arXiv subjects

Paolo Pagano

Publications and source records attributed to Paolo Pagano.

At least 19 recordsLinked to original sources

arXiv2026

Numerical simulations of waves and turbulence in coronal loops: observables and spectra

We investigate numerically the time evolution of velocity and magnetic field fluctuations in a coronal loop, focusing on the dynamics due to both phase mixing and turbulent cascade. The intensity, doppler velocity and non-thermal broadening are synthesized from numerical results in order to establish if the upcoming Multi-slit Solar Explorer (MUSE) mission could reveal the presence of those phenomena in the solar corona through its unprecedented high-resolution spectroscopic observations. The loop is represented by a cylindrical pressure-balanced magnetic structure with a transverse density and magnetic field inhomogeneity. The initial perturbation is a superposition of a torsional Alfvén wave and a transverse turbulent component with different tunable weights. In order to reconstruct plasma emission features we calculate moments of the Fe IX 171 Å spectral line. 2D maps obtained by integrating the emission along the assumed line of sight are calculated for the emission intensity $I_0$, the Doppler shift $I_1$ and the non-thermal broadening $I_2$, for several values of the model parameters. Finally, we simulate MUSE spectrograph by considering a resolution of $312$ km $\times$ $312$ km. We observe how intensity maps show the formation of longitudinal threads. The generation of small-scale fluctuations mainly takes place in the inhomogeneity region at the loop boundary, where the effects of phase mixing and non-thermal broadening are stronger. 1D power spectra of intensity and Doppler shift maps are calculated and compared with the corresponding spectra of density and line-of-sight velocity component. The agreement observed between the spectral indexes of the intensity power spectra at MUSE resolution and the one computed from the full 3D density field indicates that spectra of $I_0$ can be used to infer information on the spectrum of density inside a loop.

physics.plasm-ph↗

arXiv2026

On the cool nature of coronal nanojets

Reconnection-driven nanoflares are widely considered a leading mechanism for coronal-loop heating, but their direct fingerprints in the tenuous coronal plasma remain elusive. The recently discovered coronal nanojets offer a potential probe of reconnection dynamics, but their extreme collimation, directionality and multi-wavelength visibility are not fully understood. Here we present a 3D rMHD simulation that unprecedentedly reproduces the key properties of nanojets, offering a viable model to explain their nature. These results provide a unified picture in which nanojet morphology, dynamics and detectability are contingent on the thermodynamic environment of reconnection. Together, our results point to a cool origin of coronal nanojets, where cool and dense material permits narrow, multi-band jet signatures to emerge from reconnection.

astro-ph.SR↗

arXiv2025

3D MHD simulations of coronal loops heated via magnetic braiding I. Continuous driving

The nature and detailed properties of the heating of the million-degree solar corona are important issues that are still largely unresolved. Nanoflare heating might be dominant in active regions and quiet Sun, although direct signatures of such small-scale events are difficult to observe in the highly conducting, faint corona. The aim of this work is to test the theory of coronal heating by nanoflares in braided magnetic field structures. We analyze a 3D MHD model of a multistrand flux tube in a stratified solar atmosphere, driven by twisting motions at the boundaries. We show how the magnetic structure is maintained at high temperature and for an indefinite time, by intermittent episodes of local magnetic energy release due to reconnection. We forward-modelled optically thin emission with SDO/AIA and MUSE and compared the synthetic observations with the intrinsic coronal plasma properties, focusing on the response to impulsive coronal heating. Currents build up and their impulsive dissipation into heat are also investigated through different runs. In this first paper, we describe the proliferation of heating from the dissipation of narrow current sheets in realistic simulations of braided coronal flux tubes at unprecedented high spatial resolutions.

astro-ph.SR↗

arXiv2025

3D MHD simulations of coronal loops heated via magnetic braiding II. Automatic detection of reconnection outflows and statistical analysis of their properties

Recent observations of fast and bursty ``nanojets'' suggest novel diagnostics of nanoflare heating in the solar corona. The aim of this work is to investigate the presence and properties of reconnection outflows, similar to observed nanojets, in numerical simulations, and explore their relationship with the nanoflare properties. This work explores their potential as diagnostics for nanoflare heating in observations. We developed an algorithm of Reconnection Outflows Automatic Detection (ROAD) in 3D MHD simulations of coronal loops. We applied the algorithm to a 3D MHD stratified coronal loop model heated by magnetic reconnection and analyzed the statistical properties of the jets produced at reconnection sites, over about one solar hour. The magnetic structure is maintained at high temperature and for an indefinite time by intermittent episodes of local magnetic energy release due to reconnection.

astro-ph.SR↗

arXiv2025

Alfvén wave propagation, reflection and trapping in the solar wind

Alfvén waves are known to be important carriers of magnetic energy that could play a role in coronal heating and/or solar wind acceleration. As these waves are efficient energy carriers, how they are dissipated still remains one of the key challenges. Using a series of 1.5-D magnetohydrodynamic (MHD) simulations, we explore wave energy trapping associated with field-aligned density enhancements. We examine the parameters which govern the wave reflection and trapping. The goal of our simulations is to find optimal conditions for wave trapping, which would ultimately promote the energisation of the solar atmosphere. In agreement with previous studies, we find that maximum wave reflections happen only for a narrow range of density enhancement widths, namely when it is comparable to the Alfvén wave wavelength. In our paper, we explain this scale-selectivity using a semi-analytical model that demonstrates the importance of wave interference effects. As expected, we find that spatially extended regions of density inhomogeneities favour enhanced wave reflection and trapping. However, wave interference causes saturation of the reflected energy for very extended regions of varying density.

astro-ph.SR↗

arXiv2022

Tracing the ICME plasma with a MHD simulation

The determination of the chemical composition of interplanetary coronal mass ejection (ICME) plasma is an open issue. More specifically, it is not yet fully understood how remote sensing observations of the solar corona plasma during solar disturbances evolve into plasma properties measured in situ away from the Sun. The ambient conditions of the background interplanetary plasma are important for space weather because they influence the evolutions, arrival times, and geo-effectiveness of the disturbances. The Reverse In situ and MHD APproach (RIMAP) is a technique to reconstruct the heliosphere on the ecliptic plane (including the magnetic Parker spiral) directly from in situ measurements acquired at 1 AU. It combines analytical and numerical approaches, preserving the small-scale longitudinal variability of the wind flow lines. In this work, we use RIMAP to test the interaction of an ICME with the interplanetary medium. We model the propagation of a homogeneous non-magnetised (i.e. with no internal flux rope) cloud starting at 800 km s-1 at 0.1 AU out to 1.1 AU. Our 3D magnetohydrodynamics (MHD) simulation made with the PLUTO MHD code shows the formation of a compression front ahead of the ICME, continuously driven by the cloud expansion. Using a passive tracer, we find that the initial ICME material does not fragment behind the front during its propagation, and we quantify the mixing of the propagating plasma cloud with the ambient solar wind plasma, which can be detected at 1 AU.

astro-ph.SR↗

arXiv2022

Connecting Solar Orbiter remote-sensing observations and Parker Solar Probe in-situ measurements with a numerical MHD reconstruction of the Parker spiral

As a key feature, NASA's Parker Solar Probe (PSP) and ESA-NASA's Solar Orbiter (SO) missions cooperate to trace solar wind and transients from their sources on the Sun to the inner interplanetary space. The goal of this work is to accurately reconstruct the interplanetary Parker spiral and the connection between coronal features observed remotely by the Metis coronagraph on-board SO and those detected in situ by PSP at the time of the first PSP-SO quadrature of January 2021. We use the Reverse In-situ and MHD Approach (RIMAP), a hybrid analytical-numerical method performing data-driven reconstructions of the Parker spiral. RIMAP solves the MHD equations on the equatorial plane with the PLUTO code, using the measurements collected by PSP between 0.1 and 0.2 AU as boundary conditions. Our reconstruction connects density and wind speed measurements provided by Metis (3-6 solar radii) to those acquired by PSP (21.5 solar radii) along a single streamline. The capability of our MHD model to connect the inner corona observed by Metis and the super Alfvénic wind measured by PSP, not only confirms the research pathways provided by multi-spacecraft observations, but also the validity and accuracy of RIMAP reconstructions as a possible test bench to verify models of transient phenomena propagating across the heliosphere, such as coronal mass ejections, solar energetic particles and solar wind switchbacks.

astro-ph.SR↗

arXiv2021

Understanding the Origins of Problem Geomagnetic Storms Associated With "Stealth" Coronal Mass Ejections

Geomagnetic storms are an important aspect of space weather and can result in significant impacts on space- and ground-based assets. The majority of strong storms are associated with the passage of interplanetary coronal mass ejections (ICMEs) in the near-Earth environment. In many cases, these ICMEs can be traced back unambiguously to a specific coronal mass ejection (CME) and solar activity on the frontside of the Sun. Hence, predicting the arrival of ICMEs at Earth from routine observations of CMEs and solar activity currently makes a major contribution to the forecasting of geomagnetic storms. However, it is clear that some ICMEs, which may also cause enhanced geomagnetic activity, cannot be traced back to an observed CME, or, if the CME is identified, its origin may be elusive or ambiguous in coronal images. Such CMEs have been termed "stealth CMEs." In this review, we focus on these "problem" geomagnetic storms in the sense that the solar/CME precursors are enigmatic and stealthy. We start by reviewing evidence for stealth CMEs discussed in past studies. We then identify several moderate to strong geomagnetic storms (minimum Dst < -50 nT) in solar cycle 24 for which the related solar sources and/or CMEs are unclear and apparently stealthy. We discuss the solar and in situ circumstances of these events and identify several scenarios that may account for their elusive solar signatures. These range from observational limitations (e.g., a coronagraph near Earth may not detect an incoming CME if it is diffuse and not wide enough) to the possibility that there is a class of mass ejections from the Sun that have only weak or hard-to-observe coronal signatures. In particular, some of these sources are only clearly revealed by considering the evolution of coronal structures over longer time intervals than is usually considered. We also review a variety of numerical modelling approaches...

astro-ph.SR↗

arXiv2021

Modelling of asymmetric nanojets in coronal loops

Observations of reconnection jets in the solar corona are emerging as a possible diagnostic to study highly elusive coronal heating. Such nanojets can be observed in coronal loops and they have been linked to nanoflares. However, while models successfully describe the bilateral post-reconnection magnetic slingshot effect that leads to the jets, observations reveal that nanojets are unidirectional, or highly asymmetric, with only the jet travelling inward with respect to the coronal loop's curvature being clearly observed. The aim of this work is to address the role of the curvature of the coronal loop in asymmetric reconnection jets. In order to do so, we first use a simplified analytical model where we estimate the post-reconnection tension forces based on the local intersection angle between the pre-reconnection magnetic field lines and on their post-reconnection retracting length towards new equilibria. Second, we use a simplified numerical magnetohydrodynamic (MHD) model to study how two opposite propagating jets evolve in curved magnetic field lines. Our analytical model demonstrates that in the post-reconnection reorganised magnetic field, the inward directed magnetic tension is inherently stronger (up to 3 orders of magnitude) than the outward directed one and that, with a large enough retracting length, a regime exists where the outward directed tension disappears, leading to no outward jet at large, observable scales. Our MHD numerical model provides support for these results proving also that in the following time evolution the inward jets are consistently more energetic. The degree of asymmetry is also found to increase for small-angle reconnection and for more localised reconnection regions. This work shows that the curvature of the coronal loops plays a role in the asymmetry of the reconnection jets and inward directed jets are more likely to occur and more energetic.

astro-ph.SR↗

arXiv2021

Acoustic wave properties in footpoints of coronal loops in 3D MHD simulations

Acoustic waves excited in the photosphere and below might play an integral part in the heating of the solar chromosphere and corona. However, it is yet not fully clear how much of the initially acoustic wave flux reaches the corona and in what form. We investigate the wave propagation, damping, transmission, and conversion in the lower layers of the solar atmosphere using 3D numerical MHD simulations. A model of a gravitationally stratified expanding straight coronal loop, stretching from photosphere to photosphere, is perturbed at one footpoint by an acoustic driver with a period of 370 seconds. For this period acoustic cutoff regions are present below the transition region (TR). About 2% of the initial energy from the driver reach the corona. The shape of the cutoff regions and the height of the TR show a highly dynamic behavior. Taking only the driven waves into account, the waves have a propagating nature below and above the cutoff region, but are standing and evanescent within the cutoff region. Studying the driven waves together with the background motions in the model reveals standing waves between the cutoff region and the TR. These standing waves cause an oscillation of the TR height. In addition, fast or leaky sausage body-like waves might have been excited close to the base of the loop. These waves then possibly convert to fast or leaky sausage surface-like waves at the top of the main cutoff region, followed by a conversion to slow sausage body-like waves around the TR.

astro-ph.SR↗

arXiv2021

Probing the physics of the solar atmosphere with the Multi-slit Solar Explorer (MUSE): I. Coronal Heating

The Multi-slit Solar Explorer (MUSE) is a proposed NASA MIDEX mission, currently in Phase A, composed of a multi-slit EUV spectrograph (in three narrow spectral bands centered around 171A, 284A, and 108A) and an EUV context imager (in two narrow passbands around 195A and 304A). MUSE will provide unprecedented spectral and imaging diagnostics of the solar corona at high spatial (<0.5 arcsec), and temporal resolution (down to ~0.5s) thanks to its innovative multi-slit design. By obtaining spectra in 4 bright EUV lines (Fe IX 171A , Fe XV 284A, Fe XIX-Fe XXI 108A) covering a wide range of transition region and coronal temperatures along 37 slits simultaneously, MUSE will for the first time be able to "freeze" (at a cadence as short as 10 seconds) with a spectroscopic raster the evolution of the dynamic coronal plasma over a wide range of scales: from the spatial scales on which energy is released (~0.5 arcsec) to the large-scale often active-region size (170 arcsec x 170 arcsec) atmospheric response. We use advanced numerical modeling to showcase how MUSE will constrain the properties of the solar atmosphere on the spatio-temporal scales (~0.5 arcsec, ~20 seconds) and large field-of-view on which various state-of-the-art models of the physical processes that drive coronal heating, solar flares and coronal mass ejections (CMEs) make distinguishing and testable predictions. We describe how the synergy between MUSE, the single-slit, high-resolution Solar-C EUVST spectrograph, and ground-based observatories (DKIST and others) can address how the solar atmosphere is energized, and the critical role MUSE plays because of the multi-scale nature of the physical processes involved. In this first paper, we focus on how comparisons between MUSE observations and theoretical models will significantly further our understanding of coronal heating mechanisms.

astro-ph.SR↗

arXiv2021

Determining the source and eruption dynamics of a stealth CME using NLFFF modelling and MHD simulations

Coronal mass ejections (CMEs) that exhibit weak or no eruption signatures in the low corona, known as stealth CMEs, are problematic as upon arrival at Earth they can lead to geomagnetic disturbances that were not predicted by space weather forecasters. We investigate the origin and eruption of a stealth event that occurred on 2015 January 3 that was responsible for a strong geomagnetic storm upon its arrival at Earth. To simulate the coronal magnetic field and plasma parameters of the eruption we use a coupled approach. This approach combines an evolutionary nonlinear force-free field model of the global corona with a MHD simulation. The combined simulation approach accurately reproduces the stealth event and suggests that sympathetic eruptions occur. In the combined simulation we found that three flux ropes form and then erupt. The first two flux ropes, which are connected to a large AR complex behind the east limb, erupt first producing two near-simultaneous CMEs. These CMEs are closely followed by a third, weaker flux rope eruption in the simulation that originated between the periphery of AR 12252 and the southern polar coronal hole. The third eruption coincides with a faint coronal dimming, which appears in the SDO/AIA 211 A observations, that is attributed as the source responsible for the stealth event and later the geomagnetic disturbance at 1 AU. The incorrect interpretation of the stealth event being linked to the occurrence of a single partial halo CME observed by LASCO/C2 is mainly due to the lack of STEREO observations being available at the time of the CMEs. The simulation also shows that the LASCO CME is not a single event but rather two near-simultaneous CMEs. These results show the significance of the coupled data-driven simulation approach in interpreting the eruption and that an operational L5 mission is crucial for space weather forecasting.

astro-ph.SR↗

arXiv2020

A Transparent Distributed Ledger-based Certificate Revocation Scheme for VANETs

Among the available communication systems, vehicular networks are emerging as one of the most promising and yet most challenging instantiations of mobile ad-hoc network technologies. The deployment of such networks in large scale requires the enforcement of stringent security mechanisms that need to abide by the technical, societal, legal, and economical requirements of Intelligent Transportation Systems. Authentication is an effective process for validating user identity in vehicular netoworks. However, it cannot guarantee the network security by itself. Available industrial standards do not consider methods to promptly revoke misbehaving vehicles. The only available protection consists on the \textit{revocation by expiry}, which tolerates the misbehaving vehicle to remain trusted in the system for a long time (e.g. 3 months with certificate pre-loading according to EU security policy). This poses a huge yet dangerous limitation to the security of the vehicular ecosystem. In this work we propose a Distributed Ledger-based Certificate Revocation Scheme for Vehicular Ad-hoc Networks (VANETs) that harnesses the advantages of the underlying Distributed Ledger Technology (DLT) to implement a privacy-aware revocation process that is fully transparent to all participating entities and meets the critical message processing times defined by EU and US standards. An experimental validation and analysis demonstrates the effectiveness and efficiency of the proposed scheme, where the DLT streamlines the revocation operation overhead and delivers an economic solution against cyber-attacks in vehicular systems.

cs.CR↗

arXiv2019

First Determination of 2D Speed Distribution within the Bodies of Coronal Mass Ejections with Cross-Correlation Analysis

The determination of the speed of Coronal Mass Ejections (CMEs) is usually done by tracking brighter features (such as the CME front and core) in visible light coronagraphic images and by deriving unidimensional profiles of the CME speed as a function of altitude or time. Nevertheless, CMEs are usually characterized by the presence of significant density inhomogeneities propagating outward with different radial and latitudinal projected speeds, resulting in a complex evolution eventually forming the Interplanetary CME. In this work, we demonstrate for the first time how coronagraphic image sequences can be analyzed with cross-correlation technique to derive 2D maps of the almost instantaneous plasma speed distribution within the body of CMEs. The technique is first tested with the analysis of synthetic data, and then applied to real observations. Results from this work allow to characterize the distribution and time evolution of kinetic energy inside CMEs, as well as the mechanical energy (combined with the kinetic and potential energy) partition between the core and front of the CME. In the future, CMEs will be observed by two channels (VL and UV Ly-$α$) coronagraphs, such as Metis on-board ESA Solar Orbiter mission as well as Ly-$α$ Solar Telescope (LST) on-board Chinese Advanced Space-based Solar Observatory (ASO-S) mission. Our results will help the analysis of these future observations, helping in particular to take into account the 2D distribution of Ly-$α$ Doppler dimming effect.

astro-ph.SR↗

arXiv2019

Contribution of observed multi frequency spectrum of Alfvén waves to coronal heating

Whilst there are observational indications that transverse MHD waves carry enough energy to maintain the thermal structure of the solar corona, it is not clear whether such energy can be efficiently and effectively converted into heating. Phase-mixing of Alfvén waves is considered a candidate mechanism, as it can develop transverse gradient where magnetic energy can be converted into thermal energy. However, phase-mixing is a process that crucially depends on the amplitude and period of the transverse oscillations, and only recently have we obtained a complete measurement of the power spectrum for transverse oscillations in the corona. We aim to investigate the heating generated by phase-mixing of transverse oscillations triggered by buffeting of a coronal loop that follows from the observed coronal power spectrum as well as the impact of these persistent oscillations on the structure of coronal loops. We consider a 3D MHD model of an active region coronal loop and we perturb its footpoints with a 2D horizontal driver that represents a random buffeting motion of the loop footpoints. Our driver is composed of 1000 pulses superimposed to generate the observed power spectrum. We find that the heating supply from the observed power spectrum in the solar corona through phase mixing is not sufficient to maintain the million degree active region solar corona. We also find that the development of Kelvin-Helmholtz instabilities could be a common phenomenon in coronal loops that could affect their apparent life time. This study concludes that is unlikely that phase-mixing of Alfvén waves resulting from an observed power spectrum of transverse coronal loop oscillations can heat the active region solar corona. However, transverse waves could play an important role in the development of small scale structures.

astro-ph.SR↗

arXiv2018

In-situ generation of transverse MHD waves from colliding flows in the solar corona

Transverse MHD waves permeate the solar atmosphere and are a candidate for coronal heating. However, the origin of these waves is still unclear. In this work, we analyse coordinated observations from \textit{Hinode}/SOT and \textit{IRIS} of a prominence/coronal rain loop-like structure at the limb of the Sun. Cool and dense downflows and upflows are observed along the structure. A collision between a downward and an upward flow with an estimated energy flux of $10^{7}-10^{8}$~erg~cm$^{-2}$~s$^{-1}$ is observed to generate oscillatory transverse perturbations of the strands with an estimated $\approx40~$km~s$^{-1}$ total amplitude, and a short-lived brightening event with the plasma temperature increasing to at least $10^{5}~$K. We interpret this response as sausage and kink transverse MHD waves based on 2D MHD simulations of plasma flow collision. The lengths, density and velocity differences between the colliding clumps and the strength of the magnetic field are major parameters defining the response to the collision. The presence of asymmetry between the clumps (angle of impact surface and/or offset of flowing axis) is crucial to generate a kink mode. Using the observed values we successfully reproduce the observed transverse perturbations and brightening, and show adiabatic heating to coronal temperatures. The numerical modelling indicates that the plasma $β$ in this loop-like structure is confined between $0.09$ and $0.36$. These results suggest that such collisions from counter-streaming flows can be a source of in-situ transverse MHD waves, and that for cool and dense prominence conditions such waves could have significant amplitudes.

astro-ph.SR↗

arXiv2017

Contribution of mode coupling and phase-mixing of Alfvén waves to coronal heating

Phase-mixing of Alfvén waves in the solar corona has been identified as one possible candidate to explain coronal heating. While this scenario is supported by observations of ubiquitous oscillations in the corona carrying sufficient wave energy and by theoretical models that have described the concentration of energy in small scale structures, it is still unclear whether this wave energy can maintain the million degree solar corona. The aim of this work is to assess how much energy can be converted by a phase-mixing process triggered by the propagation of Alfvénic waves in a cylindric coronal structure, such as a coronal loop, and to estimate the impact on the coronal heating. To this end, we run 3D MHD simulations of a magnetised cylinder where the Alfvén speed varies through a boundary shell and a footpoint driver is set to trigger kink modes which mode couple to torsional Alfvén modes in the boundary shell. These Alfvén waves are expected to phase-mix and the system allows us to study the subsequent thermal energy deposition. We run a reference simulation to explain the main process and then we vary simulation parameters. When we take into consideration high values of magnetic resistivity and strong footpoint drivers, we find i) that phase-mixing leads to a temperature increase of the order of $10^5$ K or less, depending on the structure of the boundary shell, ii) that this energy is able to balance the radiative losses only in the localised region involved in the heating, iii) and how the boundary layer and the persistence of the driver influence the thermal structure of the system. Our conclusion is that due to the extreme physical parameters we adopted and the moderate impact on the heating of the system, it is unlikely that phase-mixing can contribute on a global scale to the heating of the solar corona.

astro-ph.SR↗

arXiv2015

Future capabilities of CME polarimetric 3D reconstructions with the METIS instrument: A numerical test

Understanding the 3D structure of coronal mass ejections (CMEs) is crucial for understanding the nature and origin of solar eruptions. To derive information on the 3D structure of CMEs from white-light (total and polarized brightness) images, the polarization ratio technique is widely used. The soon-to-be-launched METIS coronagraph on board Solar Orbiter will use this technique to produce new polarimetric images. We determine the accuracy at which the position of the centre of mass, direction and speed of propagation, and the column density of the CME can be determined along the line of sight. We perform a 3D MHD simulation of a flux rope ejection where a CME is produced. From the simulation we (i) synthesize the corresponding METIS white-light (total and polarized brightness) images and (ii) apply the polarization ratio technique to these synthesized images and compare the results with the known density distribution from the MHD simulation. We find that the polarization ratio technique reproduces with high accuracy the position of the centre of mass along the line of sight. However, some errors are inherently associated with this determination. The polarization ratio technique also allows information to be derived on the real 3D direction of propagation of the CME. In addition, we find that the column density derived from white-light images is accurate and we propose an improved technique where the combined use of the polarization ratio technique and white-light images minimizes the error in the estimation of column densities. Our method allows us to thoroughly test the performance of the polarization ratio technique and allows a determination of the errors associated with it, which means that it can be used to quantify the results from the analysis of the forthcoming METIS observations in white light (total and polarized brightness).

astro-ph.SR↗