arXiv subjects

Gherardo Valori

Publications and source records attributed to Gherardo Valori.

At least 19 recordsLinked to original sources

arXiv2026

A stray light analysis for SO/PHI-HRT and an updated comparison of the inferred magnetic field with SDO/HMI

Context. The High Resolution Telescope of the Polarimetric and Helioseismic Imager on Solar Orbiter (SO/PHI-HRT) operates in an extreme observational environment, observing the Sun as close as $0.28$ au. The high thermal load and large illuminating field puts high demands on the instrument in terms of both imaging performance and false light control. Aims. To characterise the amount of stray light (false light) within SO/PHI-HRT, apply a correction, and re-compare the data products with the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory (SDO/HMI). Methods. We analyse solar limb profiles and a Mercury transit to quantify the amount of stray light and add a correction term when partially reconstructing the SO/PHI-HRT images. For the comparison with SDO/HMI we use data from the 2023 March Solar Orbiter inferior conjunction and compare the magnetic fields on a pixel-by-pixel basis. Results. Increased continuum intensity contrast in the quiet Sun, and darker intensity levels are found in strong magnetic features. Consequently, much stronger fields are inferred in these features. Comparing the stray light corrected data with that from the standard SDO/HMI data products results in a much closer agreement across all vector magnetic field components, particularly when the cadence and noise levels are identical. In most solar features, SO/PHI-HRT infers stronger fields than the SDO/HMI line-of-sight magnetograms. Compared to the vector magnetic field from SDO/HMI the two are very well aligned, with only slight differences in the strongest field regions (where $|\mathbf{B}|>1600$ G or $|\mathbf{B_{\text{LOS}}}|>1300$ G).

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arXiv2026

The Non-Eruptive Reconfiguration of a Quiescent Filament After a Nearby Active Region Emergence

The unpredictability of solar filament eruptions presents major challenges for forecasting space weather, as such eruptions frequently drive coronal mass ejections (CMEs) that impact the heliosphere. While nearby flux emergence is often linked to their destabilisation, the specific characteristics of both the emerging flux and the filament that determine whether an eruption occurs remain unclear. We report observations of a quiescent filament that did not erupt following the nearby emergence of active region NOAA 13270 and a subsequent C-class flare in April 2023. Our analysis combines multi-viewpoint extreme ultraviolet (EUV) imaging and X-ray imaging with EUV spectroscopy, radio imaging and measurements of, and extrapolations from, the photospheric magnetic field. We identify the formation of a coronal null point and fan-spine topology at the interface between the active region and filament which exhibited persistent slow reconnection, indicated by chromospheric brightenings, persistent radio emission, and plasma upflows. Our results indicate that ongoing reconnection and jets can relieve magnetic stress and enable filament stability, even when under strong perturbation. We suggest that the orientation of emerging flux relative to the ambient field is a critical parameter in filament evolution, and provide observational constraints for models of filament stability and eruption.

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arXiv2025

3D structures of the base of small-scale recurrent jets revealed by Solar orbiter

Solar jets, characterized by small-scale plasma ejections along open magnetic field lines or the legs of large-scale coronal loops, play a crucial role in the dynamics of the solar atmosphere. Although spectral and EUV images have been widely used to analyze the formation and evolution of jets, the detailed 3D structure at the base of the jet has not been studied in detail due to the limitations in the spatial resolution of observations. Solar Orbiter enables us to investigate the structure of solar jets with much higher spatial and temporal resolutions and from a different angle than from Earth. By combining observations made by instruments onboard Solar Orbiter with data from the SDO, we analyzed recurrent solar jets originating in a mixed-polarity region near an active region. Additionally, we employed potential field and magnetohydrostatic extrapolation techniques to determine the magnetic field topology associated with the jets. The jets display dynamic, multi-strand outflows emanating from compact bright kernels above the magnetic inversion line, with apparent speeds exceeding 100 km/s. Magnetic field evolution reveals continuous flux cancellation at the jet footpoints. Throughout the sequence, base flows are confined within quasi-separatrix layers, with the highest velocities and temperatures located near coronal null points. Over four eruptions, the magnetic topology evolves from a simple fan-spine configuration with a single null to a more complex dome-shaped base containing multiple nulls with separatrix curtain, accompanied by a morphological transition from narrow, well-collimated spire to broader, fragmented outflows. These results provide the first direct observational evidence that dynamic changes in null-point geometry modulate jet morphology and energetics via successive reconnection episodes.

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arXiv2025

Solar Orbiter reveals persistent magnetic reconnection in medium-scale filament eruptions

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 reveal persistent magnetic reconnection events in a failed filament eruption. We identify magnetic reconnections between the filament and surrounding magnetic field structures, with both frequency and type far exceeding previous observations. 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.

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arXiv2025

Active region upflows in various coronal structures and their coupling to the lower atmosphere

Plasma upflows with a Doppler shift exceeding -10 km/s at active region (AR) boundaries are considered potential sources of the nascent slow solar wind. We investigate the driving mechanisms of a pair of coronal upflow regions on the western and eastern peripheries of an AR, which have different magnetic topologies and surroundings. It is aimed to explore how these upflows couple to the lower atmosphere. Using observations of the Fe XII 19.51 nm line from Hinode, we identified two upflow regions at the western and eastern boundaries of a decaying AR. Context images for the two regions were obtained by the High Resolution Imager (HRI) telescope of the Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter mission. Other instruments on Solar Orbiter and other observatories provide diagnostics to the lower atmosphere. Potential Field Source Surface (PFSS) extrapolations were used to examine the magnetic field configuration associated with the AR upflows. The eastern upflow region, located over the AR moss, displays small-scale dynamic fibril structures, whereas the western region hosts fan-like loops. We found blueshifted Ne VIII emission at the eastern site, in contrast to redshifted Ne VIII profiles in the west. Magnetic field extrapolations reveal a pseudostreamer topology connecting both these regions. Moreover, low transition-region lines show systematically reduced redshift below the eastern footpoint. The observations support the scenario in which both upflows are driven by pressure imbalances created by coronal reconnection, leading to a continuous upflow above approximately 0.6 MK (i.e., Ne VIII line formation temperature). Meanwhile, mass flows in the lower transition region beneath the eastern upflow region appear to respond passively to the pressure-driven coronal upflows.

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arXiv2025

First coordinated observations between Solar Orbiter and the Daniel K. Inouye Solar Telescope

Solar Orbiter and the Daniel K. Inouye Solar Telescope (DKIST) are two of the newest facilities available to the solar physics community. The first coordinated observations of the Sun by these two facilities occurred over the course of one week in October 2022. The returned data are open-access and will provide a valuable resource to researchers in the field. We provide an overview of the datasets collected by Solar Orbiter and DKIST through this coordination and discuss their scientific potential. Our aim is to demonstrate how these unique high-resolution coordinated observations, as well as similar observations obtained through subsequent campaigns, can help tackle important science questions in the field. Between 18 and 24 October 2022, Solar Orbiter and DKIST observed a decayed active region simultaneously. During this period, Solar Orbiter's separation angle with Earth decreased from 77° to 51°, enabling stereoscopic observations with DKIST. From Solar Orbiter, observations are provided by the Extreme Ultraviolet Imager (EUI), Polarimetric and Helioseismic Imager (PHI), and the Spectral Imaging of the Coronal Environment (SPICE) instruments. Meanwhile, DKIST observed using the Cryogenic Near Infrared Spectropolarimeter (CryoNIRSP), the Visible Broadband Imager (VBI), and the Visible Spectropolarimeter (ViSP). The first coordinated Solar Orbiter and DKIST campaign was a success, collecting observations multiple times over the week. We focus on three specific topics as representative examples, namely, coronal loop physics, the formation and evolution of the small-scale active region brightenings, and coronal rain dynamics. These open-access observations, and others like them, should help the solar physics community tackle key questions in the field. Such stereoscopic coordinated observations open up a new era in the analysis of the solar atmosphere.

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arXiv2024

Observation of Alfvén Wave Reflection in the Solar Chromosphere: Ponderomotive Force and First Ionization Potential Effect

We investigate the propagation of Alfvén waves in the solar chromosphere, distinguishing between upward and downward propagating waves. We find clear evidence for the reflection of waves in the chromosphere and differences in propagation between cases with waves interpreted to be resonant or nonresonant with the overlying coronal structures. This establishes the wave connection to coronal element abundance anomalies through the action of the wave ponderomotive force on the chromospheric plasma, which interacts with chromospheric ions but not neutrals, thereby providing a novel mechanism of ion-neutral separation. This is seen as a "First Ionization Potential Effect" when this plasma is lifted into the corona, with implications elsewhere on the Sun for the origin of the slow speed solar wind and its elemental composition.

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arXiv2023

Understanding the Relationship between Solar Coronal Abundances and F10.7 cm Radio Emission

Sun-as-a-star coronal plasma composition, derived from full-Sun spectra, and the F10.7 radio flux (2.8 GHz) have been shown to be highly correlated (r = 0.88) during solar cycle 24. However, this correlation becomes nonlinear during increased solar magnetic activity. Here, we use co-temporal, high spatial resolution, multi-wavelength images of the Sun to investigate the underlying causes of the non-linearity between coronal composition (FIP bias) and F10.7 solar index correlation. Using the Karl G. Jansky Very Large Array (JVLA), Hinode/EIS (EUV Imaging Spectrometer), and the Solar Dynamic Observatory (SDO), we observed a small active region, AR 12759, throughout the solar atmosphere from the photosphere to the corona. Results of this study show that the magnetic field strength (flux density) in active regions plays an important role in the variability of coronal abundances, and it is likely the main contributing factor to this non-linearity during increased solar activity. Coronal abundances above cool sunspots are lower than in dispersed magnetic plage regions. Strong magnetic concentrations are associated with stronger F10.7 cm gyroresonance emission. Considering that as the solar cycle moves from minimum to maximum, the size of sunspots and their field strength increase with gyroresonance component, the distinctly different tendencies of radio emission and coronal abundances in the vicinity of sunspots is the likely cause of saturation of Sun-as-a-star coronal abundances during solar maximum, while the F10.7 index remains well correlated with the sunspot number and other magnetic field proxies.

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arXiv2022

Changes of Magnetic Energy and Helicity in Solar Active Regions from Major Flares

Magnetic free energy powers solar flares and coronal mass ejections (CMEs), and the buildup of magnetic helicity might play a role in the development of unstable structures that subsequently erupt. To better understand the roles of energy and helicity in large flares and eruptions, we have characterized the evolution of magnetic energy and helicity associated with 21 X-class flares from 2010 to 2017. Our sample includes both confined and eruptive events, with 6 and 15 in each category, respectively. Using HMI vector magnetic field observations from several hours before to several hours after each event, we employ (a) the Differential Affine Velocity Estimator for Vector Magnetograms (DAVE4VM) to determine the photospheric fluxes of energy and helicity, and (b) non-linear force-free field (NLFFF) extrapolations to estimate the coronal content of energy and helicity in source-region fields. Using Superposed Epoch analysis (SPE), we find, on average: (1) decreases in both magnetic energy and helicity, in both photospheric fluxes and coronal content, that persist for a few hours after eruptions, but no clear changes, notably in relative helicity, for confined events; (2) significant increases in the twist of photospheric fields in eruptive events, with twist uncertainties too large in confined events to constrain twist changes (and lower overall twist in confined events); and (3) on longer time scales (event time +12 hours), replenishment of free magnetic energy and helicity content to near pre-event levels for eruptive events. For eruptive events, magnetic helicity and free energy in coronal models clearly decrease after flares, with the amounts of decrease proportional to each region's pre-flare content.

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arXiv2021

Disambiguation of Vector Magnetograms by Stereoscopic Observations from the Solar Orbiter/Polarimetric and Helioseismic Imager (PHI) and the Solar Dynamic Observatory (SDO)/Helioseismic and Magnetic Imager (HMI)

Spectropolarimetric reconstructions of the photospheric vector magnetic field are intrinsically limited by the so-called 180$^\circ$ ambiguity in the orientation of the transverse component. The successful launch and operation of Solar Orbiter has made the removal of the 180$^\circ$ ambiguity possible using solely observations obtained from two different vantage points. While the exploitation of such a possibility is straightforward in principle, it is less so in practice and it is therefore important to assess the accuracy and limitations, as a function of both the satellites orbits and measurement principles. In this work we present a stereoscopic disambiguation method (SDM) and discuss a thorough testing of its accuracy in applications to modeled active regions and quiet Sun observations. In a first series of tests, we employ magnetograms extracted from three different numerical simulations as test fields, and model observations of the magnetograms from different angles and distances. In these more idealized tests, the SDM is proven to to reach a 100% disambiguation accuracy when applied to moderately-to-well resolved fields. Even in the case of disambiguation of quiet Sun magnetograms with significant under-resolved scale, the SDM provides an accuracy between 82% and 98% depending on the field strength. The accuracy of the SDM is found to be mostly sensitive to the variable resolution of Solar Orbiter on its highly elliptic orbit, as well as to the intrinsic scale of the observed field. Finally, as a more realistic test, we consider magnetograms that are obtained using a radiative transfer inversion code and the SOPHISM instrument simulator applied to a 3D simulation of a pore, and present a preliminary discussion of the effect of the viewing angle on the observed field.

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arXiv2021

Evolution of Plasma Composition in an Eruptive Flux Rope

Magnetic flux ropes are bundles of twisted magnetic field enveloping a central axis. They harbor free magnetic energy and can be progenitors of coronal mass ejections (CMEs), but identifying flux ropes on the Sun can be challenging. One of the key coronal observables that has been shown to indicate the presence of a flux rope is a peculiar bright coronal structure called a sigmoid. In this work, we show Hinode EUV Imaging Spectrometer (EIS) observations of sigmoidal active region 10977. We analyze the coronal plasma composition in the active region and its evolution as the sigmoid (flux rope) forms and erupts as a CME. Plasma with photospheric composition was observed in coronal loops close to the main polarity inversion line during episodes of significant flux cancellation, suggestive of the injection of photospheric plasma into these loops driven by photospheric flux cancellation. Concurrently, the increasingly sheared core field contained plasma with coronal composition. As flux cancellation decreased and the sigmoid/flux rope formed, the plasma evolved to an intermediate composition in between photospheric and typical active region coronal compositions. Finally, the flux rope contained predominantly photospheric plasma during and after a failed eruption preceding the CME. The Hence, plasma composition observations of active region 10977 strongly support models of flux rope formation by photospheric flux cancellation forcing magnetic reconnection first at the photospheric level then at the coronal level.

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arXiv2021

Localised acceleration of energetic particles by a weak shock in the solar corona

Globally-propagating shocks in the solar corona have long been studied to quantify their involvement in the acceleration of energetic particles. However, this work has tended to focus on large events associated with strong solar flares and fast coronal mass ejections (CMEs), where the waves are sufficiently fast to easily accelerate particles to high energies. Here we present observations of particle acceleration associated with a global wave event which occurred on 1 October 2011. Using differential emission measure analysis, the global shock wave was found to be incredibly weak, with an Alfvén Mach number of ~1.008-1.013. Despite this, spatially-resolved type III radio emission was observed by the Nançay RadioHeliograph at distinct locations near the shock front, suggesting localised acceleration of energetic electrons. Further investigation using a magnetic field extrapolation identified a fan structure beneath a magnetic null located above the source active region, with the erupting CME contained within this topological feature. We propose that a reconfiguration of the coronal magnetic field driven by the erupting CME enabled the weak shock to accelerate particles along field lines initially contained within the fan and subsequently opened into the heliosphere, producing the observed type III emission. These results suggest that even weak global shocks in the solar corona can accelerate energetic particles via reconfiguration of the surrounding magnetic field.

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arXiv2021

Plasma Upflows Induced by Magnetic Reconnection Above an Eruptive Flux Rope

One of the major discoveries of Hinode's Extreme-ultraviolet Imaging Spectrometer (EIS) is the presence of upflows at the edges of active regions. As active regions are magnetically connected to the large-scale field of the corona, these upflows are a likely contributor to the global mass cycle in the corona. Here we examine the driving mechanism(s) of the very strong upflows with velocities in excess of 70 km/s, known as blue-wing asymmetries, observed during the eruption of a flux rope in AR 10977 (eruptive flare SOL2007-12-07T04:50). We use Hinode/EIS spectroscopic observations combined with magnetic-field modeling to investigate the possible link between the magnetic topology of the active region and the strong upflows. A Potential Field Source Surface (PFSS) extrapolation of the large-scale field shows a quadrupolar configuration with a separator lying above the flux rope. Field lines formed by induced reconnection along the separator before and during the flux-rope eruption are spatially linked to the strongest blue-wing asymmetries in the upflow regions. The flows are driven by the pressure gradient created when the dense and hot arcade loops of the active region reconnect with the extended and tenuous loops overlying it. In view of the fact that separator reconnection is a specific form of the more general quasi-separatrix (QSL) reconnection, we conclude that the mechanism driving the strongest upflows is, in fact, the same as the one driving the persistent upflows of approx. 10 - 20 km/s observed in all active regions.

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arXiv2021

Matching temporal signatures of solar features to their corresponding solar wind outflows

The role of small-scale coronal eruptive phenomena in the generation and heating of the solar wind remains an open question. Here, we investigate the role played by coronal jets in forming the solar wind by testing whether temporal variations associated with jetting in EUV intensity can be identified in the outflowing solar wind plasma. This type of comparison is challenging due to inherent differences between remote-sensing observations of the source and in situ observations of the outflowing plasma, as well as travel time and evolution of the solar wind throughout the heliosphere. To overcome these, we propose a novel algorithm combining signal filtering, two-step solar wind ballistic backmapping, window shifting, and Empirical Mode Decomposition. We first validate the method using synthetic data, before applying it to measurements from the Solar Dynamics Observatory, and Wind spacecraft. The algorithm enables the direct comparison of remote sensing observations of eruptive phenomena in the corona to in situ measurements of solar wind parameters, among other potential uses. After application to these datasets, we find several time windows where signatures of dynamics found in the corona are embedded in the solar wind stream, at a time significantly earlier than expected from simple ballistic backmapping, with the best performing in situ parameter being the solar wind mass flux.

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arXiv2021

The Evolution of Plasma Composition During a Solar Flare

We analyse the coronal elemental abundances during a small flare using Hinode/EIS observations. Compared to the pre-flare elemental abundances, we observed a strong increase in coronal abundance of Ca XIV 193.84 Å, an emission line with low first ionisation potential (FIP < 10 eV), as quantified by the ratio Ca/Ar during the flare. This is in contrast to the unchanged abundance ratio observed using Si X 258.38 Å/S X 264.23 Å. We propose two different mechanisms to explain the different composition results. Firstly, the small flare-induced heating could have ionised S, but not the noble gas Ar, so that the flare-driven Alfvén waves brought up Si, S and Ca in tandem via the ponderomotive force which acts on ions. Secondly, the location of the flare in strong magnetic fields between two sunspots may suggest fractionation occurred in the low chromosphere, where the background gas is neutral H. In this region, high-FIP S could behave more like a low-FIP than a high-FIP element. The physical interpretations proposed generate new insights into the evolution of plasma abundances in the solar atmosphere during flaring, and suggests that current models must be updated to reflect dynamic rather than just static scenarios.

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arXiv2020

A new trigger mechanism for coronal mass ejections: the role of confined flares and photospheric motions in the formation of hot flux ropes

Context: Many previous studies have shown that the magnetic precursor of a coronal mass ejection (CME) takes the form of a magnetic flux rope, and a subset of them have become known as `hot flux ropes' due to their emission signatures in $\sim$10 MK plasma. Aims: We seek to identify the processes by which these hot flux ropes form, with a view of developing our understanding of CMEs and thereby improving space weather forecasts. Methods: Extreme-ultraviolet observations were used to identify five pre-eruptive hot flux ropes in the solar corona and study how they evolved. Confined flares were observed in the hours and days before each flux rope erupted, and these were used as indicators of episodic bursts of magnetic reconnection by which each flux rope formed. The evolution of the photospheric magnetic field was observed during each formation period to identify the process(es) that enabled magnetic reconnection to occur in the $β<1$ corona and form the flux ropes. Results: The confined flares were found to be homologous events and suggest flux rope formation times that range from 18 hours to 5 days. Throughout these periods, fragments of photospheric magnetic flux were observed to orbit around each other in sunspots where the flux ropes had a footpoint. Active regions with right-handed (left-handed) twisted magnetic flux exhibited clockwise (anticlockwise) orbiting motions, and right-handed (left-handed) flux ropes formed. Conclusions: We infer that the orbital motions of photospheric magnetic flux fragments about each other bring magnetic flux tubes together in the corona, enabling component reconnection that forms a magnetic flux rope above a flaring arcade. This represents a novel trigger mechanism for solar eruptions and should be considered when predicting solar magnetic activity.

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arXiv2020

The Magnetic Environment of a Stealth Coronal Mass Ejection

Interest in stealth coronal mass ejections (CMEs) is increasing due to their relatively high occurrence rate and space weather impact. However, typical CME signatures such as extreme-ultraviolet dimmings and post-eruptive arcades are hard to identify and require extensive image processing techniques. These weak observational signatures mean that little is currently understood about the physics of these events. We present an extensive study of the magnetic field configuration in which the stealth CME of 3 March 2011 occurred. Three distinct episodes of flare ribbon formation are observed in the stealth CME source active region (AR). Two occurred prior to the eruption and suggest the occurrence of magnetic reconnection that builds the structure which will become eruptive. The third occurs in a time close to the eruption of a cavity that is observed in STEREO-B 171A data; this subsequently becomes part of the propagating CME observed in coronagraph data. We use both local (Cartesian) and global (spherical) models of the coronal magnetic field, which are complemented and verified by the observational analysis. We find evidence of a coronal null point, with field lines computed from its neighbourhood connecting the stealth CME source region to two ARs in the northern hemisphere. We conclude that reconnection at the null point aids the eruption of the stealth CME by removing field that acted to stabilise the pre-eruptive structure. This stealth CME, despite its weak signatures, has the main characteristics of other CMEs, and its eruption is driven by similar mechanisms.

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arXiv2020

Study of the spatial association between an active region jet and a nonthermal type~${\rm III}$ radio burst

We aim to investigate the spatial location of the source of an active region (AR) jet and its relation with associated nonthermal type~III radio emission. An emission measure (EM) method was used to study the thermodynamic nature of the AR jet. The nonthermal type~{\rm III} radio burst observed at meterwavelength was studied using the Murchison Widefield Array (MWA) radio imaging and spectroscopic data. The local configuration of the magnetic field and the connectivity of the source region of the jet with open magnetic field structures was studied using a nonlinear force-free field (NLFFF) extrapolation and potential field source surface (PFSS) extrapolation respectively. The plane-of-sky velocity of the AR jet was found to be $\sim$136~km/s. The EM analysis confirmed the presence of low temperature 2~MK plasma for the spire, whereas hot plasma, between 5-8 MK, was present at the footpoint region which also showed the presence of Fe~{\sc xviii} emission. A lower limit on the electron number density was found to be 1.4$\times$10$^{8}$ cm$^{-3}$ for the spire and 2.2$\times$10$^{8}$~cm$^{-3}$ for the footpoint. A temporal and spatial correlation between the AR jet and nonthermal type III burst confirmed the presence of open magnetic fields. An NLFFF extrapolation showed that the photospheric footpoints of the null point were anchored at the location of the source brightening of the jet. The spatial location of the radio sources suggests an association with the extrapolated closed and open magnetic fields although strong propagation effects are also present. The multi-scale analysis of the field at local, AR, and solar scales confirms the interlink between different flux bundles involved in the generation of the type III radio signal with flux transferred from a small coronal hole to the periphery of the sunspot via null point reconnection with an emerging structure.

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