arXiv subjects

Giulio Del Zanna

Publications and source records attributed to Giulio Del Zanna.

At least 19 recordsLinked to original sources

arXiv2026

UK White Paper on Space-based total solar eclipse observations: structure and dynamics of the solar atmosphere

Our Sun is uniquely placed to enable a detailed study of astrophysical plasmas and how they are governed by the magnetic fields that thread through them. On the one hand, magnetic fields confine plasma and determine plasma heating, flows, and energisation. On the other hand, magnetic fields and their evolution give rise to the most violent eruptions in the Solar System. Understanding the details of how energy is built up and released, and the impact of these physical processes on the plasma, remain key open questions that directly map to UKRI's science strategy through the STFC Solar System Advisory Panel's roadmap for Solar System research goals: What are the causes, consequences and predictability of solar magnetic variability and the solar cycle? What are the structures, dynamics and energetics of the Sun? What are the underlying processes that drive Sun-planet connections? And what are the fundamental processes at work in the Solar System? As laid out in this White Paper, the Moon-Enabled Sun Occultation Mission (MESOM) directly addresses these questions and in doing so delivers several Pillars of the National Space Strategy.

astro-ph.IM↗

arXiv2025

Understanding Solar Flares and Energetic Events: Open questions, observational requirements, and instrumental needs over the coming decade

Solar flares are the largest energy-release events in the Solar System, allowing us to study fundamental physical phenomena under extreme conditions. Those include magnetic reconnection, particle acceleration, radiation transport, and various plasma physics processes, all of which occur throughout the heliosphere and rest of the Universe. Flares and eruptive events are also components of geo-effective space weather. Their impacts from a space weather perspective are numerous, such as harm to satellites, disruption to GPS, communications and power systems, and impacts on passenger air travel. A comprehensive understanding of solar flares is therefore not just a compelling science problem, but also important for national security and infrastructure. This white paper (WP) addresses critical open science questions related to solar flares. Key observations and capabilities required to make significant advancements over the coming decade are identified. The UK has a robust and vibrant solar flare community. We are key partners in international collaborations, and also provide instrumentation for existing and upcoming ESA/NASA/JAXA space missions. Continuing this effort over the coming decade is vital to maintain UK leadership in this field, achieve Solar System Advisory Panel roadmap goals, and to work under the UK Space Agency's National Space Strategy Pillars. Several complementary WPs have been submitted that discuss instruments or concepts that would directly address the observational requirements we describe (including: SPARK, solar optimised IFUs, Solar-C/EUVST, and OSIRIS), as well as required numerical modelling efforts and infrastructure.

astro-ph.IM↗

arXiv2025

UK White Paper on Magnetic Reconnection

Magnetic reconnection powers explosive releases of magnetic energy, heating and particle acceleration throughout the plasma universe. Knowledge of this universal process is vital to understanding the Heliosphere, as it plays a key role in solar flares, coronal mass ejections, coronal heating, solar wind acceleration, geomagnetic storms, and interactions between the solar wind and planetary magnetospheres. As such, reconnection underpins multiple science objectives of multiple future space missions. The UK plays a leading role in this international field, through a combination of in situ measurements from Earth's magnetosphere and the solar wind, observations of the solar corona and chromosphere, and world-class numerical simulations and theory. This white paper identifies: Nine priority science objectives for reconnection research in the next decade; Recommendations to guide investment in theory, simulations and infrastructure; Mission priorities and required measurements to ensure the UK maintains and improves its world-class credentials in reconnection science.

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arXiv2025

Evidence of Extreme Ultraviolet Resonant Excitation in the Middle Corona During A Solar Flare

We present observations of an eruptive solar flare on 2016 January 6 that occurred behind the solar limb from the perspective of the Earth, but was well observed by STEREO and the Solar Extreme Ultraviolet Monitor on the Mars Atmosphere and {Volatile} EvolutioN (MAVEN) mission. Light curves showing the evolution of the flare's irradiance as a function of time taken by MAVEN are well correlated with the brightness evolution of fan structures observed in the PROBA2 SWAP 174 Å passband, suggesting that the radiance of structures near the flare site was influenced by emission from the flare. Because SWAP did not directly observe the flare itself, this event represents a rare opportunity to study the evolution of emission near a flare without the risk of instrumental scattered light contaminating the observations. We analyze this evolution and implement a simple model to explore the possibility that resonant excitation (or resonant scattering) plays an important role in driving coronal EUV emission during flaring events. Our modeling shows that for a large flare, resonant excitation could increase emission from nearby structures by about 45%, consistent with our findings that the involved structures observed by SWAP increased in brightness by about 60% during the flare. We conclude that resonant excitation may play an important role in driving coronal EUV emission under certain circumstances and should be accounted for in models and emission-based analysis tools.

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arXiv2025

Reduced atomic models for large-scale computations: Fe XIII near-infrared lines

Accurate atomic models for astrophysical plasma can be very complex, requiring thousands of states. However, for a variety of applications such as large-scale forward models of the Stokes parameters of a spectral line in the solar corona, it is necessary to build much reduced atomic models. We present two examples of such models, focused on the two near-infrared Fe XIII lines observed on the ground at 10750, 10801 Angstroms. These lines are primary diagnostics for a range of missions (especially the Daniel K. Inouye Solar Telescope, DKIST) to measure electron densities and magnetic fields in the solar corona. We calculate the Stokes parameters for a range of coronal conditions using CHIANTI (for intensities) and P-CORONA (for intensities and polarization), and use P-CORONA and a realistic global MHD simulation to show that the reduced models provide accurate results, typically to within 5% those obtained with larger models. Reduced models provide a significant decrease (over three orders of magnitude) in the computational time in spectropolarimetric calculations. The methods we describe are general and can be applied to a range of conditions and other ions.

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arXiv2024

Modelling Stellar Irradiances I: the transition regions of FGKM stars

We employed advanced ionisation equilibrium models that we developed in Dufresne et. al. (2024), which include charge transfer and density effects, to model UV stellar irradiances for a sample of stars. Our sample includes $ε$ Eridani (K2 V), $α$ Centauri A (G2 V), Procyon (F5 V) and Proxima Centauri (M5.5 Ve). We measured line fluxes from STIS datasets and used O IV and O V as density diagnostics to find the formation pressure of ions in the transition region (TR) and adopted a simple Differential Emission Measure (DEM) modelling. Our findings indicate significant improvements in modelling spectral lines from anomalous ions such as Si IV, C IV and N V of the Li- and Na-like sequences, which produce the strongest lines in the UV. For example, the Si IV lines were under-predicted by a factor of five and now are within 40% the observed fluxes. The improved models allow us to obtain for the first time reliable estimates of some stellar chemical abundances in the TR. We compared our results with available photospheric abundances in the literature and found no evidence for the First Ionisation Potential (FIP) effect in the TR of our stellar sample. Finally, we compared our results with the solar TR that can also be described by photospheric abundances.

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arXiv2024

Effect of Systematic Uncertainties on Density and Temperature Estimates in Coronae of Capella

We estimate the coronal density of Capella using the O VII and Fe XVII line systems in the soft X-ray regime that have been observed over the course of the Chandra mission. Our analysis combines measures of error due to uncertainty in the underlying atomic data with statistical errors in the Chandra data to derive meaningful overall uncertainties on the plasma density of the coronae of Capella. We consider two Bayesian frameworks. First, the so-called pragmatic-Bayesian approach considers the atomic data and their uncertainties as fully specified and uncorrectable. The fully-Bayesian approach, on the other hand, allows the observed spectral data to update the atomic data and their uncertainties, thereby reducing the overall errors on the inferred parameters. To incorporate atomic data uncertainties, we obtain a set of atomic data replicates, the distribution of which captures their uncertainty. A principal component analysis of these replicates allows us to represent the atomic uncertainty with a lower-dimensional multivariate Gaussian distribution. A $t$-distribution approximation of the uncertainties of a subset of plasma parameters including a priori temperature information, obtained from the temperature-sensitive-only Fe XVII spectral line analysis, is carried forward into the density- and temperature-sensitive O VII spectral line analysis. Markov Chain Monte Carlo based model fitting is implemented including Multi-step Monte Carlo Gibbs Sampler and Hamiltonian Monte Carlo. Our analysis recovers an isothermally approximated coronal plasma temperature of $\approx$5 MK and a coronal plasma density of $\approx$10$^{10}$ cm$^{-3}$, with uncertainties of 0.1 and 0.2 dex respectively.

astro-ph.SR↗

arXiv2024

Determining the nanoflare heating frequency of an X-ray Bright Point observed by MaGIXS

Nanoflares are thought to be one of the prime candidates that can heat the solar corona to its multi-million kelvin temperature. Individual nanoflares are difficult to detect with the present generation instruments, however their presence can be inferred by comparing simulated nanoflare-heated plasma emissions with the observed emission. Using HYDRAD coronal loop simulations, we model the emission from an X-ray bright point (XBP) observed by the Marshall Grazing Incidence X-ray Spectrometer (MaGIXS), along with nearest-available observations from the Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO) and X-Ray Telescope (XRT) onboard Hinode observatory. The length and magnetic field strength of the coronal loops are derived from the linear-force-free extrapolation of the observed photospheric magnetogram by Helioseismic and Magnetic Imager (HMI) onboard SDO. Each loop is assumed to be heated by random nanoflares, whose magnitude and frequency are determined by the loop length and magnetic field strength. The simulation results are then compared and matched against the measured intensity from AIA, XRT, and MaGIXS. Our model results indicate the observed emissions from the XBP under study could be well matched by a distribution of nanoflares with average delay times 1500 s to 3000 s, which suggest that the heating is dominated by high-frequency events. Further, we demonstrate the high sensitivity of MaGIXS and XRT to diagnose the heating frequency using this method, while AIA passbands are found to be the least sensitive.

astro-ph.SR↗

arXiv2023

Dark Halos around Solar Active Regions. I. Emission properties of the Dark Halo around NOAA 12706

Dark areas around active regions (ARs) have been first observed in chromospheric lines more than a century ago and are now associated to the Hα fibril vortex around ARs. Nowadays, large areas surrounding ARs with reduced emission relative to the Quiet Sun (QS) are also observed in spectral lines emitted in the transition-region (TR) and low-corona. For example, they are clearly seen in the SDO/AIA 171 Å images. We name these chromospheric and TR/coronal dark regions as Dark Halos (DHs). Coronal DHs are poorly studied and, because their origin is still unknown, to date it is not clear if they are related to the chromospheric fibrillar ones. Furthermore, they are often mistaken for Coronal Holes (CHs). Our goal is to characterize the emission properties of a DH by combining, for the first time, chromospheric, TR and coronal observations in order to provide observational constraints for future studies on the origin of DHs. This study also aims at investigating the different properties of DHs and CHs and at providing a quick-look recipe to distinguish between them. We study the DH around AR NOAA 12706 and the southern CH, that were on the disk on 2018 April 22, by analyzing IRIS full-disk mosaics, SDO/AIA filtergrams and SDO/HMI magnetograms. Fibrils are observed all around the AR core in the chromospheric Mg II h&k IRIS mosaics, most clearly in the h3 and k3 features. The TR emission in the DH is much lower compared to QS area, unlike in the CH. Moreover, the DH is much more extended in the low-corona than in the chromospheric Mg II h3 and k3 images. Finally, the intensities, emission measure, spectral profile, non-thermal velocity and average magnetic field strength measurements clearly show that DHs and CHs exhibit different characteristics and therefore should be considered as distinct types of structures on the Sun.

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arXiv2023

Multi-wavelength observations of a B-class flare using XSM, AIA, and XRT

We present multi-wavelength observations by Chandrayaan-2/XSM, SDO/AIA and Hinode/XRT of a B-class flare observed on 25th February, 2021, originating from an active region (AR 12804) near the North-West limb. The microflare lasts for approx 30 mins and is composed of hot loops reaching temperatures of 10 MK. We report excellent agreement (within 20 percent) for the average effective temperatures obtained at the flare peak from all the three instruments, which have different temperature sensitivities. The XRT filter combination of Be-thin and Be-med provides an excellent opportunity to measure the high-temperatures in such microflare events. The elemental abundances during the evolution of the microflare are also studied and observed to drop towards photospheric values at the flare peak time, compared to coronal values during the rise and decay phase. This is consistent with previous XSM studies.

astro-ph.SR↗

arXiv2023

CHIANTI -- an atomic database for emission lines -- Paper XVII: Version 10.1, revised ionization and recombination rates and other updates

The CHIANTI atomic database provides sets of assessed data used for simulating spectral observations of astrophysical plasmas. This article describes updates that will be released as version~10.1 of the database. A key component of CHIANTI is the provision of ionization and recombination rates that are used to compute the ionization balance of a plasma over a range of temperatures. Parameters for calculating the ionization rates of all stages of ions from H through Zn were compiled and inserted into the CHIANTI database in 2009. These were based on all measurements that were available at the time and supplemented with distorted wave calculations. Since then, there have been a number of new laboratory measurements for ions that produce spectral lines that are commonly observed. Parameters have been fit to these new measurements to provide improved ability to reproduce the ionization cross sections and rate coefficients, and these are added to the database. CHIANTI 10.1 also includes new recombination rates for the phosphorus isoelectronic sequence, and the updated ionization and recombination rates have been used to calculate a new ionization equilibrium file. In addition, CHIANTI 10.1 has new electron collision and radiative datasets for eight ions in the nitrogen and oxygen isoelectronic sequences, and updated energy level and wavelength data for seven other ions.

physics.atom-ph↗

arXiv2023

Evolution of elemental abundances in hot active region cores from Chandrayaan-2 XSM observations

The First Ionization Potential (FIP) bias, whereby elemental abundances for low FIP elements in different coronal structures vary from their photospheric values and may also vary with time, has been widely studied. In order to study the temporal variation, and to understand the physical mechanisms giving rise to the FIP bias, we have investigated the hot cores of three ARs using disk-integrated soft X-ray spectroscopic observation with the Solar X-ray Monitor (XSM) onboard Chandrayaan-2. Observations for periods when only one AR was present on the solar disk were used so as to ensure that the AR was the principal contributor to the total X-ray intensity. The average values of temperature and EM were ~3 MK and 3.0E46/cm3 respectively. Regardless of the age and activity of the AR, the elemental abundances of the low FIP elements, Al, Mg, and Si were consistently higher than their photospheric values. The average FIP bias for Mg and Si was ~3, whereas the FIP bias for the mid-FIP element, S, was ~1.5. However, the FIP bias for the lowest FIP element, Al, was observed to be higher than 3, which, if real, suggests a dependence of the FIP bias of low FIP elements on their FIP value. Another major result from our analysis is that the FIP bias of these elements is established in within ~10 hours of emergence of the AR and then remains almost constant throughout its lifetime.

astro-ph.SR↗

arXiv2023

A benchmark study of atomic models for the transition region against quiet Sun observations

The use of the coronal approximation to model line emission from the solar transition region has led to discrepancies with observations over many years, particularly for Li- and Na-like ions. Studies have shown that a number of atomic processes are required to improve the modelling for this region, including the effects of high densities, solar radiation and charge transfer on ion formation. Other non-equilibrium processes, such as time dependent ionisation and radiative transfer, are also expected to play a role. A set of models which include the three relevant atomic processes listed above in ionisation equilibrium has recently been built. These new results cover the main elements observed in the transition region. To assess the effectiveness of the results, the present work predicts spectral line intensities using differential emission measure modelling. Although limited in some respects, this differential emission measure modelling does give a good indication of the impact of the new atomic calculations. The results are compared to predictions of the coronal approximation and to observations of the average, quiet Sun from published literature. Significant improvements are seen for the line emission from Li- and Na-like ions, inter-combination lines and many other lines. From this study, an assessment is made of how far down into the solar atmosphere the coronal approximation can be applied, and the range over which the new atomic models are valid.

astro-ph.SR↗

arXiv2023

Defining the Middle Corona

The middle corona, the region roughly spanning heliocentric altitudes from $1.5$ to $6\,R_\odot$, encompasses almost all of the influential physical transitions and processes that govern the behavior of coronal outflow into the heliosphere. Eruptions that could disrupt the near-Earth environment propagate through it. Importantly, it modulates inflow from above that can drive dynamic changes at lower heights in the inner corona. Consequently, this region is essential for comprehensively connecting the corona to the heliosphere and for developing corresponding global models. Nonetheless, because it is challenging to observe, the middle corona has been poorly studied by major solar remote sensing missions and instruments, extending back to the Solar and Heliospheric Observatory (SoHO) era. Thanks to recent advances in instrumentation, observational processing techniques, and a realization of the importance of the region, interest in the middle corona has increased. Although the region cannot be intrinsically separated from other regions of the solar atmosphere, there has emerged a need to define the region in terms of its location and extension in the solar atmosphere, its composition, the physical transitions it covers, and the underlying physics believed to be encapsulated by the region. This paper aims to define the middle corona and give an overview of the processes that occur there.

astro-ph.SR↗

arXiv2023

Role of small-scale impulsive events in heating the X-ray bright points of the quiet Sun

Small-scale impulsive events, known as nanoflares, are thought to be one of the prime candidates that can keep the solar corona hot at its multi-million Kelvin temperature. Individual nanoflares are difficult to detect with the current generation instruments; however, their presence can be inferred through indirect techniques such as a Differential Emission Measure (DEM) analysis. Here we employ this technique to investigate the possibility of nanoflare heating of the quiet corona during the minimum of solar cycle 24. During this minimum, active regions (ARs) were absent on the solar-disk for extended periods. In the absence of ARs, X-ray bright points (XBP) are the dominant contributor to disk-integrated X-rays. We estimate the DEM of the XBPs using observations from the Solar X-ray Monitor (XSM) onboard the Chandrayaan-2 orbiter and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamic Observatory. XBPs consist of small-scale loops associated with bipolar magnetic fields. We simulate such XBP loops using the EBTEL hydrodynamic code. The lengths and magnetic field strengths of these loops are obtained through a potential field extrapolation of the photospheric magnetogram. Each loop is assumed to be heated by random nanoflares having an energy that depends on the loop properties. The composite nanoflare energy distribution for all the loops has a power-law slope close to -2.5. The simulation output is then used to obtain the integrated DEM. It agrees remarkably well with the observed DEM at temperatures above 1 MK, suggesting that the nanoflare distribution, as predicted by our model, can explain the XBP heating.

astro-ph.SR↗

arXiv2022

The First Flight of the Marshall Grazing Incidence X-ray Spectrometer (MaGIXS)

The Marshall Grazing Incidence X-ray Spectrometer (MaGIXS) sounding rocket experiment launched on July 30, 2021 from the White Sands Missile Range in New Mexico. MaGIXS is a unique solar observing telescope developed to capture X-ray spectral images, in the 6 - 24 Angstrom wavelength range, of coronal active regions. Its novel design takes advantage of recent technological advances related to fabricating and optimizing X-ray optical systems as well as breakthroughs in inversion methodologies necessary to create spectrally pure maps from overlapping spectral images. MaGIXS is the first instrument of its kind to provide spatially resolved soft X-ray spectra across a wide field of view. The plasma diagnostics available in this spectral regime make this instrument a powerful tool for probing solar coronal heating. This paper presents details from the first MaGIXS flight, the captured observations, the data processing and inversion techniques, and the first science results.

astro-ph.SR↗

arXiv2022

Formation and Thermodynamic Evolution of plasmoids in active region jets

We have carried out a comprehensive study of the temperature structure of plasmoids, which successively occurred in recurrent active region jets. The multithermal plasmoids were seen to be travelling along the multi-threaded spire as well as at the footpoint region in the EUV/UV images recorded by the Atmospheric Imaging Assembly (AIA). The Differential Emission Measure (DEM) analysis was performed using EUV AIA images, and the high-temperature part of the DEM was constrained by combining X-ray images from the X-ray telescope (XRT/Hinode). We observed a systematic rise and fall in brightness, electron number densities and the peak temperatures of the spire plasmoid during its propagation along the jet. The plasmoids at the footpoint (FPs) (1.0-2.5 MK) and plasmoids at the spire (SPs) (1.0-2.24 MK) were found to have similar peak temperatures, whereas the FPs have higher DEM weighted temperatures (2.2-5.7 MK) than the SPs (1.3-3.0 MK). A lower limit to the electron number densities of plasmoids - SPs (FPs) were obtained that ranged between 3.4-6.1$\times$10$^{8}$ (3.3-5.9$\times$10$^{8}$) cm$^{-3}$ whereas for the spire, it ranged from 2.6-3.2$\times$10$^{8}$ cm$^{-3}$. Our analysis shows that the emission of these plasmoids starts close to the base of the jet(s), where we believe that a strong current interface is formed. This suggests that the blobs are plasmoids induced by a tearing-mode instability.

astro-ph.SR↗

arXiv2022

Soft X-ray Spectral Diagnostics of Multi-thermal Plasma in Solar Flares with Chandrayaan-2 XSM

Spectroscopic observations in X-ray wavelengths provide excellent diagnostics of the temperature distribution in solar flare plasma. The Solar X-ray Monitor (XSM) onboard the Chandrayaan-2 mission provides broad-band disk integrated soft X-ray solar spectral measurements in the energy range of 1-15 keV with high spectral resolution and time cadence. In this study, we analyse X-ray spectra of three representative GOES C-class flares obtained with the XSM to investigate the evolution of various plasma parameters during the course of the flares. Using the soft X-ray spectra consisting of the continuum and well-resolved line complexes of major elements like Mg, Si, and Fe, we investigate the validity of the isothermal and multi-thermal assumptions on the high temperature components of the flaring plasma. We show that the soft X-ray spectra during the impulsive phase of the high intensity flares are inconsistent with isothermal models and are best fitted with double peaked differential emission measure distributions where the temperature of the hotter component rises faster than that of the cooler component. The two distinct temperature components observed in DEM models during the impulsive phase of the flares suggest the presence of the directly heated plasma in the corona and evaporated plasma from the chromospheric footpoints. We also find that the abundances of low FIP elements Mg, Si, and Fe reduces from near coronal to near photospheric values during the rising phase of the flare and recovers back to coronal values during decay phase, which is also consistent with the chromospheric evaporation scenario.

astro-ph.SR↗