arXiv subjects

Andrea Francesco Battaglia

Publications and source records attributed to Andrea Francesco Battaglia.

17 recordsLinked to original sources

arXiv2026

Solar Orbiter observations of solar energetic electron events associated with hard microflares

Generally, large solar flares accelerate electrons to high energies more efficiently than microflares. However, some microflares, known as hard microflares (HMFs), also produce high-energy electrons, as indicated by their flat hard X-ray (HXR) spectra. These events are typically associated with footpoints located in or at the edge of sunspots. The mechanisms behind this efficient acceleration, and their connection to solar energetic electrons (SEEs), remain unclear. We compare, for the first time, HXR spectra of HMFs with in-situ electron spectra of associated SEEs using Solar Orbiter STIX and EPD observations. This provides insight into acceleration processes and the transport of high-energy electrons into interplanetary space. We identify eight HMFs observed jointly by Solar Orbiter and Earth-based instruments that are associated with SEEs, confirmed through timing and magnetic connectivity analysis. Each event is studied using HXR spectroscopy, SEE velocity-dispersion analysis, and in-situ electron spectral analysis. Seven of eight events show consistent timing between flare HXR emission and inferred electron injection, as well as good agreement with magnetic connectivity estimates. The known correlation between HXR photon and in-situ electron spectral indices extends to HMFs, which occupy the hard end of the distribution, even compared to larger flares. We conclude that HMFs produce prompt SEEs with hard spectra, demonstrating efficient electron acceleration without requiring large flare energy release. Their magnetic configuration, involving open field lines from the sunspot, suggests they may be an important contributor to filling the heliosphere with energetic particles.

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arXiv2026

ZIMPOL detects scattering polarization in He I D$_3$ during a solar flare

Spectropolarimetric observations of solar flares in the He I D$_3$ line at 5876 Å are extremely rare, and their diagnostic potential remains largely unexplored. We report the first unambiguous detection of linear polarization in He I D$_3$ during a solar flare. Using the high-precision ZIMPOL polarimeter at the IRSOL observatory in Locarno (Switzerland), we tracked the temporal evolution of the He I D$_3$ Stokes profiles throughout the M7 GOES-class flare that occurred on 3 May 2023 at 10:45 UT. We analyzed the time evolution of the maximum in linear polarization and the absorption depth of the intensity profile. Both the fractional linear polarization, which peaks at $6\times10^{-4}$, and the absorption depth increase rapidly before gradually decaying, with their maxima occurring approximately 5 minutes after the peaks in GOES X-ray flux and SDO/AIA 304 Å emission. From the evolving He I D$_3$ core position, we also derived the temporal evolution of the plasma bulk velocities. The intensity profiles exhibiting strong absorption seems to originate from the flare ribbons. The time evolution of all Stokes parameters in the 3 May 2023 event was driven by changes in ortho-helium density prompted by the different phases of the flare. Our analysis suggests that the observed He I D$_3$ linear polarization is likely not dominated by the theorized impact polarization, as it exhibits neither spatial correspondence with electron precipitation sites nor temporal synchronization with the impulsive phase. Instead, the signals are consistent with scattering polarization produced by anisotropic radiation pumping. We conclude that scattering polarization signal on the order of $0.01\%$ can be produced in the He I D$_3$ line during solar flares. This can provide constraints for flare models.

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arXiv2025

Unveiling Spatiotemporal Properties of the Quasi-periodic Pulsations in the Balmer Continuum at 3600 Å in an X-class Solar White-light Flare

Quasi-periodic pulsations (QPPs) in the Balmer continuum of solar white-light flares (WLFs) are rarely reported, and accurately pinpointing the spatial source of flaring QPPs remains a significant challenge. We present spatiotemporal characteristics of QPPs of an X2.8 two-ribbon solar WLF (SOL2023-12-14T17:02), which was well observed by the White-light Solar Telescope (WST) aboard the Advanced Space-based Solar Observatory, with high-cadence imaging (1--2 s) in the Balmer continuum at 3600 Å. Combined with additional multi-instrument data, we find that the enhancement of the WLF in both Balmer and Paschen continua shows strong spatiotemporal correlation with hard X-ray (HXR) emissions. Notably, the pulses in the WST Balmer continuum exhibited a near-zero time lag with most HXR pulses, whereas soft X-ray and extreme ultraviolet emissions showed a lag of 2--3 s. Interestingly, quasi-harmonic QPPs with periods of $\sim$11 s and $\sim$20 s were observed in multiple wavelengths in the rising phase of the white-light continuum. Furthermore, we employed Fourier transform to spatially locate the QPPs around 11 and 20 s, revealing that they primarily originated from the east flare ribbon, which exhibited the most substantial continuum enhancement. More interestingly, we find that the west ribbon contributed significantly to the 11-second QPP but had a weaker contribution to the 20-second QPP. Moreover, the occurrence of quasi-harmonic QPPs is temporally coincident with the rapid elongation and separation motions of flare ribbons. Possible mechanisms for the quasi-harmonic QPPs have been discussed. These observations provide valuable insights into QPP modeling for solar and stellar flares.

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arXiv2025

New insights into the proton precipitation sites in solar flares

The Reuven Ramaty High Energy Solar Spectrocopy Imager (RHESSI) $γ$-ray observations of the extraordinary GOES X25 flare SOL2003-10-28T11:10 are revisited to investigate previously reported conclusions that flare-accelerated electrons and protons precipitate along spatially separated flare loops. In contrast to previous works which reconstructed 2.223 MeV images over extended time periods ($\sim$20 minutes), we selected shorter integration times of the order of 2 to 3 minutes. Using simulations of the 2.223 MeV profile in combination with observations of the prompt $γ$-ray lines from the INTEGRAL mission, we obtain two separated integration time ranges representing the peak of the flare and the start of the decay, respectively. The resulting $γ$-ray images are then compared to GONG white-light (WL) observations to identify where along the flaring ribbons electrons and protons precipitation occurs. We point out that previously reported results comparing RHESSI hard X-ray (HXR) and $γ$-ray images only hold if the relative time evolution in the two energy ranges is the same. As the decay times for the 28 October 2003 is different at the considered two energy ranges (200-300 keV and around 2.223 MeV), the previously published conclusion that electrons and protons precipitate at different locations is an overstatement. Using shorter integration times reveals that the $γ$-ray and HXR sources spatially coincide with the WL flare ribbons. Our key conclusion is that electron and proton precipitation sites coincide with the flare ribbons, suggesting that the electron and proton precipitation sites are the same, at least within RHESSI's imaging capabilities. This result solves the twenty-years-long mystery around the previously reported different electron and proton precipitation sites.

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arXiv2024

The observational evidence that all microflares that accelerate electrons to high energies are rooted in sunspots

In general, large solar flares are more efficient at accelerating high-energy electrons than microflares. Nonetheless, we sometimes observe microflares that accelerate electrons to high energies. We statistically characterize 39 microflares with strikingly hard spectra in the hard X-ray (HXR) range, which means that they are efficient in accelerating high-energy electrons. We refer to these events as "hard microflares." The statistical analysis is built upon spectral and imaging information from STIX, combined with EUV and magnetic field maps from SDO. The key observational result is that all hard microflares in this dataset have one of the footpoints 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 10 times weaker. Approximately 78% of 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. 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. 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.

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arXiv2024

Efficiency of solar microflares in accelerating electrons when rooted in a sunspot

We investigate two microflares of GOES classes A9 and C1 (after background subtraction) observed by STIX onboard Solar Orbiter with exceptionally strong nonthermal emission. We complement the hard X-ray imaging and spectral analysis by STIX with co-temporal observations in the (E)UV and visual range by AIA and HMI, in order to investigate what makes these microflares so sufficient in high-energy particle acceleration. We performed case studies of two microflares observed by STIX on October 11, 2021 and November 10, 2022 that showed unusually hard microflare X-ray spectra with power-law indices of the electron flux distributions delta = 2.98 and delta = 4.08 during their nonthermal peaks and photon energies up to 76 keV and 50 keV respectively. For both events under study, we found that one footpoint is located within a sunspot covering areas with mean magnetic flux densities in excess of 1500 G, suggesting that the hard electron spectra are caused by the strong magnetic fields in which the flare loops are rooted. In addition, we revisited the unusually hard RHESSI microflare initially published by Hannah et al. (2008b) and found that in this event also one flare kernel was located within a sunspot, which corroborates the result from the two hard STIX microflares under study. We conclude that the characteristics of the strong photospheric magnetic fields inside sunspot umbrae and penumbrae where the flare loops are rooted play an important role in the generation of exceptionally hard X-ray spectra in these microflares.

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arXiv2024

The White-light Emissions in Two X-class Flares Observed by ASO-S and CHASE

The white-light continuum emissions in solar flares (i.e., white-light flares) are usually observed on the solar disk but, in a few cases, off the limb. Here we present on-disk as well as off-limb continuum emissions at 3600 Å (in the Balmer continuum) in an X2.1 flare (SOL2023-03-03T17:52) and an X1.5 flare (SOL2023-08-07T20:46), respectively, observed by the White-light Solar Telescope (WST) on the Advanced Space-based Solar Observatory (ASO-S). These continuum emissions are seen at the ribbons for the X2.1 flare and on loops during the X1.5 event, in which the latter also appears in the decay phase. These emissions also show up in the pseudo-continuum images at Fe I λ6173 from the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO). In addition, the ribbon sources in the X2.1 flare exhibit significant enhancements in the Fe I line at 6569.2 Å and the nearby continuum observed by the Chinese Hα Solar Explorer (CHASE). It is found that the on-disk continuum emissions in the X2.1 flare are related to a nonthermal electron-beam heating either directly or indirectly, while the off-limb emissions in the X1.5 flare are associated with thermal plasma cooling or due to Thomson scattering. These comprehensive continuum observations can provide good constraints on flare energy deposition models, which helps well understand the physical mechanism of white-light flares.

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arXiv2023

The existence of hot X-ray onsets in solar flares

It is well known among the scientific community that solar flare activity often begins well before the main impulsive energy release. Our aim is to investigate the earliest phase of four distinct flares observed by Solar Orbiter/STIX and determine the relationships of the newly heated plasma to flare structure and dynamics. The analysis focuses on four events that were observed from both Earth and Solar Orbiter, which allows for a comparison of STIX observations with those of GOES/XRS and SDO/AIA. The early phases of the events were studied using STIX and GOES spectroscopic analysis to investigate the evolution of the physical parameters of the plasma, including the isothermal temperature and emission measure. Furthermore, to determine the location of the heated plasma, STIX observations were combined with AIA images. The events with clear emission prior to the impulsive phase show elevated temperatures ($> 10\,\mathrm{MK}$) from the very beginning, which indicates that energy release started before any detection by STIX. Although the temperature shows little variation during the initial phase, the emission measure increases by about two orders of magnitude, implying a series of incrementally greater energy releases. The spectral analysis of STIX and GOES from the very first time bins suggests that the emission has a multi-thermal nature, with a hot component of more than $10\,\mathrm{MK}$. This analysis confirms the existence of "hot onsets," with STIX detecting the hot onset pattern even earlier than GOES. These elevated temperatures imply that energy release actually begins well before any detection by STIX. Therefore, hot onsets may be significant in the initiation, early development, or even prediction of solar flares.

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arXiv2023

Multi-point study of the energy release and transport in the 28 March 2022, M4-flare using STIX, EUI, and AIA during the first Solar Orbiter nominal mission perihelion

We present a case study of an M4-class flare on 28 March 2022, near Solar Orbiter's first science perihelion (0.33 AU). Solar Orbiter was 83.5° west of the Sun-Earth line, making the event appear near the eastern limb, while Earth-orbiting spacecraft observed it near the disk center. The timing and location of the STIX X-ray sources were related to the plasma evolution observed in the EUV by the Extreme Ultraviolet Imager (EUI) on Solar Orbiter and the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory, and to the chromospheric response observed in 1600 Å by AIA. We performed differential emission measure (DEM) analysis to further characterize the flaring plasma at different subvolumes. The pre-flare magnetic field configuration was analyzed using a nonlinear force-free (NLFF) extrapolation. In addition to the two classical hard X-ray (HXR) footpoints at the ends of the flaring loops, later in the event we observe a nonthermal HXR source at one of the anchor points of the erupting filament. The evolution of the AIA 1600~Å footpoints indicates that this change in footpoint location represents a discontinuity in an otherwise continuous westward motion of the footpoints throughout the flare. The NLFF extrapolation suggests that strongly sheared field lines close to, or possibly even part of, the erupting filament reconnected with a weakly sheared arcade during the first HXR peak. The remainder of these field lines reconnected later in the event, producing the HXR peak at the southern filament footpoint. Our results show that the reconnection between field lines with very different shear in the early phase of the flare plays a crucial role in understanding the motion of the HXR footpoint during later parts of the flare. This generalizes simpler models, such as whipping reconnection, which only consider reconnection propagating along uniformly sheared arcades.

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arXiv2023

The data center for the Spectrometer and Telescope for Imaging X-rays (STIX) onboard Solar Orbiter

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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arXiv2023

STIX imaging I -- Concept

Aims. To provide a schematic mathematical description of the imaging concept of the Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter. The derived model is the fundamental starting point for both the interpretation of STIX data and the description of the data calibration process. Methods. We describe the STIX indirect imaging technique which is based on spatial modulation of the X-ray photon flux by means of tungsten grids. We show that each of 30 STIX imaging sub-collimators measures a complex Fourier component of the flaring X-ray source corresponding to a specific angular frequency. We also provide details about the count distribution model, which describes the relationship between the photon flux and the measured pixel counts. Results. We define the image reconstruction problem for STIX from both visibilities and photon counts. 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.

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arXiv2022

Identifying the energy release site in a Solar microflare with a jet

One of the main science questions of the Solar Orbiter and Parker Solar Probe missions deals with understanding how electrons in the lower solar corona are accelerated and how they subsequently access interplanetary space. We aim to investigate the electron acceleration and energy release sites as well as the manner in which accelerated electrons access the interplanetary space in the case of the SOL2021-02-18T18:05 event, a GOES A8 class microflare associated with a coronal jet. This study takes advantage of three different vantage points, Solar Orbiter, STEREO-A, and Earth, with observations ranging from radio to X-ray. Multi-wavelength timing analysis combined with UV/EUV imagery and X-ray spectroscopy by Solar Orbiter/STIX (Spectrometer/Telescope for Imaging X-rays) is used to investigate the origin of the observed emission during different flare phases. The event under investigation satisfies the classical picture of the onset time of the acceleration of electrons coinciding with the jet and the radio type III bursts. This microflare features prominent hard X-ray nonthermal emission down to at least 10 keV and a spectrum that is much harder than usual for a microflare with a spectral index of 2.9. From Earth's vantage point, the microflare is seen near the limb, revealing the coronal energy release site above the flare loop in EUV, which, from STIX spectroscopic analysis, turns out to be hot (at roughly the same temperature of the flare). Moreover, this region is moving toward higher altitudes over time (about 30 km/s). During the flare, the same region spatially coincides with the origin of the coronal jet. We conclude that the energy release site observed above-the-loop corresponds to the electron acceleration site, corroborating that interchange reconnection is a viable candidate for particle acceleration in the low corona on field lines open to interplanetary space.

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arXiv2022

Solar flare hard X-rays from the anchor points of an eruptive filament

Context. We present an analysis of a GOES M1.8 flare with excellent observational coverage in UV, EUV, and X-ray, including observations from the instruments IRIS, SDO with AIA, Hinode/EIS, Hinode/XRT, and Solar Orbiter with the Spectrometer/Telescope for Imaging X-rays (STIX). Hard X-ray emission is often observed at the footpoints of flare loops and is occasionally observed in the corona. In this flare, four nonthermal hard X-ray sources are seen. Aim. Our aim is to understand why we can observe four individual nonthermal sources in this flare and how we can characterize the physical properties of these four sources. Methods. We used the multiwavelength approach to analyze the flare and characterize the four sources. To do this, we combined imaging at different wavelengths and spectroscopic fitting in the EUV and X-ray range. Results. The flare is eruptive with an associated coronal mass ejection, and it shows the classical flare picture of a heated flare loop seen in EUV and X-rays, and two nonthermal hard X-ray footpoints at the loop ends. In addition to the main flare sources, we observed two outer sources in the UV, EUV, and nonthermal X-ray range located away from the main flare loop to the east and west. The two outer sources are clearly correlated in time, and they are only seen during the first two minutes of the impulsive phase, which lasts a total of about four minutes. Conclusions. Based on the analysis, we determine that the outer sources are the anchor points of an erupting filament. The hard X-ray emission is interpreted as flare-accelerated electrons that are injected upward into the filament and then precipitate along the filament toward the chromosphere, producing Bremsstrahlung. While sources like this have been speculated to exist, this is the first report of their detection.

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arXiv2022

The Lyman-$α$ Emission in a C1.4 Solar Flare Observed by the Extreme Ultraviolet Imager aboard Solar Orbiter

The hydrogen Lyman-$α$ (H {\sc i} Ly$α$) emission during solar flares has rarely been studied in spatially resolved images and its physical origin has not been fully understood. In this paper, we present novel Ly$α$ images for a C1.4 solar flare (SOL2021-08-20T22:00) from the Extreme Ultraviolet Imager aboard Solar Orbiter, together with multi-waveband and multi-perspective observations from the Solar Terrestrial Relations Observatory Ahead and the Solar Dynamics Observatory spacecraft. It is found that the Ly$α$ emission has a good temporal correlation with the thermal emissions at 1--8 Å and 5--7 keV, indicating that the flaring Ly$α$ is mainly produced by a thermal process in this small event. However, nonthermal electrons play a minor role in generating Ly$α$ at flare ribbons during the rise phase of the flare, as revealed by the hard X-ray imaging and spectral fitting. Besides originating from flare ribbons, the Ly$α$ emission can come from flare loops, likely caused by plasma heating and also cooling that happen in different flare phases. It is also found that the Ly$α$ emission shows fairly similar features with the He {\sc ii} 304 Å emission in light curve and spatio-temporal variation along with small differences. These observational results improve our understanding of the Ly$α$ emission in solar flares and also provide some insights for investigating the Ly$α$ emission in stellar flares.

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arXiv2022

Forward-fitting STIX visibilities

Aima. To determine to what extent the problem of forward fitting visibilities measured by the Spectrometer/Telescope Imaging X-rays (STIX) on-board Solar Orbiter is more challenging with respect to the same problem in the case of previous hard X-ray solar imaging missions; to identify an effective optimization scheme for parametric imaging for STIX. Methods. This paper introduces a Particle Swarm Optimization (PSO) algorithm for forward fitting STIX visibilities and compares its effectiveness with respect to the standard simplex-based optimization algorithm used so far for the analysis of visibilities measured by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). This comparison is made by considering experimental visibilities measured by both RHESSI and STIX, and synthetic visibilities generated by accounting for the STIX signal formation model. Results. We found out that the parametric imaging approach based on PSO is as reliable as the one based on the simplex method in the case of RHESSI visibilities. However, PSO is significantly more robust when applied to STIX simulated and experimental visibilities. Conclusions. Standard deterministic optimization is not effective enough for forward-fitting the few visibilities sampled by STIX in the angular frequency plane. Therefore a more sofisticated optimization scheme must be introduced for parametric imaging in the case of the Solar Orbiter X-ray telescope. The forward-fitting routine based on PSO we introduced in this paper proved to be significantly robust and reliable, and could be considered as an effective candidate tool for parametric imaging in the STIX context.

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arXiv2021

STIX X-ray microflare observations during the Solar Orbiter commissioning phase

The Spectrometer/Telescope for Imaging X-rays (STIX) is the HXR instrument onboard Solar Orbiter designed to observe solar flares over a broad range of flare sizes, between 4-150 keV. We report the first STIX observations of microflares recorded during the instrument commissioning phase in order to investigate the STIX performance at its detection limit. This first result paper focuses on the temporal and spectral evolution of STIX microflares occuring in the AR12765 in June 2020, and compares the STIX measurements with GOES/XRS, SDO/AIA, and Hinode/XRT. For the observed microflares of the GOES A and B class, the STIX peak time at lowest energies is located in the impulsive phase of the flares, well before the GOES peak time. Such a behavior can either be explained by the higher sensitivity of STIX to higher temperatures compared to GOES, or due to the existence of a nonthermal component reaching down to low energies. The interpretation is inconclusive due to limited counting statistics for all but the largest flare in our sample. For this largest flare, the low-energy peak time is clearly due to thermal emission, and the nonthermal component seen at higher energies occurs even earlier. This suggests that the classic thermal explanation might also be favored for the majority of the smaller flares. In combination with EUV and SXR observations, STIX corroborates earlier findings that an isothermal assumption is of limited validity. Future diagnostic efforts should focus on multi-wavelength studies to derive differential emission measure distributions over a wide range of temperatures to accurately describe the energetics of solar flares. Commissioning observations confirm that STIX is working as designed. As a rule of thumb, STIX detects flares as small as the GOES A class. For flares above the GOES B class, detailed spectral and imaging analyses can be performed.

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arXiv2021

The Alfvénic nature of chromospheric swirls

We investigate the evolution and origin of small-scale chromospheric swirls by analyzing numerical simulations of the quiet solar atmosphere, using the radiative magnetohydrodynamic code CO$5$BOLD. We are interested in finding their relation with magnetic field perturbations and in the processes driving their evolution. For the analysis, the swirling strength criterion and its evolution equation are applied in order to identify vortical motions and to study their dynamics. We introduce a new criterion, the magnetic swirling strength, which allows us to recognize torsional perturbations in the magnetic field. We find a strong correlation between swirling strength and magnetic swirling strength, in particular in intense magnetic flux concentrations, which suggests a tight relation between vortical motions and torsional magnetic field perturbations. Furthermore, we find that swirls propagate upward with the local Alfvén speed as unidirectional swirls, in the form of pulses, driven by magnetic tension forces alone. In the photosphere and low chromosphere, the rotation of the plasma co-occurs with a twist in the upwardly directed magnetic field that is in the opposite direction of the plasma flow. All together, these are characteristics of torsional Alfvén waves. We also find indications of an imbalance between the hydrodynamic and magnetohydrodynamic baroclinic effects being at the origin of the swirls. At the base of the chromosphere, we find a net upwardly directed Poynting flux, which is mostly associated with large and complex swirling structures that we interpret as the superposition of various small-scale vortices. We conclude that the ubiquitous swirling events observed in simulations are tightly correlated with perturbations of the magnetic field. At photospheric and chromospheric levels, they form Alfvén pulses that propagate upward and may contribute to chromospheric heating.

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