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Oscillatory Phase and Acoustic Travel-Time Inconsistencies Measured between SDO/HMI and GONG Dopplergrams

This study reveals that systematic artifacts in both SDO/HMI and GONG Dopplergrams, including persistent phase-shift anomalies and east-west travel-time asymmetries, significantly compromise the accuracy of helioseismic meridional-circulation measurements.

Original authors: Junwei Zhao, Ruizhu Chen, S. P. Rajaguru, Shukur Kholikov

Published 2026-06-08

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CC BY 4.0

Original authors: Junwei Zhao, Ruizhu Chen, S. P. Rajaguru, Shukur Kholikov

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Sun as a giant, glowing drum. Just like a drumhead vibrates when struck, the Sun's surface ripples with sound waves caused by turbulent gas churning beneath it. Scientists use these ripples, a field called helioseismology, to "see" inside the Sun, much like how doctors use ultrasound to see inside a human body.

This paper is essentially a report card on two different "ears" listening to that drum:

SDO/HMI: A high-tech camera on a NASA satellite orbiting Earth.
GONG: A global network of ground-based telescopes.

Both instruments are supposed to be listening to the exact same solar sounds to map out the meridional circulation—a giant, slow river of gas flowing from the Sun's equator toward its poles. This flow is crucial because it helps drive the Sun's magnetic engine, which creates sunspots and solar storms.

The Problem: Two Listeners, Two Stories

The researchers took data from both instruments over 14 years (2010–2024) and compared them side-by-side. They expected the two instruments to agree, like two people listening to the same song. Instead, they found a "ghost in the machine."

Here is what they discovered, broken down with simple analogies:

1. The "Northwest Ghost" (Phase Anomalies)

When the scientists compared the timing of the sound waves arriving at the same spot on the Sun from both instruments, they found a weird glitch.

The Analogy: Imagine two people recording a concert. Most of the time, the music matches perfectly. But in the northwest corner of the stage, one recorder suddenly starts playing the music slightly out of sync with the other.
The Reality: There is a specific patch on the Sun's northwest side where the SDO and GONG instruments disagree on the timing of the waves. This "phase shift" isn't random; it's a persistent error that has actually been getting worse over the last 14 years. It's like a clock that is slowly losing time, but only in one specific room of the house.

2. The "Diagonal Slope" (The Fake Flow)

After removing the obvious differences (like the fact that one camera is in space and the other is on Earth), a strange pattern remained.

The Analogy: Imagine looking at a flat, calm lake. If you tilt your head slightly, the water might look like it's sloping downhill from one corner to the other, even though it's actually flat.
The Reality: The data showed a gradual "slope" in the timing of the waves, running from the northeast to the southwest. If a scientist didn't know about this glitch, they would look at the data and think, "Oh, the Sun has a giant current of gas flowing southward on the western side!"
The Catch: This "flow" is likely an illusion created by the instruments, not a real river of gas. It's a systematic error that tricks the math into seeing movement where there is none.

3. The East vs. West Mismatch

In a perfect world, the Sun's flow should look the same whether you measure it on the left (east) or right (west) side of the center.

The Analogy: If you measure the speed of a river by standing on the left bank and the right bank, you should get the same number.
The Reality: Both instruments gave different answers for the left side compared to the right side. The SDO/HMI satellite showed a bigger difference between the two sides than the ground-based GONG network did. This suggests that both instruments have their own unique "biases" or flaws that mess up the measurements.

The Big Conclusion

The paper concludes that we currently cannot fully trust our measurements of the Sun's internal gas flows because both major instruments are carrying "systematic artifacts" (hidden errors).

The "Northwest Ghost" and the "Diagonal Slope" are confusing the data, making it look like the Sun's internal flows are changing or moving in ways they might not actually be.
Because these errors change over time (they get worse or shift), it is very hard to fix them with a simple math correction.
The fact that both instruments show similar weird patterns suggests the problem might be deeper than just a broken camera; it might be something about how we interpret the data or how the instruments interact with the Sun's complex physics.

In short: We are trying to map the Sun's internal currents, but our two best maps have smudges and fake lines on them. Until we figure out what causes these smudges, our picture of how the Sun's magnetic engine works remains a bit blurry.

Technical Summary: Oscillatory Phase and Acoustic Travel-Time Inconsistencies Measured between SDO/HMI and GONG Dopplergrams

Problem Statement
Determining the internal profile of the Sun's meridional circulation remains a subject of significant debate in helioseismology. While near-surface flows are well-measured, internal profiles derived from different research teams using data from the Helioseismic and Magnetic Imager (SDO/HMI), the Michelson Doppler Imager (MDI), and the Global Oscillation Network Group (GONG) yield inconsistent results, ranging from single-cell to double-cell circulation patterns. Previous studies have identified a systematic bias where SDO/HMI-derived flows appear stronger southward than those from other instruments, and a persistent offset in acoustic travel times exists between SDO/HMI and GONG data. The origin of these discrepancies—whether they stem from instrumental artifacts, center-to-limb (CtoL) effects, or genuine solar variability—remains unclear, hindering accurate characterization of solar interior dynamics.

Methodology
To investigate the origin of these discrepancies, the authors analyzed contemporaneous Dopplergrams from SDO/HMI and GONG spanning 14 years (May 2010 to April 2024). The data were processed using identical procedures to ensure consistency:

Data Preparation: Continuous 24-hour full-disk Dopplergrams were remapped to Postel's projection, tracked at the Snodgrass rate, and spatially binned to a uniform resolution of $0.36^{\circ}$ pixel $^{-1}$ .
Relative Phase Shift Analysis: The authors computed relative oscillatory phase shifts ( $\delta\phi$ ) between the two instruments at identical solar locations. This was achieved by extracting time sequences from both datasets, computing their cross-correlation in the Fourier domain, and averaging over the frequency range of 3.0–4.0 mHz (where signal-to-noise is optimal).
Travel-Time Measurements: Acoustic travel-time shifts ( $\delta\tau$ ) were measured along the north-south direction on both the eastern and western sides of the solar central meridian. These measurements were compared between instruments and across the central meridian to identify asymmetries and systematic offsets.
Temporal Evolution: The study examined the temporal evolution of phase anomalies and travel-time discrepancies over the 14-year period, including annual and monthly variations.

Key Results
The analysis revealed several systematic inconsistencies that challenge current helioseismic interpretations:

Persistent Phase Anomalies: A distinct area of phase-shift anomalies was identified in the northwestern quadrant of the solar disk (approximately $0^{\circ}$ – $20^{\circ}$ longitude and $15^{\circ}$ – $35^{\circ}$ latitude). The magnitude of these anomalies has shown an increasing trend from 2010 to 2024, with amplitudes reaching $\sim7.0$ mrad (corresponding to $\sim0.3$ seconds in travel time).
Across-Disk Phase Trend: After removing the axisymmetric CtoL component, the relative phase-shift maps displayed a systematic, gradual decrease from the northeastern to the southwestern quadrant. This trend creates a gradient that, if interpreted as a flow, would mimic a southward flow stronger on the western side of the central meridian and suggest faster rotation in the southern hemisphere compared to the northern.
Travel-Time Inconsistencies:
- Instrumental Offset: A significant, persistent offset exists between SDO/HMI and GONG travel-time measurements, even after CtoL corrections.
- East-West Asymmetry: Contrary to expectations, travel-time measurements on the eastern and western sides of the central meridian are not consistent for either instrument. The discrepancies are more pronounced in SDO/HMI data but are statistically significant in GONG data as well.
- Correlation: The temporal evolution of the east-west travel-time asymmetry shows a strong correlation between the two independent instruments, suggesting the cause is not purely instrumental but may involve unknown factors coupled with solar signals.

Significance and Claims
The paper concludes that both SDO/HMI and GONG carry systematic artifacts that significantly affect meridional circulation measurements. The authors assert that:

Systematic Bias: The observed phase anomalies and east-west asymmetries are likely responsible for the conflicting meridional circulation profiles reported in previous literature.
Correction Challenges: The time-varying nature of the phase anomalies and the non-stationary behavior of the east-west asymmetry pose major challenges for accurate characterization and correction.
Limitations of Current Proxies: The findings undermine the validity of using equatorial east-west travel times as a stationary proxy for the CtoL effect, as the phase anomalies exhibit across-disk trends that do not necessarily share the same behavior as the equatorial region.
Modest Attribution: The study emphasizes that because the phase maps represent relative measurements between instruments, it remains difficult to definitively assign the specific source of the error to one instrument versus the other. However, the presence of systematic errors in both datasets is evident.

The authors do not propose a specific correction method or a new inversion technique but highlight that these unresolved systematic issues must be addressed to achieve a reliable determination of the Sun's internal meridional circulation.

The Problem: Two Listeners, Two Stories

1. The "Northwest Ghost" (Phase Anomalies)

2. The "Diagonal Slope" (The Fake Flow)

3. The East vs. West Mismatch

The Big Conclusion

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