a direct combination of the measurements. The LoS velocity distributions are evaluated and a clear linear relation is found between the two instruments with a slope of 0.96 and a correlation of 92%. We find that the signals form at similar heights, with a separation of 7±14 km, which is larger than previous estimates. A close-up look at the penumbra of a sunspot and its Evershed flow is presented. We conclude that the signals inferred by SO/PHI-HRT and SDO/HMI show very good agreement and high correlation when instrumental and a

What carries the argument

Alignment and remapping of simultaneous observations to enable pixel-by-pixel comparison of the four-hour time series after accounting for large-scale velocities.

Load-bearing premise

That the alignment, remapping, and removal of large-scale velocities fully remove all geometric and instrumental differences so that any remaining discrepancy is due only to formation-height offset rather than residual calibration or solar-evolution effects during the 4-hour sequence.

What would settle it

Repeating the comparison with a different alignment or remapping procedure and finding a much lower correlation or larger height separation.

Figures

Figures reproduced from arXiv: 2510.25515 by A. Feller, A. Gandorfer, A. Ulyanov, D. Calchetti, D. Orozco S\'uarez, F.J. Bail\'en, G. Valori, H. Strecker, J. Blanco Rodr\'iguez, J. Hirzberger, J.S. Castellanos Dur\'an, J. Schou, J. Sinjan, K. Albert, S.K. Solanki, T.L. Riethm\"uller, T. Oba, X. Li.

**Figure 1.** Figure 1: Top left panel: intensity continuum as measured by SO/PHIHRT. A 10 Mm scale is given as reference. Top right panel: LoS velocity as measured by SO/PHI-HRT. Bottom left panel: LoS magnetic field as measured by SO/PHI-HRT. Bottom right panel: LoS magnetic field as measured by SDO/HMI. The yellow box shows the SO/PHI-HRT FoV. The black arrows in the top left panel indicate the solar north (N) and east (E). N… view at source ↗

**Figure 2.** Figure 2: k-ω diagrams. Top left panel: Power spectrum of the remapped SO/PHI-HRT time series (units: m 2 /s 2 ). Top right panel: Power spectrum of the SDO/HMI time series interpolated to 60 s cadence (units: m 2 /s 2 ). Bottom left panel: phase difference between SO/PHI-HRT and SDO/HMI where positive values means that SO/PHI-HRT is lagging behind SDO/HMI (units: degrees). The red dashed box shows the area used to… view at source ↗

**Figure 3.** Figure 3: Scatter plot distribution of the velocity field values as measured by the remapped SO/PHI-HRT (x axis) and SDO/HMI (y axis). The color scale shows the number of points in each bin on a logarithmic scale. The black line shows an orthogonal least square linear fit to the data points, while the contours depict the results of a 2-D Gaussian distribution fit. The slope and offset obtained by the linear fit, th… view at source ↗

**Figure 4.** Figure 4: Temporal variation of the fitting results. The dots represent the results of the fits obtained frame by frame, whereas the dashed lines show the linear trends of these results. The error bars are not shown because they are comparable with the marker size. Top panel: slope (left axis, blue) and offset (right axis, orange) obtained by the linear fit. Middle panel: x (left axis, green) and y (right axis, red)… view at source ↗

**Figure 5.** Figure 5: Spatial dependence of the temporal correlation between the remapped SO/PHI-HRT and SDO/HMI signals (top panel), and of the slope (middle panel) and offset (bottom panel) obtained by the linear fit. The blue and red contours show the -200 and +200 G values of the SO/PHI-HRT LoS magnetic field respectively. The FoV is shown as a yellow box in [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗

**Figure 6.** Figure 6: Scatter plot distribution of the velocity field values as measured by the remapped SO/PHI-HRT (x axis) and SDO/HMI (y axis) in the sunspot penumbra. The colors and lines are defined as in [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗

**Figure 7.** Figure 7: Left panel: remapped SO/PHI-HRT LoS velocity signal around the sunspot averaged in time (oriented as in [PITH_FULL_IMAGE:figures/full_fig_p007_7.png] view at source ↗

read the original abstract

The Polarimetric and Helioseismic Imager (SO/PHI) onboard Solar Orbiter is a spectropolarimeter scanning the Fe I line at 617.3 nm, providing data of the solar photosphere. The same line is sampled by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) and many other on-ground instruments. In this paper, we aim at assessing the consistency between line-of-sight (LoS) velocity measurements from the two instruments. Reliable measurements of up and down flows from SO/PHI are crucial and unique when Solar Orbiter is facing the far side of the Sun. Also, a combination of measurements from two vantage points to study horizontal flows must rely on consistent observations. For this purpose, we compare the LoS velocity measured by SO/PHI's High Resolution Telescope (SO/PHI-HRT) and SDO/HMI on 29 March 2023, when Solar Orbiter was crossing the Sun-Earth line at a distance of 0.39 au from the Sun. Because such co-alignments are rare, this configuration offered an almost unique opportunity to directly compare data products from both telescopes. The data are aligned and remapped to allow a pixel-by-pixel comparison of the whole time series of 4 hours length. Temporal and spatial variations are considered for a direct combination of the measurements. The LoS velocity distributions are evaluated and a clear linear relation is found between the two instruments with a slope of 0.96 and a correlation of 92%. We find that the signals form at similar heights, with a separation of 7$\pm$14 km, which is larger than previous estimates. A close-up look at the penumbra of a sunspot and its Evershed flow is presented. We conclude that the signals inferred by SO/PHI-HRT and SDO/HMI show very good agreement and high correlation when instrumental effects and large-scale velocities on the Sun are properly accounted for.

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.

Desk Editor's Note private letter to a colleague

This gives a useful new cross-calibration point for SO/PHI-HRT and SDO/HMI velocities during their 2023 alignment, with clear agreement numbers but a height offset that rests on strong assumptions about residuals.

read the letter

This paper reports a direct comparison of line-of-sight velocity measurements from SO/PHI-HRT on Solar Orbiter and SDO/HMI during their alignment on 29 March 2023. The main result is that the two instruments show good agreement, with a slope of 0.96 and 92% correlation after proper alignment and removal of large-scale velocities. They also estimate that the signals form at heights separated by about 7 km with a large uncertainty of 14 km. What stands out is the new quantitative cross-calibration for this specific pair of space instruments at 0.39 au. Prior work had ground-based comparisons, but this is a rare space-based matchup on the same Fe I line. The authors handle the remapping to a common grid and look at both the overall statistics and a detailed view of the Evershed flow in a sunspot penumbra. That practical focus is helpful for anyone who needs to trust velocity data from these observatories when Solar Orbiter is on the far side or when combining viewpoints. The softer part is the interpretation of the residual differences as purely a height offset. The paper assumes that after subtracting large-scale flows and aligning the images, any pixel differences come from the small height separation. But solar granulation and supergranulation evolve over the 4-hour period, and the two instruments have different perspectives and independent calibrations. Residuals from those could easily affect the height estimate at the level of the quoted 14 km error bar. The abstract says instrumental effects are accounted for, yet without the precise functional form of the subtraction or the misalignment metric, it's difficult to confirm that the height difference is the unique explanation. This kind of paper is for researchers in solar physics who work with photospheric velocity fields or plan to use data from both missions together. A reader looking for empirical checks on instrument consistency would get direct value from the numbers and the example. It deserves a serious referee because the data come from flagship instruments and the comparison is observationally grounded, even if some processing details need verification. I recommend putting it through peer review.

Referee Report

2 major / 2 minor

Summary. The paper compares line-of-sight velocity measurements from SO/PHI-HRT on Solar Orbiter and SDO/HMI on 29 March 2023 during a rare Sun-Earth line crossing at 0.39 au. After aligning and remapping the 4-hour time series to a common grid and subtracting large-scale velocities, the authors report a linear relation (slope 0.96) with 92% correlation between the two instruments' Fe I 617.3 nm velocities. They infer a formation-height separation of 7±14 km and present a close-up analysis of penumbral Evershed flows, concluding that the signals agree well once instrumental effects and large-scale flows are accounted for.

Significance. If the results hold, the work supplies a direct empirical test of velocity consistency between two space-based spectropolarimeters sampling the same line from different heliocentric distances. This is valuable for multi-viewpoint studies, far-side observations, and stereoscopic flow analyses that combine SO/PHI and HMI data. The near-unity slope and high correlation, together with the quantified height offset, offer a practical benchmark for cross-calibration.

major comments (2)

[Results section (height-inference paragraph)] Results section (height-inference paragraph): The claim that residual velocity differences arise solely from a 7±14 km formation-height offset assumes that alignment, remapping, and large-scale subtraction have eliminated all geometric, calibration, and evolutionary contributions. Over 4 hours, granulation and supergranulation evolve on shorter timescales, and the differing projection angles from 0.39 au versus 1 au can leave residuals at tens of m s⁻¹. The manuscript should supply the explicit functional form of the large-scale subtraction and report the post-subtraction rms velocity residuals in quiet-Sun regions to show they lie below the threshold that would bias the height difference within the stated uncertainty.
[Methods section (data alignment and remapping)] Methods section (data alignment and remapping): The alignment and remapping procedure is described at a high level, but the cross-correlation metric used to quantify residual misalignment, the stray-light correction applied to each instrument, and the precise method by which the 7 km height separation is extracted from the velocity differences are not given in sufficient detail. These steps are load-bearing for the pixel-by-pixel comparison and the height attribution; without them the central claim cannot be fully verified from the text.

minor comments (2)

[Abstract] Abstract: The statement that the derived height separation is 'larger than previous estimates' lacks citations or quantitative comparison; adding one sentence referencing the earlier values would improve context.
[Figure captions] Figure captions: Captions for velocity maps and scatter plots should explicitly state the final spatial sampling after remapping and the exact temporal window used for each panel.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thorough review and valuable suggestions. Below we respond to each major comment. We will revise the manuscript to include the additional details requested, which will enhance the clarity and verifiability of our results.

read point-by-point responses

Referee: [Results section (height-inference paragraph)] Results section (height-inference paragraph): The claim that residual velocity differences arise solely from a 7±14 km formation-height offset assumes that alignment, remapping, and large-scale subtraction have eliminated all geometric, calibration, and evolutionary contributions. Over 4 hours, granulation and supergranulation evolve on shorter timescales, and the differing projection angles from 0.39 au versus 1 au can leave residuals at tens of m s⁻¹. The manuscript should supply the explicit functional form of the large-scale subtraction and report the post-subtraction rms velocity residuals in quiet-Sun regions to show they lie below the threshold that would bias the height difference within the stated uncertainty.

Authors: We agree that the assumptions underlying the height inference require more explicit justification to rule out other contributions. In the revised manuscript we will provide the explicit functional form of the large-scale subtraction (a low-order polynomial fit to the remapped velocity fields after alignment) and report the post-subtraction rms velocity residuals measured in quiet-Sun regions. We will also add a short discussion of granulation evolution and projection-angle effects over the 4-hour interval, noting that the observed 92% correlation supports that any residuals remain below the level that would affect the reported height difference within its stated uncertainty. revision: yes
Referee: [Methods section (data alignment and remapping)] Methods section (data alignment and remapping): The alignment and remapping procedure is described at a high level, but the cross-correlation metric used to quantify residual misalignment, the stray-light correction applied to each instrument, and the precise method by which the 7 km height separation is extracted from the velocity differences are not given in sufficient detail. These steps are load-bearing for the pixel-by-pixel comparison and the height attribution; without them the central claim cannot be fully verified from the text.

Authors: We acknowledge that the Methods section would benefit from greater specificity. In the revision we will expand the description to include the cross-correlation metric used to assess residual misalignment, the stray-light corrections applied to the SO/PHI-HRT and SDO/HMI data, and the precise procedure used to derive the 7±14 km formation-height separation from the aligned velocity differences. These additions will allow full verification of the pixel-by-pixel comparison and height attribution. revision: yes

Circularity Check

0 steps flagged

Direct observational comparison; height offset extracted from residuals but not circular by construction

full rationale

The paper performs a direct empirical comparison of line-of-sight velocities from SO/PHI-HRT and SDO/HMI after alignment, remapping to a common grid, and subtraction of large-scale flows over the 4-hour sequence. The reported slope of 0.96 and 92% correlation are straightforward statistical outputs from the pixel-by-pixel data matching. The formation-height separation of 7±14 km is derived as an interpretation of residual differences between the two datasets, but this does not reduce any claimed prediction or first-principles result to its own inputs by construction. No self-definitional steps, fitted parameters renamed as predictions, or load-bearing self-citations appear in the derivation chain. The analysis remains an observational matching exercise whose central claims are independently verifiable against the raw aligned time series.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard assumptions of solar spectroscopy (Fe I 617.3 nm line formation) and on the validity of the alignment/remapping procedure; no new free parameters are introduced beyond the reported height offset, which is derived from the comparison itself.

free parameters (1)

formation height separation
The 7±14 km offset is extracted from the velocity comparison data rather than taken from prior literature.

axioms (1)

domain assumption After alignment and large-scale velocity removal, residual differences are dominated by formation-height offset rather than uncorrected instrumental or evolutionary effects.
Invoked to interpret the linear relation and height difference as physical rather than artifactual.

pith-pipeline@v0.9.0 · 6009 in / 1442 out tokens · 26468 ms · 2026-05-18T03:16:54.256387+00:00 · methodology

discussion (0)

Reference graph

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