Solar flares release a tremendous amount of magnetic energy that subsequently manifests in several forms; the bulk of this energy is transported through the Sun’s atmosphere and explosively heats the chromosphere. While hard X-ray observations have pointed to flare-accelerated electrons as a primary means by which energy is transported following flares, alternative processes undoubtedly act alongside, or even instead of, those energetic electrons. To shed light on this we analysed flare-optimized, high-cadence Solar Orbiter observations. Footpoints from two flare ribbons were observed by the Spectral Imaging of the Coronal Environment (SPICE) instrument. Curiously, those footpoints exhibited contrasting behaviour: one had short-lived yet strong decreases in the Lyman β/Lyman γ line intensity ratio, whereas the other exhibited a more prolonged, moderate dip in that ratio. These observations were compared to synthetic spectra from radiation hydrodynamic simulations of flares driven by various energy transport mechanisms. This revealed that one footpoint was driven by energetic particle precipitation, while the other was driven by enhanced thermal heat flux. The implication is that energetic particles do not dominate along the entirety of flare ribbons. Critically, we must now focus on understanding where, when and why different mechanisms dominate in solar flare energy transport. High-resolution flare footpoint observations in the extreme ultraviolet and X-rays were taken by Solar Orbiter. Combined with simulations, the results reveal that the dominant mechanism carrying flare energy through the Sun’s atmosphere can vary on small spatial scales.

Flare ribbons form when energy released by coronal magnetic reconnection is deposited in the low solar atmosphere, so by studying the dynamics of flare ribbons, one obtains an indirect measurement of reconnection. Our aim is to quantify the spatial and temporal scales of substructures in the Extreme Ultraviolet (EUV) flare ribbons, known as kernels, as a probe of the spatial extent and duration of energy injection during the impulsive phase of solar flares. Unprecedented observations of an M2.5 GOES-class flare from the March 2024 major flare campaign of Solar Orbiter were used. These data were obtained at high-cadence in short-exposure mode with the Extreme Ultraviolet Imager's high-resolution telescope, HRI_EUV. Individual kernels were automatically identified using a classical computer vision algorithm. Size distributions of ribbon kernels were derived, and an average light curve of individual kernels was extracted. The EUV flare kernels were small (≲ 60 pixels≈ 1 Mm^2) and a significant fraction were unresolved at a plate scale of 135 km/pix. Furthermore, we derived surprisingly short EUV kernel heating times of less than a few seconds. The average profile exhibits a sharp rise of 1.7±0.3 s from half-maximum, requiring an additional 2.3^+0.7_-0.4 s to return to its reference value. Our findings indicate that approximately half of the kernels were unresolved in this flare, despite the enhanced angular resolution offered by Solar Orbiter's proximity to the Sun at 0.38 AU here. Furthermore, we show that energy was only injected in a localised region (≲ 1 Mm^2) of flare ribbons for less than a few seconds. These results necessitate an in-depth investigation into the implications of such small-scale and transient injections on the energy flux deposited in solar flares, and the resulting response of the solar atmosphere.

Aims. Diagnosing solar flare conditions is essential for understanding coronal energy release. Using combined microwave and X-ray data, we aim to reconstruct 3D maps of the magnetic fields and plasma parameters in the SOL2021-05-07 flare. Methods. We used imaging spectroscopy from the Expanded Owens Valley Solar Array (EOVSA) to derive spatial maps of the magnetic field strength, as well as the thermal and nonthermal electron densities, along with the power-law index of nonthermal electrons via gyrosynchrotron modeling. Simultaneous X-ray observations from Hinode/X-Ray Telescope (Hinode/XRT) and Solar Orbiter/Spectrometer Telescope for Imaging X-rays (SolO/STIX), taken from different vantage points, enable a stereoscopic reconstruction of the flaring loop. By correlating the positions of microwave and thermal X-ray sources, we associated the 3D coordinates with the microwave-derived plasma parameters. Results. We derived observational 3D maps of magnetic field strength, Alfv & eacute;n speed, and plasma beta in a flaring volume, revealing a magnetically dominated environment. These spatially resolved diagnostics provide valuable constraints for models of magnetic reconnection and flare dynamics, representing a step toward a realistic 3D characterization of energy release in solar eruptive events.

Context. The Spectrometer/Telescope for Imaging X-rays (STIX) on board the ESA Solar Orbiter mission and the Hard X-ray Imager (HXI) aboard the Advanced Space-based Solar Observatory (ASO-S) satellite provide for the first time systematic coverage of solar flare hard X-ray sources from different vantage points. This unprecedented configuration enables the reconstruction of the three-dimensional (3D) hard X-ray intensity distribution of the thermal emission in solar flares. Aims. The main objectives of this study are twofold: (1) to perform 3D reconstructions of the thermal hard X-ray source of a solar flare using stereoscopic Fourier data (visibilities) provided by STIX and HXI; and (2) to investigate the evolution in time of the reconstructed 3D source morphology. Methods. The sets of 2D visibilities measured by STIX and HXI represent a sampling of the 3D Fourier transform of the flaring hard X-ray source on two planes orthogonal to the instruments’ lines of sight. Therefore, the 3D reconstruction problem is analogous to the standard 2D imaging problem and can be addressed with similar techniques. In this case, we performed 3D reconstructions by means of the iterative space reconstruction algorithm (ISRA). Results. We consider the SOL2024-10-03T12:12 event observed by STIX and HXI from largely different vantage points separated by 85.7°. We performed 3D reconstructions of the flaring thermal emission at a 10 s cadence, and we determined the X-ray source height and radial velocity over time. As a consistency test, we show that the morphology and location of our 3D reconstructions are consistent with the corresponding 2D images independently obtained from STIX and HXI data. Conclusions. The proposed 3D reconstruction methodology provides reliable results for events with a separation angle close to 90°. However, the dynamic range of the 3D reconstructions is limited by the low number of observed visibilities (as in the 2D case) and by the limited number of vantage points on the flaring events.

Solar Orbiter conducted a series of flare-optimised observing campaigns in 2024 using the Major Flare Solar Orbiter Observing Plan (SOOP). Dedicated observations were performed during two distinct perihelia intervals in March/April and October, during which over 22 flares were observed, ranging from B- to M-class. These campaigns leveraged high-resolution and high-cadence observations from the mission’s remote-sensing suite, including the High-Resolution EUV Imager (EUI/HRIEUV), the Spectrometer/Telescope for Imaging X-rays (STIX), the Spectral Imaging of the Coronal Environment (SPICE) spectrometer, and the High Resolution Telescope of the Polarimetric and Helioseismic Imager (PHI/HRT), as well as coordinated ground-based and Earth-orbiting observations. EUI/HRIEUV, operating in short-exposure modes, provided two-second-cadence, non-saturated EUV images, revealing structures and dynamics on scales not previously observed. Simultaneously, STIX captured hard X-ray imaging and spectroscopy of accelerated electrons, while SPICE acquired EUV slit spectroscopy to probe chromospheric and coronal responses. Together, these observations offer an unprecedented view of magnetic reconnection, energy release, particle acceleration, and plasma heating across a broad range of temperatures and spatial scales. These campaigns have generated a rich dataset that will be the subject of numerous future studies addressing Solar Orbiter’s top-level science goal: “How do solar eruptions produce energetic particle radiation that fills the heliosphere?”. This paper presents the scientific motivations, operational planning, and observational strategies behind the 2024 flare campaigns, along with initial insights into the observed flares. We also discuss lessons learned for optimizing future Solar Orbiter Major Flare campaigns and provide a resource for researchers aiming to utilize these unique observations.

The first simultaneous observations of the Fe xviii 974.86 Å and Fe xx 721.56 Å forbidden lines from the Spectral Imaging of the Coronal Environment (SPICE) spectrograph on Solar Orbiter are presented. The lines were observed from the postflare loops of an M2.5 class solar flare that peaked at 23:49 UT on 2024 March 23. The Fe xx /Fe xviii ratio is used to derive a temperature spacetime map for the flaring period, with values ranging from 8 to 20 MK. The map reveals repeated episodes of heating at the SPICE slit location over a 30 minute period. For one location with the brightest emission, the plasma cooled from 10 to 8 MK in 5 minutes, which is longer than the expected conductive cooling time of 170 s, suggesting continued background heating during the cooling period. Doppler shifts of between 0 and +10 km s ^−1 were obtained with precisions of 1–4 km s ^−1 , but the accuracies are lower due to uncertainties over the absolute wavelength calibration; hence, we cannot conclude there are plasma flows in the flare loops. The widths of the two lines were found to be close to the instrumental widths with no evidence of nonthermal broadening although this result is limited by the instrument resolution. The Fe xviii and Fe xx lines have high signal-to-noise with only a 5 s exposure time, demonstrating that the lines will be valuable for high-cadence flare studies with SPICE.

Solar filament eruptions play a key role in driving space weather, yet their fine-scale evolution remains poorly understood due to observational limitations. Using unprecedented high-resolution observations from Solar Orbiter's Extreme Ultraviolet Imager (105 km/pixel) and Polarimetric and Helioseismic Imager, we detect persistent magnetic reconnection events in a failed filament eruption. We identify magnetic reconnections between the filament and the surrounding magnetic field structures, with a higher frequency and a greater variety of types than previously observed. These reconnections significantly affect the filament stability and eruption dynamics, leading to sequential coronal jets and failed eruptions. We propose a "persistent magnetic cutting" concept, highlighting how persistent small-scale magnetic reconnections cumulatively affect filament stability during its evolution.

Solar eruptive events, including solar flares and coronal mass ejections (CMEs), are typically characterised by energetically significant X-ray emissions from flare-accelerated electrons and hot thermal plasmas. Occulted events, where the main flare is blocked by the solar limb, provide an opportunity to observe and analyse the X-ray emissions specifically associated with CMEs. This study investigates the X-ray and extreme ultraviolet (EUV) emissions associated with a large filament eruption and CME that occurred on February 15, 2022. This event was highly occulted from the three vantage points of Solar Orbiter, STEREO-A, and Earth. We utilised X-ray observations from the Spectrometer/Telescope for Imaging X-rays (STIX) and EUV observations from the Full Sun Imager (FSI) of the Extreme Ultraviolet Imager (EUI) on-board Solar Orbiter, supplemented by multi-viewpoint observations from STEREO-A/EUVI. This enabled a comprehensive analysis of the X-ray emissions in relation to the filament structure observed in EUV. We used STIX's imaging and spectroscopy capabilities to characterise the X-ray source associated with the eruption. Our analysis reveals that the X-ray emissions associated with the occulted eruption originated from an altitude exceeding 0.3Rsun above the main flare site. The X-ray time-profile showed a sharp increase and exponential decay, and consisted of both a hot thermal component at 17 MK and non-thermal emissions (>11.4 keV) characterised by an electron spectral index of 3.9. Imaging analysis showed an extended X-ray source that coincided with the EUV emission as observed from EUI, and was imaged until the source grew to a size larger than the imaging limit of STIX (180 arcsec). The findings demonstrate that STIX combined with EUI provides a unique and powerful tool for examining the energetic properties of the CME component of solar energetic eruptions.

Models of particle acceleration in solar eruptive events suggest that roughly equal energy may go into accelerating electrons and ions. However, while previous solar X-ray spectroscopic imagers have transformed our understanding of electron acceleration, only one resolved image of γ-ray emission from solar accelerated ions has ever been produced. This paper outlines a new satellite instrument concept—the large imaging spectrometer for solar accelerated nuclei (LISSAN)—with the capability not only to observe hundreds of events over its lifetime, but also to capture multiple images per event, thereby imaging the dynamics of solar accelerated ions for the first time. LISSAN provides spectroscopic imaging at photon energies of 40 keV–100 MeV on timescales of ≲10 s with greater sensitivity and imaging capability than its predecessors. This is achieved by deploying high-resolution scintillator detectors and indirect Fourier imaging techniques. LISSAN is suitable for inclusion in a multi-instrument platform such as an ESA M-class mission or as a smaller standalone mission. Without the observations that LISSAN can provide, our understanding of solar particle acceleration, and hence the space weather events with which it is often associated, cannot be complete.

Context. In general, large solar flares are more efficient at accelerating high-energy electrons than microflares. Nonetheless, sometimes microflares that accelerate electrons to high energies are observed. Their origin is unclear. Aims. We statistically characterized microflares with strikingly hard spectra in the hard X-ray (HXR) range, which means that they are efficient at accelerating high-energy electrons. We refer to these events as "hard microflares". Methods. We selected 39 hard microflares, based on their spectral hardness estimated from the Solar Orbiter/STIX HXR quicklook light curves in two energy bands. The statistical analysis is built on spectral and imaging information from STIX combined with extreme ultraviolet (EUV) and magnetic field maps from SDO/AIA and SDO/HMI. Results. The key observational result is that all hard microflares in this dataset have one of the footpoint rooted directly within a sunspot (either in the umbra or the penumbra). This clearly indicates that the underlying magnetic flux densities are large. For the events with the classic two-footpoints morphology, the absolute value of the mean line-of-sight magnetic flux density (and vector magnetic field strength) at the footpoint rooted within the sunspot ranges from 600 to 1800 G (1500 to 2500 G), whereas the outer footpoint measures from 10 to 200 G (100 to 400 G), therefore about ten times weaker. In addition, approximately 78% of the hard microflares, which exhibited two HXR footpoints, have similar or even stronger HXR flux from the footpoint rooted within the sunspot. This contradicts the magnetic mirroring scenario. The median footpoint separation, measured through HXR observations, is approximately 24 Mm, which aligns with regular events of similar GOES classes. In addition, about 74% of the events could be approximated by a single-loop geometry, demonstrating that hard microflares typically have a relatively simple morphology. Out of these events, around 54% exhibit a relatively flat flare loop geometry. Conclusions. We conclude that all hard microflares are rooted in sunspots, which implies that the magnetic field strength plays a key role in efficiently accelerating high-energy electrons, with hard HXR spectra associated with strong fields. This key result will allow us to further constrain our understanding of the electron acceleration mechanisms in flares and space plasmas.

Aims: This paper aims to demonstrate the importance of short-exposure extreme ultraviolet (EUV) observations of solar flares in the study of particle acceleration, heating and energy partition in flares. This work highlights the observations now available from the Extreme Ultraviolet Imager (EUI) instrument suite on board Solar Orbiter while operating in short-exposure mode. Methods: A selection of noteworthy flares observed simultaneously by the Spectrometer Telescope for Imaging X-rays (STIX) and the Full Sun Imager of EUI (EUI/FSI) are detailed. New insights are highlighted and potential avenues of investigation are demonstrated, including forward-modelling the atmospheric response to a non-thermal beam of electrons using the RADYN 1D hydrodynamic code, in order to compare the predicted and observed EUV emission. Results: The examples given in this work demonstrate that short-exposure EUI/FSI observations are providing important diagnostics during flares. A dataset of more than 9000 flares observed by STIX (from November 2022 until December 2023) with at least one short-exposure EUI/FSI 174 Å image is currently available. The observations reveal that the brightest parts of short-exposure observations consist of substructure in flaring ribbons that spatially overlap with the hard X-ray emission observed by STIX in the majority of cases. We show that these observations provide an opportunity to further constrain the electron energy flux required for flare modelling, among other potential applications.

Particle acceleration, and the thermalisation of energetic particles, are fundamental processes across the universe. Whilst the Sun is an excellent object to study this phenomenon, since it is the most energetic particle accelerator in the Solar System, this phenomenon arises in many other astrophysical objects, such as active galactic nuclei, black holes, neutron stars, gamma ray bursts, solar and stellar coronae, accretion disks and planetary magnetospheres. Observations in the Extreme Ultraviolet (EUV) are essential for these studies but can only be made from space. Current spectrographs operating in the EUV use an entrance slit and cover the required field of view using a scanning mechanism. This results in a relatively slow image cadence in the order of minutes to capture inherently rapid and transient processes, and/or in the spectrograph slit ‘missing the action’. The application of image slicers for EUV integral field spectrographs is therefore revolutionary. The development of this technology will enable the observations of EUV spectra from an entire 2D field of view in seconds, over two orders of magnitude faster than what is currently possible. The Spectral Imaging of the Solar Atmosphere (SISA) instrument is the first integral field spectrograph proposed for observations at ∼180 Å combining the image slicer technology and curved diffraction gratings in a highly efficient and compact layout, while providing important spectroscopic diagnostics for the characterisation of solar coronal and flare plasmas. SISA’s characteristics, main challenges, and the on-going activities to enable the image slicer technology for EUV applications are presented in this paper.

HXI on ASO-S and STIX onboard Solar Orbiter are the first simultaneously operating solar hard X-ray imaging spectrometers. ASO-S’s low Earth orbit and Solar Orbiter’s periodic displacement from the Sun–Earth line enables multi-viewpoint solar hard X-ray spectroscopic imaging analysis for the first time. Here, we demonstrate the potential of this new capability by reporting the first results of 3D triangulation of hard X-ray sources in the SOL2023-12-31T21:55 X5 flare. HXI and STIX observed the flare near the east limb with an observer separation angle of 18°. We triangulated the brightest regions within each source, which enabled us to characterise the large-scale hard X-ray geometry of the flare. The footpoints were found to be in the chromosphere within uncertainty, as expected, while the thermal looptop source was centred at an altitude of 15.1 ± 1 Mm. Given the footpoint separation, this implies a more elongated magnetic-loop structure than predicted by a semi-circular model. These results show the strong diagnostic power of joint HXI and STIX observations for understanding the 3D geometry of solar flares. We conclude by discussing the next steps required to fully exploit their potential.

Solar Orbiter's four in situ instruments have recorded numerous energetic electron events at heliocentric distances between 0.5 and 1 au. We analyze energetic electron fluxes, spectra, pitch-angle distributions, associated Langmuir waves, and type III solar radio bursts for three events to understand what causes modifications in the electron flux and identify the origin and characteristics of features observed in the electron spectrum. We investigate what electron beam properties and solar wind conditions are associated with Langmuir wave growth and spectral breaks in the electron peak flux as a function of energy. We observe velocity dispersion and quasilinear relaxation in the electron flux caused by the resonant wave–particle interactions in the deca-keV range, at the energies at which we observe breaks in the electron spectrum, cotemporal with the local generation of Langmuir waves. We show, via the evolution of the electron flux at the time of the event, that these interactions are responsible for the spectral signatures observed around 10 and 50 keV, confirming the results of simulations by Kontar and Reid. These signatures are independent of pitch-angle scattering. Our findings highlight the importance of using overlapping FOVs when working with data from different sensors. In this work, we exploit observations from all in situ instruments to address, for the first time, how the energetic electron flux is modified by the beam–plasma interactions and results in specific feature appearing in the local spectrum. Our results, corroborated with numerical simulations, can be extended to a wider range of heliocentric distances.

The Spectrometer and Telescope for Imaging X-rays (STIX) on board Solar Orbiter observes solar X-ray emission in the range of 4 – 150 keV and produces spectra and images of solar flares over a wide range of flare magnitudes. During nominal operation, STIX continuously generates data. A constant data flow requires fully automated data-processing pipelines to process and analyze the data, and a data platform to manage, visualize, and distribute the data products to the scientific community. The STIX Data Center has been built to fulfill these needs. In this paper, we outline its main components to help the community better understand the tools and data it provides. The STIX Data Center is operated at the University of Applied Sciences and Arts Northwestern Switzerland (FHNW) and consists of automated processing pipelines and a data platform. The pipelines process STIX telemetry data, perform common analysis tasks, and generate data products at different processing levels. They have been designed to operate fully automatically with minimal human intervention. The data platform provides web-based user interfaces and application programmable interfaces for searching and downloading STIX data products. The STIX Data Center has been operating successfully for more than two years. The platform facilitates instrument operations and provides vital support to STIX data users.

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Particle acceleration is a fundamental process arising in many astrophysical objects, including active galactic nuclei, black holes, neutron stars, gamma-ray bursts, accretion disks, solar and stellar coronae, and planetary magnetospheres. Its ubiquity means energetic particles permeate the Universe and influence the conditions for the emergence and continuation of life. In our solar system, the Sun is the most energetic particle accelerator, and its proximity makes it a unique laboratory in which to explore astrophysical particle acceleration. However, despite its importance, the physics underlying solar particle acceleration remain poorly understood. The SPARK mission will reveal new discoveries about particle acceleration through a uniquely powerful and complete combination of γ-ray, X-ray, and EUV imaging and spectroscopy at high spectral, spatial, and temporal resolutions. SPARK’s instruments will provide a step change in observational capability, enabling fundamental breakthroughs in our understanding of solar particle acceleration and the phenomena associated with it, such as the evolution of solar eruptive events. By providing essential diagnostics of the processes that drive the onset and evolution of solar flares and coronal mass ejections, SPARK will elucidate the underlying physics of space weather events that can damage satellites and power grids, disrupt telecommunications and GPS navigation, and endanger astronauts in space. The prediction of such events and the mitigation of their potential impacts are crucial in protecting our terrestrial and space-based infrastructure.

We provide a mathematical description of the imaging concept of the Spectrometer/Telescope for Imaging X-rays (STIX) onboard Solar Orbiter . Specifically, we describe the STIX indirect-imaging technique, which is based on spatial modulation of the X-ray photon flux by means of tungsten grids, and we show that each of 30 STIX imaging sub-collimators measures a Fourier component of the flaring X-ray source corresponding to a specific two-dimensional angular frequency. We also provide details about the count-distribution model, which describes the relationship between the photon flux and the measured pixel counts. The derived imaging model is the fundamental starting point both for the interpretation of STIX data and for the description of the data-calibration process. Finally, we provide an overview of the algorithms implemented for the solution of the imaging problem and a comparison of the results obtained with these different methods in the case of the SOL2022-03-31T18 flaring event.

The NDCube 2 API is a Python application programming interface (API) for storing and manipulating N-dimensional coordinate-aware astronomical data. While there are Python packages for handling astronomical data and coordinate transformations separately and for handling specific combinations of dimensions and transformations, none provide a unified and agnostic way of handling them simultaneously. This leads to a proliferation of different APIs for conducting the same analysis tasks on similar types of observations and introduces technical barriers between multi-instrument studies and cross-community collaboration. In this paper, we outline how the NDCube 2 API and its implementation in the open-source, community-developed ndcube package, together with the AstroPy WCS API, help to solve this problem. We discuss the guiding principles underpinning the API design and provide examples of how it is already being used to serve broad sections of the astronomy community, including agency-funded missions. The aim of this paper is to help users better understand the purpose and potential of the NDCube 2 API and ndcube package and hence how to more effectively deploy them in scientific analyses and software development.