Paolo Pagano

Context. Magnetic reconnection is seen as the lead mechanism to explain the persistent and bursty heating of the solar corona that keeps its thermal structure. Lately observations and modelling of magnetic reconnection outflows, i.e. nanojets, have empowered our capabilities to scrutinise this mechanisms; however, more needs to be understood on the relation between the energetic and kinematic events. Aims. The aim of this paper is to study in detail the properties of the heating events (nanoflares) and their kinematic counterparts (nanojets) in order to augment detection capabilities of magnetic reconnection events. Methods. We use magnetohydrodynamic simulations of the interaction of two magnetic flux tubes where magnetic reconnection occurs and such reconnection outflows are generated. We vary key parameters, such as the critical current and the size of the domain to study the heating and slingshot mechanisms. Results. We find that the nanojets’ properties can give away a lot of information about the magnetic reconnection that generated the nanojets, although it is difficult to directly link the amount of heating and the jet speeds. We also find that such reconnection model can explain a wide range of heating events. Conclusions. Magnetic reconnection is still invariably linked to both heating and outflows and our simulations show the complex interplay between these two byproducts of the magnetic reconnection and how their properties can be studied through observations.

Context. Coronal loops are often regarded as the building blocks of the solar corona; however, the heating mechanism and complex dynamics resulting from the turbulent nature of the plasma and the presence of waves are still unclear. Aims. We numerically investigated the time evolution of velocity and magnetic field fluctuations in a coronal loop, focusing on the dynamics due to both phase mixing and turbulent cascades. We synthesized the intensity, Doppler velocity, and nonthermal broadening from numerical results to assess whether the upcoming Multi-slit Solar Explorer (MUSE) mission can reveal the presence of those phenomena in the solar corona through its unprecedented high-resolution spectroscopic observations. Methods. We studied the time evolution of velocity and magnetic field fluctuations in coronal loops using numerical 3D magnetohydrodynamics (MHD) simulations. 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. We included heating effects due to viscous and resistive dissipation. To reconstruct plasma emission features, we calculated moments of the Fe IX 171 Å spectral line, synthesized from the simulations. We calculated 2D maps obtained by integrating the emission along the assumed line of sight for the emission intensity I ₀ , the Doppler shift I ₁ , and the nonthermal broadening I ₂ as functions of time for several values of the model parameters. Finally, we simulated the MUSE spectrograph by considering a resolution of 312 km × 312 km. Results. We observe that intensity maps show the formation of longitudinal threads during the time evolution. The generation of small-scale fluctuations takes place mainly in the inhomogeneity region at the loop boundary, where the effects of phase mixing and nonthermal broadening are stronger. In these regions the presence of transverse oscillations is clearly visible in I ₁ and I ₂ maps. We calculated 1D power spectra of intensity and Doppler shift maps and compared them with the corresponding spectra of density and the line-of-sight velocity component. At large spatial scales (i.e., between 300 km and 3000 km) the spectral indexes of density and intensity spectra are very similar, while the spectrum of I ₁ is steeper. Conclusions. We discuss these results in the context of forward modeling of observations of fluctuation dynamics in coronal loops, in particular with the MUSE spectrograph. The resolution attained by MUSE will allow us to obtain power spectra of coronal loops with a resolution never achieved before. The agreement observed between the spectral indices of the intensity power spectra at MUSE resolution and those computed from the full 3D density field indicates that the spectra of I ₀ can be used to infer information about the spectra of density fluctuations inside a loop.

We present a comprehensive investigation of the fast coronal mass ejection (CME) event of 2022 September 5 by combining remote sensing observations, in situ measurements, and numerical magnetohydrodynamic (MHD) simulations. The CME, one of the most energetic of the recent solar cycle, was observed by SOHO and STEREO-A in the corona and intercepted in situ by the Parker Solar Probe (PSP) at ∼0.07 au and Solar Orbiter at ∼0.71 au. Using multiviewpoint coronagraphic data, we reconstruct the 3D geometry and kinematics of the CME-driven shock, deriving its propagation speed, direction, and initial size. These results are used to constrain a data-driven MHD simulation based on the RIMAP framework, which incorporates realistic solar wind conditions reconstructed from PSP and WIND measurements. The simulation reproduces key features of the shock and CME evolution detected at PSP, with a good quantitative agreement. For the Solar Orbiter, the model captures qualitative features of the shock passage, but does not fully reproduce the detailed temporal evolution of the event. The analysis highlights the importance of ambient solar wind structure and CME geometry in shaping shock propagation and evolution. We find that a simple cone model is insufficient to explain the observed duration of the event, requiring the inclusion of a velocity “tail” in the CME profile. Our results demonstrate the capability of combining multispacecraft observations with data-constrained MHD modeling to investigate CME evolution across the inner heliosphere and provide critical inputs for understanding shock-driven particle acceleration in extreme solar events.

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 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 1 solar hr. The magnetic structure is maintained at a high temperature and for an indefinite time by intermittent episodes of local magnetic energy release due to reconnection.

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 synthesize the optically thin emission with SDO/AIA and MUSE and compare the synthetic observations with the intrinsic coronal plasma properties, focusing on the response to impulsive coronal heating. Currents’ buildup 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.

Solar coronal loops are magnetically confined plasma structures whose properties are closely linked to the ambient magnetic field. Using 3D magnetohydrodynamic simulations of kink-unstable coronal flux tubes, we investigate how the intensity of the ambient magnetic field influences the thermal regime of coronal loops. The simulations, performed with the PLUTO code, explore field strengths between 10 and 80 G under continuous photospheric twisting that drives magnetic reconnection and heating through anomalous resistivity. After an initial transient, the system reaches a statistically steady state characterized by persistent nanoflare-like energy release. We find that the differential emission measure distributions increase in magnitude and shift to higher temperatures with increasing magnetic field strength, while being largely insensitive to other parameters such as resistivity and resolution. Extending the classical Rosner–Tucker–Vaiana scaling laws, we derive a scaling relation for the loop temperature, T ∝ B ^4/7 , which quantitatively matches the simulation results. This study demonstrates that the strength of the ambient coronal magnetic field is the primary factor determining the thermal state of coronal loops, providing a physically grounded connection between magnetic field measurements and observed coronal temperatures.

Context. Outflows perpendicular to the guide field are believed to be a possible signature of magnetic reconnection in the solar corona. Specifically, outflows can help detect the occurrence of ubiquitous small-angle magnetic reconnection. Aims. The aim of this work is to identify possible diagnostic techniques of such outflows in hot coronal loops with the Atmospheric Image Assembly (AIA) on board the Solar Dynamics Observatory and the forthcoming MUltislit Solar Explorer (MUSE), in a realistically dynamic coronal loop environment where a magnetohydrodynamic (MHD) avalanche is occurring. Methods. We considered a 3D MHD model of two magnetic flux tubes, including a stratified, radiative, and thermal-conducting atmosphere, twisted by footpoint rotation. The faster rotating flux tube becomes kink-unstable and soon involves the other one in the avalanche. The turbulent decay of this magnetic structure on a global scale leads to the formation, fragmentation, and dissipation of current sheets, driving impulsive heating akin to a nanoflare storm. We captured a clear outflow from a reconnection episode soon after the initial avalanche and synthesised its emission as detectable with AIA and MUSE. Results. The outflow has a maximum temperature around 8 MK, total energy of 10²⁴ erg, velocity of a few hundred km/s, and duration of less than 1 min. We show the emission in the AIA 94 Å channel (Fe XVIII line) and in the MUSE 108 Å Fe XIX spectral line. Conclusions. This outflow shares many features with nanojets recently detected at lower temperatures. However, its low emission measure makes its detection difficult with AIA, while Doppler shifts can be measured with MUSE. Conditions become different in the later steady-state phase, when the flux tubes are filled with denser and relatively cooler plasma.

Context. Magnetohydrodynamic (MHD) instabilities, such as the kink instability, can trigger the chaotic fragmentation of a twisted magnetic flux tube into small-scale current sheets that dissipate as aperiodic impulsive heating events. In turn, the instability could propagate as an avalanche to nearby flux tubes and lead to a nanoflare storm. Our previous work was devoted to related 3D MHD numerical modeling, which included a stratified atmosphere from the solar chromosphere to the corona, tapering magnetic field, and solar gravity for curved loops with the thermal structure modelled by plasma thermal conduction, along with optically thin radiation and anomalous resistivity for 50 Mm flux tubes. Aims. Using 3D MHD modeling, this work addresses predictions for the extreme-ultraviolet (EUV) imaging spectroscopy of such structure and evolution of a loop, with an average temperature of 2–2.5 MK in the solar corona. We set a particular focus on the forthcoming MUSE mission, as derived from the 3D MHD modeling. Methods. From the output of the numerical simulations, we synthesized the intensities, Doppler shifts, and non-thermal line broadening in 3 EUV spectral lines in the MUSE passbands: Fe IX 171 Å, Fe XV 284 Å, and Fe XIX 108 Å, emitted by ∼1 MK, ∼2 MK, and ∼10 MK plasma, respectively. These data were detectable by MUSE, according to the MUSE expected pixel size, temporal resolution, and temperature response functions. We provide maps showing different view angles (front and top) and realistic spectra. Finally, we discuss the relevant evolutionary processes from the perspective of possible observations. Reults. We find that the MUSE observations might be able to detect the fine structure determined by tube fragmentation. In particular, the Fe IX line is mostly emitted at the loop footpoints, where we might be able to track the motions that drive the magnetic stressing and detect the upward motion of evaporating plasma from the chromosphere. In Fe XV, we might see the bulk of the loop with increasing intensity, with alternating filamentary Doppler and non-thermal components in the front view, along with more defined spots in the topward view. The Fe XIX line is very faint within the chosen simulation parameters; thus, any transient brightening around the loop apex may possibly be emphasized by the folding of sheet-like structures, mainly at the boundary of unstable tubes. Conclusions. In conclusion, we show that coronal loop observations with MUSE can pinpoint some crucial features of MHD-modeled ignition processes, such as the related dynamics, helping to identify the heating processes.

Solar wind switchbacks are polarity reversals of the magnetic field, recently frequently measured by Parker Solar Probe inside 0.2 AU. In this Letter we show that magnetic switchbacks, similar to those observed by PSP, are reproduced by injecting a time-limited collimated high-speed stream in the Parker spiral. We performed a 2D magnetohydrodynamics simulation with the PLUTO code of a slightly inclined jet at 1000 km s⁻¹ between 5 and 60 R_⊙. The jet rapidly develops a field inversion at its wings and, at the same time, it is bent by the Parker spiral. The match with the radial outward wind field creates two asymmetric switchbacks, one that bends to the anti-clockwise and one that bends to the clockwise direction in the ecliptic plane, with the last one being the most extended. The simulation shows that such S-shaped magnetic features travel with the jet and persist for several hours and to large distances from the Sun (beyond 20 R_⊙). We show the evolution of physical quantities as they would be measured by a hypothetical detector at a fixed position when crossed by the switchback, for comparison with in situ measurements.

Context. A possible key element for large-scale energy release in the solar corona is a magnetohydrodynamic (MHD) kink instability in a single twisted magnetic flux tube. An initial helical current sheet progressively fragments in a turbulent way into smaller-scale sheets. Dissipation of these sheets is similar to a nanoflare storm. Since the loop expands in the radial direction during the relaxation process, an unstable loop can disrupt nearby stable loops and trigger an MHD avalanche. Aims. Exploratory investigations have been conducted in previous works with relatively simplified loop configurations. In this work, we address a more realistic environment that comprehensively accounts for most of the physical effects involved in a stratified atmosphere typical of an active region. The questions we investigate are whether the avalanche process will be triggered, with what timescales, and how will it develop as compared with the original, simpler approach. Methods. We used three-dimensional MHD simulations to describe the interaction of magnetic flux tubes, which have a stratified atmosphere with chromospheric layers, a thin transition region to the corona, and a related transition from high-β to dlow-β regions. The model also includes the effects of thermal conduction and of optically thin radiation. Results. Our simulations address the case where one flux tube amongst a few is twisted at the footpoints faster than its neighbours. We show that this flux tube becomes kink unstable first in conditions in agreement with those predicted by analytical models. It then rapidly affects nearby stable tubes, instigating significant magnetic reconnection and dissipation of energy as heat. In turn, the heating brings about chromospheric evaporation as the temperature rises up to about 10⁷ K, close to microflare observations. Conclusions. This work confirms, in more realistic conditions, that avalanches are a viable mechanism for the storing and release of magnetic energy in plasma confined in closed coronal loops as a result of photospheric motions.