Identifying variability of Solar Flare Energy Transport Mechanisms via Solar Orbiter’s Major Flare Campaign.

(Solar Orbiter Nugget #82 G. S. Kerr^1,2, S. Krucker^3,4, J. C. Allred⁵, J. M. Rodriguez-Gomez^2,5, A. R. Inglis^2,5, D. F. Ryan⁶, L. A. Hayes⁷, R. O. Milligan⁸, A. F. Kowalski^9,10, J. E. Plowman¹¹, P. R. Young⁵, T. A. Kucera⁵, and J. W. Brosius^2,5)

Introduction

During solar flares an enormous amount of magnetic energy is released in the corona before ultimately flowing to the lower atmosphere, where it is dissipated. A dramatic response of the plasma results, creating both large-scale flare ribbons, with localized fine-scale structure, as well as the development of the flare arcade [1]. Forward modelling this radiative response has become a key method to understand flares, and to critique our theories of how energy, radiation and mass are transported [e.g. 2,3]. 
Due to the unambiguous evidence of their presence in flares, from the hard X-ray bremsstrahlung they produce [4], flare-accelerated electrons (> tens of keV) are commonly thought of as the primary means by which energy is carried from the coronal release site to the lower atmosphere. Observations are often interpreted under that assumption.  However, it is an open question if energetic electron precipitation really dominates in every location within flare ribbons, not least because it is very rare for hard X-ray sources to be co-spatial with the entire ribbon (more commonly, they coincide with the strongest portions) [5].
In our recent publication [6], we demonstrated using Solar Orbiter Major Flare campaign observations [7], that, indeed, the dominant energy transport mechanism does vary in space. Different portions of flare ribbons are not exclusively heated by precipitating particles, and we must consider alternate forms also. 
 

Results

Figure 1. An EUI/HRI_EUV image of the flaring region from 23^rd March 2024. The large, bright loops in the north of the field of view were produced by an M2.5 class flare. The inset region highlights a microflare that occurred during the gradual phase of the M2.5 flare. The SPICE 4” slit, shown in cyan, cut through each ribbon of that microflare, providing two footpoint observations. A movie is available from [6]. 

On March 23^rd 2024 Solar Orbiter observed a very active flaring region during the Major Flare campaign, including a microflare that occurred during the gradual phase of an M-class flare. That program provided high-cadence, high-resolution, multi-wavelength observations from STIX [8], EUI/HRIEUV [9], and SPICE [10]. SPICE’s slit was fortuitously placed to cross the loops of the M-class flare, and through each of the ribbons of the microflare (Figure 1). Spectra from SPICE (operating in sit-and-stare mode) were thus available from two flare footpoints, with 5.1s cadence.  Although the soft X-ray signal was dominated by the post-flare loops from the M-class flare, there was a modest but measurable increase at in hard X-rays at high-energies (>20 keV) co-temporal with the microflare. Thus, there was evidence that particle acceleration took place during the microflare. Spectral fitting of that data provided estimates of the properties of the non-thermal electron distribution, used to drive simulation so of the flare.

Figure 2. Spacetime maps of various spectral lines observed by SPICE. In cooler lines the two footpoint sources are clear, and highlighted in the upper left panel. In the transition region and hottest lines flare loops form later, appearing between the ribbon sources. The M2.5 flare loops are also caught by SPICE, in the north of the field of view. The northern footpoint is stronger, with a very impulsive transient peak that initially rapidly decreases within 30s, followed by a longer decay. Adapted from Extended Data Figure 1 in [6] under a Creative Commons NonCommerical-NoDerivatives International License (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Spacetime maps revealed that SPICE caught a strong, transient (~30s) footpoint in the northern ribbon, and a weaker, more gradually evolving footpoint in the southern ribbon (Figure 2). Enhanced emission was present throughout chromosphere, transition region, coronal, and very hot flare plasma (spanning 10kK – 10MK). Mapping the ratio of the Lyman β -to- Lyman γ (R_βγ) intensities revealed rather different responses within each footpoint (Figure 3). The footpoint in the northern ribbon showed a sharp decrease to R_βγ<2, that was short-lived (FWHM ~ 25s). The footpoint in the southern ribbon instead exhibited a more modest decrease to R_βγ = 2-2.4, which was much longer in duration (~144s). 

Figure 3. The observed (top portion) and modelled (bottom portion) Lyβ and Lyγ ratios. The space time maps illustrates the decrease of the ratio in response to the flare, with the panels below showing cuts through each footpoint. On those panels the thick black line is the weighted mean of R_βγ, with a Gaussian fit (red-dashed) illustrating the lifetime of the decrease. Green lines show the intensity lightcurves from each source. The stronger footpoint shows a more marked decrease of R_βγ, which is transient in nature. Below the observation are four panels showing results from flare modelling. The bottom left are electron beam driven flares, in which R_βγ reaches the observed minimum values from the stronger footpoint (blue band). The histogram shows the combined results of 31 models in which the energy deposition varied in time (a 20s triangular pulse), illustrating the change in the shape of R_βγ in time versus the constant injection (leftmost panel). The bottom right panels are two examples of conduction-driven flares, where R_βγ > 2. Adapted from Figures 2 and 3 in [6] under a Creative Commons NonCommerical-NoDerivatives International License (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Comparing to a grid of radiation hydrodynamic simulations produced using the RADYN code [11,12], a range of energy transport mechanisms were modelled: non-thermal electrons, non-thermal protons, combined beams, and direct heating of the corona (with subsequent thermal conduction). It was found that the means by which energy was transported to the chromosphere was the source of this contrasting behaviour. 
Simulations in which energy was transported via non-thermal particles led to the behaviour observed in the strong footpoint, with transient R_βγ<2 that persisted for the duration of electron injection (Figure 3, bottom left). Simulations in which energy was instead transported by an enhanced heat flux from the corona (thermal conduction driven flares) led to behaviour comparable to the weaker footpoint, with longer-lived R_βγ>2 (Figure 3, bottom right). During the flares, thermal collisions populated Lyγ at the expense of Lyβ.  Due to the extended spatial extent over which this occurred, and the hotter plasma present in the upper chromosphere, this effect was more pronounced in the particle-beam driven flares. A sharper drop in R_βγ resulted in those scenarios. The temporal behaviour of the Lyman line ratio seemingly encoded information about the precipitation timescale into an individual location. 

Conclusions

The combination of high-resolution, high-cadence observations (particularly EUV spectroscopy) and detailed radiative hydrodynamic simulations, showed that within the two flare footpoints observed by SPICE different energy transport mechanisms were dominant. 
Solar flare energy transport is neither uniform nor dominated by a single process. Developing similar diagnostics for other EUV/UV/optical spectra is also likely to be fruitful, not least because such observations have significantly better spatial resolution than afforded by X-ray instruments (the typical way to diagnose energetic particles). Going forward, it is crucial that we understand when, where, and why particular processes dominate flare energetics.

This research has been published in Kerr et al. “Spatial variation of energy transport mechanisms within solar flare ribbons,” Nature Astronomy (2026). DOI:10.1038/s41550-025-02747-9, which contains a more detailed discussion of the results.

Affiliations

(1) SUPA, School of Physics and Astronomy, University of Glasgow, UK
(2) Department of Physics, The Catholic University of America, USA
(3) University of Applied Sciences and Arts Northwestern Switzerland, Switzerland
(4) Space Sciences Laboratory, University of California Berkeley, USA
(5) Heliophysics Science Division, NASA Goddard Space Flight Center, USA
(6) Mullard Space Science Laboratory, University College London, UK
(7) Astronomy & Astrophysics Section, Dublin Institute for Advanced Studies, Ireland
(8) Queen’s University Belfast, UK
(9) Department of Astrophysical and Planetary Sciences, University of Colorado Boulder, USA
(10) National Solar Observatory, USA
(11) Southwest Research Institute, USA

Acknowledgements

We gratefully acknowledge the following funding: NASA’s Early Career Investigator Program (grant number 80NSSC21K0460; G.S.K. and A.F.K.), NASA’s Heliophysics Supporting Research programme (grant number 80NSSC21K0010; G.S.K. and R.O.M.), NASA’s Heliophysics Theory, Modelling and Simulations programme (J.C.A. and G.S.K.), NASA’s SOGI programme (grant number 80NSSC24K1242; S.K.), GSFC SO/SPICE project funding (T.A.K., P.R.Y., A.R.I., J.W.B., J.M.R.-G. and G.S.K.), the PHaSER co-operative agreement (grant number 80NSSC21M0180; A.R.I., J.W.B., J.M.R.-G. and G.S.K.), a Royal Society–Research Ireland University Research Fellowship (grant number URF/R1/241775; L.A.H.), the European Office of Aerospace Research and Development (grant number FA8655-22-1-7044P00001; R.O.M.), the Science and Technology Facilities Council (grant number ST/X000923/1; R.O.M.) and SwRI’s Solar Orbiter subcontract from GSFC (grant number 80GSFC20C0053; J.E.P.). Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. The STIX instrument is an international collaboration between Switzerland, Poland, France, Czech Republic, Germany, Austria, Ireland and Italy. The EUI instrument was built by CSL, IAS, MPS, MSSL/UCL, PMOD/WRC, ROB and LCF/IO with funding from the Belgian Federal Science Policy Office (BELSPO/PRODEX PEA 4000112292 and 4000134088); the Centre National d’Etudes Spatiales (CNES); the UK Space Agency (UKSA); the Bundesministerium für Wirtschaft und Energie (BMWi) through the Deutsches Zentrum für Luftund Raumfahrt (DLR); and the Swiss Space Office (SSO). The development of SPICE has been funded by ESA member states and ESA. It was built and is operated by a multinational consortium of research institutes supported by their respective funding agencies: STFC RAL (UKSA, hardware lead), IAS (CNES, operations lead), GSFC (NASA), MPS (DLR), PMOD/WRC (Swiss Space Office), SwRI (NASA) and UiO (Norwegian Space Agency).

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