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Forward synthesis of an extreme He I 10830 Å blueshifted profile during a solar flare

This paper reports the successful forward synthesis of an extreme He I 10830 Å blueshifted profile observed during an M4.5 solar flare using the GREGOR telescope's upgraded GRIS instrument, revealing that the profile arises from a nearly continuous distribution of chromospheric velocities along the line of sight rather than independent atmospheric structures.

Original authors: Benedetta Gandolfi, Christoph Kuckein, Sergio Javier González Manrique, Juan Carlos Trelles Arjona, Manuel Collados, Ivan Milić, Sandeep Dubey, Andrés Asensio Ramos

Published 2026-10-06

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CC BY 4.0

Original authors: Benedetta Gandolfi, Christoph Kuckein, Sergio Javier González Manrique, Juan Carlos Trelles Arjona, Manuel Collados, Ivan Milić, Sandeep Dubey, Andrés Asensio Ramos

Original paper licensed under CC BY 4.0 (https://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

The Sun is not a static ball of fire but a churning, magnetic atmosphere that constantly shifts and erupts. To understand how energy moves from the deep interior to the outer layers, scientists study the light coming from different parts of the solar surface. One specific window into this process is a spectral line created by helium, a gas that exists high in the Sun's atmosphere, known as the chromosphere. Under normal conditions, this helium gas absorbs light at a very specific color, creating a small dip in the spectrum. However, when the Sun is active, such as during a solar flare, the gas can move incredibly fast. If that gas rushes toward Earth, the light it absorbs shifts toward the blue end of the spectrum, a phenomenon called a blueshift. For decades, instruments on Earth have been limited in how far they could look toward that blue end, meaning the fastest, most extreme movements of solar gas often remained invisible, hidden just beyond the edge of what could be seen.

In 2024, a team of astronomers upgraded a powerful solar telescope called GREGOR to fix this blind spot. By adjusting their equipment, they were able to capture a much wider range of colors, finally revealing the full extent of these rapid movements. On May 31, 2025, while observing a large active region on the Sun, the team witnessed an M4.5 solar flare, a significant burst of energy. During this event, they recorded a helium absorption profile that was unlike anything seen before: a deep, broad stretch of missing light that extended far into the blue, indicating gas moving at speeds that had previously been undetectable. The researchers then set out to understand exactly what kind of atmospheric structure could create such an unusual signature.

To solve this puzzle, the team used a sophisticated computer model to simulate the light passing through the solar atmosphere. They did not try to find a single, simple explanation. Instead, they built a model consisting of twenty-eight thin layers of gas stacked on top of one another, representing the line of sight from the telescope to the Sun. Each layer was assigned a different speed, ranging from nearly stationary to a staggering 280 kilometers per second toward the observer. The model also included the effects of a nearby silicon line from the lower solar surface, which became blended with the helium signal as the gas moved faster. By carefully adjusting the density and speed of each layer, the researchers successfully recreated the exact shape of the observed light curve, matching the deep, extended blue tail and the complex blending with the silicon line.

The results suggest that this extreme blueshift is not caused by a single, isolated blast of gas, nor is it a collection of many separate, unrelated clouds moving at different speeds. Instead, the smooth and systematic changes in the model parameters indicate that the solar atmosphere in this region is behaving like a continuous, accelerating flow. The gas is moving in a coherent stream where the speed increases steadily as you look higher up, creating a nearly continuous distribution of velocities. This interpretation challenges the idea that such complex profiles are just a messy pile of independent structures; rather, it points to a highly organized, rapidly accelerating column of plasma. The team found that the total amount of gas involved was substantial, with an optical depth significantly larger than what is typically seen in other studies of solar flares.

This discovery highlights the power of the new instrument upgrade, which allowed the detection of velocities that were previously out of reach. The study confirms that the most energetic solar events can drive chromospheric gas to speeds approaching 280 kilometers per second, creating spectral signatures that stretch far beyond the limits of older instruments. While the exact physical mechanism driving this specific acceleration remains a subject for further study, the modeling provides a clear picture of the atmospheric structure required to produce the observation. The work demonstrates that what looks like a single, strange line in the data is actually the spectroscopic fingerprint of a vast, continuous flow of solar material, offering a new window into the violent and dynamic nature of our star's atmosphere.

Technical Summary: Forward Synthesis of an Extreme He I 10830 Å Blueshifted Profile During a Solar Flare

Problem and Motivation
Solar flares generate highly dynamic chromospheric conditions, often resulting in complex spectral signatures in the He I 10830 Å triplet. While redshifted profiles are well-documented, extremely blueshifted absorption features (indicating rapid upflows) have historically remained undetected. This limitation stems from the spectral window of most solar spectrographs, which typically extend only to approximately 150 km s⁻¹ toward the blue. Consequently, faster upflowing events fall outside the observable range. The 2024 upgrade of the GREGOR Infrared Spectrograph (GRIS) addressed this by shifting the He I 10830 Å coverage toward shorter wavelengths to accommodate simultaneous Ca II 8542 Å observations. This modification enabled the detection of extreme blueshifted profiles previously inaccessible. The paper focuses on analyzing one such extreme profile observed during an M4.5 solar flare, characterized by absorption extending far into the blue, crossing the rest wavelength of the photospheric Si I 10827 Å line.

Methodology
The authors employed forward synthesis using the HAnle and ZEeman Light (HAZEL 2.0) code to model the observed Stokes I profile. The synthesis strategy involved the following key components:

Atmospheric Model: The spectrum was modeled using a photospheric atmosphere (VAL-C model) to synthesize the Si I 10827 Å line via the SIR code, serving as a boundary condition for the chromospheric He I synthesis.
Stacked-Slab Geometry: Recognizing that the observed profile exhibited a continuous, deep absorption extending to the blue without distinct separation into discrete components, the authors rejected a multi-component filling-factor approach. Instead, they adopted a "stacked-slab" geometry, where radiation propagates through 28 vertically stacked chromospheric slabs.
Parameter Variation: In this geometry, the line-of-sight (LOS) velocity, optical depth ( $\tau$ ), Doppler width ( $\Delta v$ ), and damping parameter ( $a$ ) were varied for each slab. The magnetic field vector was set to zero, and the source function enhancement parameter ( $\beta$ ) was fixed at unity.
Velocity Sampling: The 28 slabs were configured to reproduce the velocity gradient. Nine slabs covered the complex region near the rest wavelengths of He I and Si I, while 19 slabs covered the extended high-velocity blueshifted wing with an almost linear progression in LOS velocity (steps of $\approx$ 10 km s⁻¹), ranging from nearly rest to approximately $-280$ km s⁻¹.

Key Results
The forward synthesis successfully reproduced the major characteristics of the observed extreme blueshifted profile:

Profile Reproduction: The model matched the extended blueshifted absorption, the nearly flat absorption plateau, the deep blended core caused by the overlap of the blueshifted He I triplet and the Si I line, and the return to continuum at longer wavelengths.
Velocity Distribution: The inferred LOS velocities spanned from $+3$ km s⁻¹ to $-280$ km s⁻¹. The bottom panel of Figure 4 illustrates an almost linear variation of velocity with cumulative optical depth, suggesting a coherent velocity gradient rather than independent structures.
Optical Depth: The total optical depth of the stacked slabs was found to be $\tau \approx 6$ , significantly larger than values reported in other He I 10830 Å studies. The authors note this value is proportional to the column number density of He I atoms in the ground state, though it depends on the assumption of a constant source function.
Parameter Trends: The atmospheric parameters exhibited smooth, systematic variations. The optical depth generally decreased from 0.5 to 0.035 (with a localized enhancement in the plateau region), while the Doppler width and damping parameters showed localized adjustments corresponding to changes in the spectral morphology.

Significance and Claims
The paper claims that the stacked-slab solution should not be interpreted as a collection of independent atmospheric structures, but rather as a discrete numerical representation of a nearly continuous distribution of chromospheric velocities along the line of sight.

Physical Interpretation: The results suggest that the observed extreme blueshift is the spectroscopic signature of chromospheric plasma spanning a broad, nearly continuous range of LOS velocities, consistent with an accelerated flow.
Instrumental Impact: The study demonstrates the capability of the upgraded GRIS instrument to detect extreme Doppler shifts ( $\approx -280$ km s⁻¹) that were previously beyond the spectral range of solar spectrographs.
Modeling Approach: The work extends previous studies (which typically used 2–3 slabs) to a much more extreme case, validating the use of a multi-slab forward synthesis to model continuous velocity gradients in flare environments. The authors emphasize that the smooth evolution of atmospheric parameters supports the interpretation of a stratified, optically thick atmospheric structure rather than discrete, independent components.

The paper concludes that while the specific physical mechanism driving this velocity stratification cannot be determined solely from these observations, the inferred structure is consistent with a continuous, moderately optically thick flow. The study does not propose specific future experimental proposals or applications beyond the immediate demonstration of the instrument's new capabilities and the modeling of this specific event.

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