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CHromospheric magnetic fields in fLAREs and their evolution

Periodic Reporting for period 1 - CHLARE (CHromospheric magnetic fields in fLAREs and their evolution)

Reporting period: 2021-09-01 to 2023-08-31

This research project aims to investigate the variations of the solar magnetic field during flares, which are the most energetic events in our solar system.

Solar flares play an important role in society due to their ability to temporarily disrupt technology and infrastructure, influence communication systems, and contribute to scientific research and environmental phenomena.

Despite the significant impact of flares on modern technology, their timing and location remain unpredictable. Changes in the solar magnetic field topology are recognized as the primary cause of flares, but their underlying physics is not fully understood. Previous studies have demonstrated notable changes in the magnetic field within the photosphere during flares. However, research on the chromosphere, located higher in the atmosphere where flares have their origin, is limited due to the specialized instrumentation required by ground-based telescopes. The He I 1083.0 nm triplet is identified as the most suitable spectral range for studying the upper chromosphere.


Since there are no diagnostic tools available for analyzing the He I triplet in flares, the primary objective is to upgrade an existing tool to interpret spectral profiles of He I in emission, as they occur in flares. The secondary objective is to utilize the enhanced tool to analyze the evolution of the magnetic field vector in existing datasets for the first time. To put the results into context, additional observations will be used. Finally, new observations of flares with Europe’s largest solar telescope GREGOR are planned, as the current amount of spectropolarimetric flare observations using the He I triplet is very limited.


The HAZEL code, widely utilized in the scientific community, serves as the primary tool for robust spectral-line inversions of the He I triplet. However, it cannot interpret He I profiles in flares. The code was thoroughly upgraded to allow for inversions of the He I triplet in flares.

The chromospheric magnetic field was inferred at different stages of an M-class flare, using the upgraded inversion tool. Enhancements of up to 1000 G in the magnetic field strength in the flare are seen in the active part of the flare. The line-of-sight inclination shows significant changes at the borders of the flare.

Furthermore, we successfully inverted another chromospheric line, the Ca II 854.2 nm line, which was simultaneously observed alongside the He I triplet. Such combinations of spectral lines in flares are rare, offering significant potential for discovery science. The novel NLTE inversion code DeSIRe was employed to interpret the Ca II flare observations. Our investigation revealed that the Ca II profile, particularly under extreme atmospheric conditions such as flares, can be prone to misinterpretation by inversion codes. Comparison with the He I physical maps resolved ambiguities encountered in the Ca II inversions, highlighting the importance of multi-wavelength studies.

The combination of He I and Ca II inversions reveals plasma motions directed upward at the active front of the flare. We underscore the importance of high-cadence chromospheric instrumentation for both, space and ground-based telescopes in comprehending the nature of flares.
The widely used inversion code HAZEL was modified to analyze He I Stokes parameters observed during flares. We implemented a new parameter (beta) into the code, which addresses the emission in the profiles. The upgraded code was uploaded to github and is available to the scientific community.

The modified inversion code was applied to data of an M-class flare using a two-fold strategy. Initially, all the data was inverted with one atmospheric component, followed by inversions with two atmospheric components. The Bayesian Information Criterion (BIC) was then employed for each pixel to choose between one or two atmospheric components.

For the first time, the magnetic field was inferred using the He I triplet during a high-energetic flare. The results of the inversions are smooth, assuring that the inversion process was successful. Enhancements of up to 1000 G in the magnetic field strength in the active part of the flare are detected. The line-of-sight inclination shows significant changes at the borders of the flare.

For the analysis of the second data set #2 (X-class flare), a secondment at the University of Bern was carried out. The data was inverted using the same strategy as for data set #1. A partial success was achieved in at least half of the field-of-view. The reason might be some additional contamination of cross-talk and, even more likely, the complexity of the spectral profiles.

The M-class flare inversions were disseminated at the following international conferences/ workshops as an oral presentation: Spanish National Solar Physics Meeting (2023), Methods and techniques used in NLTE inversions Workshop (2023), Annual Meeting of the German Astronomical Society (2023), CLASP 2.1 Science Meeting (2023), and as a poster at the European Space Weather Week (2023).

In replacement of the secondment at University College London (UCL), additional ground-based data in the lower chromosphere of the Ca II 8542 Å line, simultaneously observed with the He I triplet during the M-class flare (data set #1), was analyzed. The data were inverted using the novel NLTE inversion code (DeSIRe). We successfully extracted information regarding the line-of-sight velocities of the plasma before, during, and after the flare.

These results were presented at several conferences: XV Reunion Cientifica de la SEA (2022), Spanish National Solar Physics Meeting (2023), and at the Methods and techniques used in NLTE inversions Workshop (2023).

An instrumentation upgrade for acquiring flare data with high-speed recording cameras was completed and integrated into the GREGOR telescope, Europe's largest solar telescope, now accessible to the scientific community.

Ground-based telescope images require image restoration to correct for wavefront distortion caused by Earth's atmosphere, only achieved with computationally expensive methods like MOMFBD. To address this, we devised a novel approach leveraging neural networks to accelerate image restoration, enabling significantly faster processing times compared to the conventional MOMFBD code.

New observations were conducted during the project using the GREGOR telescope. Through competitive calls, we were awarded a total of 43 observing days over the project's duration. Four days were successful to record new flare data, albeit with relatively low intensity (C-class flare).
A new tool to analyze the Stokes profiles of He I in solar flares is provided. The upgrade is part of the inversion code HAZEL. The code is available on Github.

For the first time, the magnetic field variations, as inferred from the He I triplet, are shown in an M-class flare.

A novel inversion code (DeSIRe) has been used to successfully carry out inversions of the Ca II 854.2 nm line during an M-class flare.

We found that the combination of He I and Ca II inversions, two chromospheric wavelength ranges, was crucial to better understand flares.

The results have an impact on flare models, future prediction methods, and space weather forecasting.

Image restoration accelerated by neural networks was succesfully accomplished.
Flare observations from the ground (left). Example of Stokes profiles in the flare and fit (right).