Studying coronal rain formation through thermal non-equilibrium (TNE) and thermal instability (TI) provides insights into coronal heating mechanisms. We analysed a quiescent coronal rain event using space-based observations from the High-Resolution Imager in Extreme Ultraviolet (HRIEUV) of Solar Orbiter (SolO), the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO), and the Slit-Jaw Imager (SJI) from the Interface Region Imaging Spectrograph (IRIS) from November 1st, 2023. During the coronal rain shower, the coronal loop exhibits substantial EUV variability and structural changes. Rain clumps fell at 72 − 87 km s−1 with cool EUV absorbing core sizes of ≈600 km and densities of ≈6 × 1011 cm−3 preceded by strong compressions. These mostly isothermal compressions suggest energy transfer into the rain, decelerating it and possibly reducing cooling rates – consistent with accretion braking timescales. The shower carried microflare-level energy (4.64 × 1026 erg), with clumps producing impacts that reach the lower transition region and are visible across all EUV channels and in SJI 1400 Å. The impacts generated hot rebound flows (106.2 − 106.3 K, 85 − 87 km s−1) that refilled and reheated the loop but carried less than 15 % of the clumps’ kinetic energy. We detected steady footpoint heating signatures consistent with the TNE-TI scenario, with an estimated amplitude of 10−2 ± 0.3 erg cm−3 s−1 and heating scale heights of 2 − 10 Mm, matching active region values. Coronal rain may thus serve as both a template for accretion braking and a proxy for integrated heating driving TNE-TI cycles.

The Atmosphere Imaging Assembly (AIA) 304 & Aring; channel aboard the Solar Dynamic Observatory offers an unparalleled full-disk view of cool material at T approximate to 10(5) K emitted by the He II 304 & Aring; spectral line. This opens the possibility for the in-depth and widespread analysis of the formation and evolution of small cool structures seen in the solar atmosphere. Of particular interest is the phenomenon of coronal rain, which has been linked to the overarching heating and cooling cycles of the solar corona. However, within the channel's passband, hot diffuse emission from several ions is also included, leading to comparable intensity levels to the cool emission, particularly off-limb. This makes it very difficult to disentangle cool coronal rain from this hotter material. In this paper a novel morphological approach to separating these components called DeepFilter is investigated. This approach utilises a generative machine learning algorithm that can learn how to convert the AIA 304 & Aring; images into the style of images obtained with the Interface Region Imaging Spectrograph (IRIS) 1400 & Aring;, which has a similar temperature formation peak as for He II 304 & Aring; but lacks this hot-component contamination. We find that the method produces good results, showing a clear reduction in the amount of hot-component material present in the final images while preserving the majority of the underlying cool structures. DeepFilter is compared to the recent physics-based RFit algorithm and is found to produce comparable results. Although the DeepFilter method is shown to perform worse at removing hot emission and material far from the limb, it performs comparably on other data - with the advantage of being far less data intensive - which makes it more effective for large-scale statistical analysis.

Accurate temperature diagnostics of the solar corona are necessary for detecting the heating and cooling processes, and better understanding the conversion of the magnetic energy into thermal energy. A major obstacle in this enterprise is the multi-temperature emission contained in ultraviolet (UV) and extreme UV (EUV) passbands such as those of the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO) and the Slit-Jaw Imager (SJI) of the Interface Region Imaging Spectrograph (IRIS). In this work we extend the Response Fitting (RFit) method to disambiguate between cool, warm, and hot emission in the SDO/AIA and IRIS/SJI passbands. We improve previous results for AIA 304 Å and find very good cool/hot decomposition for AIA 94 Å allowing to improve previous empirical disambiguation methods for this passband. The hot temperature coverage of AIA allows RFit to be applied across instruments. This allows to disambiguate the hot flaring emission from Fe xxi contained within the SJI 1330 Å and SJI 1400 Å passbands, supported by IRIS spectrograph (SG) results. We further estimate that the SJI 1330 Å response function lacks ≈ 50% of emission at temperatures above log T=7 . Photospheric abundances help reduce this gap to ≈ 40% . The emission peak at log T=6.2 in the SJI response functions is greatly altered at different densities, which can be used, in principle, for density diagnostics. These results are basically identical to those with the Differential Emission Measure (DEM) method, with the advantage that RFit is instantaneous, unlocking the possibility of real-time measurements. We also provide lower bounds for the hot emission in AIA 211 Å, and the very hot (flaring) emission in AIA 131 Å and AIA 193 Å, which help constrain the overestimated emission from the DEM in the hot temperature range. We apply RFit to an AIA-IRIS co-observation that includes a flare, and calculate the average relative percentage contribution for the cool-hot emission to find 17/83, 96/4, 79/21, 10/90, 65/35, 65/35, respectively for AIA 94 Å, AIA 131 Å, AIA 211 Å, AIA 304 Å, SJI 1330 Å and SJI 1400 Å. We obtain a more coherent picture of the hot temperature evolution in the log T =6.8 – 7.15 temperature range and its spatial localisation during the flare, and similarly for the cooling during the gradual phase. We further accurately detect and quantify the cool plasma from coronal rain, which is observed to increase seven-fold due to the flare-driven cooling.

Detection and characterization of small-scale energetic events such as nanoflares and nanojets remain challenging owing to their short lifetimes, small spatial extent, and relatively low energy release, despite their potential role in coronal heating. Recent observations have identified nanojets as small-scale (length ≲6.6 Mm, width ≲1 Mm), fast (∼ few 100 km s ^−1 ), and short-lived (≲30 s) ejections associated with nanoflare-scale energies, providing evidence of magnetic reconnection at small spatial scales. However, the lack of synthetic diagnostics has limited the connection between magnetohydrodynamic (MHD) models and observations. In this Letter, we present synthetic observations of the coalescence of two flux ropes, leading to nanojet-like signatures from a numerical model obtained with the MPI-AMRVAC code. We report synthetic observables in extreme-ultraviolet lines compatible with existing instruments such as Solar Dynamics Observatory/Atmospheric Imaging Assembly and the upcoming Multi-slit Solar Explorer mission and compare the synthetic observables with an existing observation of nanojets. The synthetic diagnostics of the emissivity maps, Doppler velocity, and thermal and nonthermal line broadening produce key observational properties, suggesting a plausible 3D scenario for nanojet generation where tiny flux ropes reconnect within loops. Our results provide predictions for the detectability of nanojets with current and future spectroscopic facilities and establish a bridge between MHD modeling and observations.

Elemental abundances in solar flares are observed to vary both spatially and temporally, but the underlying mechanisms remain poorly understood. The interplay between advection and the preferential acceleration of low first ionization potential (FIP) elements likely shapes the observed abundance distributions. Models of the FIP effect predict enhancements near loop footpoints that diffuse upward over time. We simulate strong evaporation events that advect this low-FIP enhancement into the corona. When the enhancement is sharply peaked, the corona does not become fractionated, exhibiting only a localized abundance peak near the loop apex that facilitates coronal rain formation. By contrast, a broad enhancement with relatively weak heating yields a uniformly fractionated corona, which is not sufficient for coronal rain formation. As the heating rate increases, the low-FIP enhanced plasma is increasingly compressed towards the loop apex, and coronal rain is able to form. These results suggest a potential observational correlation between the presence and amount of coronal rain, the strength of flare heating and the fractionation process itself. This article is part of the Theo Murphy meeting issue 'Solar atmospheric abundances in space and time'.

Small-scale eruptive phenomena in the solar corona including miniature flux ropes and associated cool plasma condensations are not fully understood despite increasing high-resolution observations. We perform forward modeling based on a 2.5D MHD simulation capturing homologous flux rope eruptions, in-situ condensation leading to a mini-prominence, and subsequent post-flare coronal rain. Synthetic diagnostics are obtained using optically-thin EUV and UV emissions, and non-LTE radiative transfer treatment for the Hα line. The synthetic EUV emission reveals the flux ropes as bright rim-like structures. The corresponding UV diagnostic shows bright region, which is co-spatial with the dark core due to embedded cool plasma (∼ tens of kK) inside the flux rope, identifying an erupting mini-prominence. Spectral synthesis of Si IV 1402.77 A indicates an upward motion of the mini-filament, and reveals the presence of two predominant velocity components during eruption. At a later stage, thermal instability in post-flare arcades produces coronal rain with temperatures of ≈ 10^4 K. The EUV diagnostics reveal brightening at the downstream of the rain blob, indicating localized heating associated with compressional effects. The Hα spectral synthesis shows enhanced absorption signatures and red-shifted profiles corresponding to downflows of the coronal rain blobs up to ≈ 23 km s^-1, whereas the Si IV 1402.77 A spectral profile shows the maximum downflow velocity of ≈50 km s^-1, highlighting the evidence of thermodynamic and kinematic structuring within the falling rain blobs. The synthetic diagnostics provide clear, multi-wavelength signatures that can guide future high-resolution observations, and highlight the importance of small-scale reconnection-driven processes in shaping the multi-thermal structure (between MK to kK) of the solar corona.

Context. Erupting flux ropes play a crucial role in powering a wide range of solar transients, including flares, jets, and coronal mass ejections. These events are driven by the release of stored magnetic energy, facilitated by the shear in complex magnetic topologies. However, the mechanisms governing the formation and eruption of flux ropes, particularly the role of magnetic shear distribution in coronal arcades, are not fully understood. Aims. We investigate how the spatial distribution of magnetic shear along coronal arcades influences the formation and evolution of eruptive flux ropes, with a focus on the evolution of mean shear during different phases of the eruption process. Methods. We employed 2.5D resistive magnetohydrodynamic (MHD) simulations incorporating nonadiabatic effects of optically thin radiative losses, magnetic field-aligned thermal conduction, and spatially varying background heating in order to realistically model the coronal environment. A stratified solar atmosphere under gravity was initialized with a non-force-free field comprising sheared arcades. We studied two different cases by varying the initial shear to analyze their resulting dynamics and the possibility of flux rope formation and eruptions. Results. Our results show that strong initial magnetic shear leads to spontaneous flux rope formation and eruption via magnetic reconnection, driven by the Lorentz force. The persistence and distribution of shear along the arcades are crucial in determining the formation and onset of flux rope instabilities. The shear distribution infers the non-potentiality distributed along arcades and demonstrates its relevance in identifying sites prone to eruptive activity. We have explored the evolution of mean shear and the relative strength between guide field and reconnection field during the pre- and post-eruption phases, with implications of bulk heating for the "hot onset" phenomena in flares, and particle acceleration. In contrast, the weaker shear case does not lead to the formation of any flux ropes. Conclusions. The spatial distribution of magnetic shear and its evolution and mean shear play a decisive role in the dynamics of flux rope formation and eruption. Our findings highlight the limitations of relying solely on footpoint shear and underscore the need for coronal-scale diagnostics. These results are relevant for understanding eruptive onset conditions and can promote a better interpretation of coronal observations from current and future missions.

Sunspots are intense regions of magnetic flux that are rooted deep below the photosphere. It is well established that sunspots host magnetohydrodynamic waves, with numerous observations showing a connection to the internal acoustic (or p- )modes of the Sun. The p- modes are fast waves below the equipartition layer and are thought to undergo a double mode conversion as they propagate upward into the atmosphere of sunspots, which can generate Alfvénic modes in the upper atmosphere. We employ 2.5D magnetohydrodynamic numerical simulations to investigate the adiabatic wave propagation and examine the resulting power spectra of coronal Alfvénic waves. A broadband wave source is used, which has a 1D power spectrum mimicking aspects of the observed p- mode power spectrum. We examine magnetoacoustic wave propagation and mode conversion from the photosphere to the corona. Frequency filtering of the upwardly propagating acoustic waves is a natural consequence of a gravitationally stratified atmosphere and plays a key role in shaping the power spectra of mode-converted waves. We demonstrate that the slow and fast magnetoacoustic waves and Alfvén waves above the equipartition layer have similarly shaped power spectra, which are modified versions of the driver spectrum. Notably, the results reveal that the coronal wave power spectra have a peak at a higher frequency than that of the underlying p -mode driver. This matches observations of coronal Alfvénic waves and further supports the role of the mode conversion process as a mechanism for Alfvénic wave generation in the Sun's atmosphere.

Magnetic reconnection is a proposed mechanism for nanojets associated with coronal heating. We investigate the characteristics of reconnection-driven nanojets just before and during a prominence eruption using the High Resolution Imager (HRI) of the Extreme Ultraviolet Imager (EUI) aboard Solar Orbiter during its perihelion on 2024 September 30. Extreme-ultraviolet (EUV) images at unprecedented high spatial and temporal resolution from HRIEUV were analysed. The dimensions and propagation speeds of nanojets were estimated and used to estimate the kinetic energies. Nanojet activity was compared with the Geostationary Operational Environmental Satellite (GOES) X-ray flux to assess its relation to flare evolution. The high spatial and temporal resolution in the EUV was found to be essential to fully capture the properties and numbers of reconnection nanojets. Approximately 120 nanojets were detected during the eruption, with 40 analysed in detail. Nanojets exhibited lengths of 200-5000 km, widths of 200-500 km, and durations of 2-12 s. Instant velocities ranged from 150-600 km s(-1) with kinetic energies reaching 1 . 56 x 10(27) erg. These nanojets are faster, longer, more energetic, and more numerous compared to previous studies. We also find clear signatures of acceleration and deceleration, reflecting magnetic tension release, and reach of new equilibria. Reconnection events during the eruption were found to be more frequent and energetically intense. Pre-flare nanojet clustering indicates small-scale reconnection may precede large eruptive activity. These results suggest that nanojets also occur in fully ionized coronal plasma, playing a role in both quiescent and eruptive solar activity.

Understanding the processes associated with coronal rain due to the thermal non-equilibrium (TNE) and thermal instability (TI) scenario can help us understand coronal heating. We aim to study the properties of a quiescent coronal rain event and its effect on the solar atmosphere. We utilise space-based data from the High-Resolution Imager in Extreme Ultraviolet of Solar Orbiter, the Atmospheric Imaging Assembly of the Solar Dynamics Observatory, and the Slit-Jaw Imager (SJI) from the Interface Region Imaging Spectrograph from November 1st, 2023. During the coronal rain shower, the coronal loop exhibits large EUV variability and drastic changes in sub-structure. Coronal rain clumps with total velocities between 72 km s^-1 and 87 km s^-1 and cool EUV absorbing core sizes of ≈600 km and densities of ≈5×10^11 cm^-3 are seen to fall with a strong compression ahead. During the compression we measure a low polytropic index with γ=1.085, suggesting the presence of molecules. The rain shower carries a total of 3.09×10^26 erg, and the clumps produce impacts seen in all EUV channels and in SJI 1400 Å. The impacts generate hot rebound flows with temperatures of 10^6.2-10^6.3K and velocities of 85-87 km s^-1, which refill and reheat the loop but carry less than 20% of the clumps' kinetic energies. We find signatures of a steady footpoint heating, in agreement with the TNE-TI scenario, with an estimated amplitude of 2.56×10^-2erg cm^-3 s^-1 in agreement with active region estimates. Coronal rain may therefore be a good proxy for the total integrated heating that gives birth to TNE-TI.

Elemental abundances in solar flares are observed to vary both spatially and temporally, but the underlying mechanisms remain poorly understood. There is an interplay between advection and the preferential acceleration of low first ionization potential (FIP) elements that likely shapes the observed abundance distributions. Models of the FIP effect predict enhancements near loop footpoints that diffuse upward over time. We simulate strong evaporation events that advect this low-FIP enhancement into the corona. When the enhancement is sharply peaked, the corona does not become fractionated, exhibiting only a localized abundance peak near the loop apex that facilitates coronal rain formation. In contrast, a broad enhancement with relatively weak heating yields a uniformly fractionated corona, which is not sufficient for coronal rain formation. As the heating rate increases, the low-FIP material is increasingly compressed toward the loop apex, and rain is able to form. These results suggest a potential observational correlation between the presence and amount of coronal rain, the strength of flare heating, and the fractionation process itself.

Space weather refers to both the dynamic conditions in interplanetary space driven by the Sun and its subsequent impact on the near-Earth environment. Space weather can disrupt or destroy ground- and space-based infrastructure. Although not fully understood, its significance is growing rapidly with our increasing dependence on modern technology.

Sergey Belov, Dmitrii Kolotkov, Patrick Antolin and Anne-Marie Broomhall discuss problems of thermal regulation in the Sun's atmosphere

Solar flares are energetic phenomena that influence coronal plasma dynamics through the magnetic reconnection-driven large-scale reconfiguration, heating and particle acceleration. Even though the energy release is usually strongly localised, it is well known that the flaring can impact a large part of the solar atmosphere through e.g. fast MHD shocks and particle acceleration. Coronal rain is a well known product of strongly stratified heating, seen in quiescent (non-flaring) and flaring conditions. This study investigates quiescent rain showers neighboring a flare site, focusing on their temporal evolution across the pre-flare, impulsive, and gradual phases. Using high-resolution imaging from the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO), we perform a quantitative comparison of rain quantity, intensity, and velocity before and after a C7.5 flare. Our results reveal an increase of approximately 27% in the average number of rain events from pre-flare to impulsive phases, suggesting a possible causal link with the flare perturbations. Besides, a significant increase in both average intensity and downflow velocity by 17% and 18%, respectively, from pre-flare to the gradual phases, suggesting a possible flare-induced density enhancement in the neighbouring coronal rain. These findings highlight the potential of using rain as a sensitive indicator of magnetic or thermodynamic changes, primarily governed by internal loop dynamics, but potentially influenced by external, flare-related perturbations.

The AIA~304 channel on board the \textit{Solar Dynamics Observatory} (SDO) offers a unique view of low transition region (TR) plasma emitting in the He II 304 ˚A line. However, when observing off-limb, the emission of the (small) cool structures in the solar atmosphere (such as spicules, coronal rain and prominence material) can be of the same order as the surrounding hot coronal emission from other spectral lines included in the 304 passband, particularly over active regions. In this paper we investigate three methods based on temperature and morphology that are able to distinguish the cool and hot emission within the 304 passband. The methods are based on the Differential Emission Measure (DEM), a linear decomposition of the AIA response functions (RFit) and the Blind Source Separation (BSS) technique. All three methods are found to produce satisfactory results in both quiescent and flaring conditions, largely removing the diffuse corona and leading to images with cool material off-limb in sharp contrast with the background. We compare our results with co-aligned IRIS data in the SJI 1400 and 2796 channels, and find the RFit method to best match the quantity and evolution of the cool material detected with \textit{IRIS}. Some differences can appear due to plasma emitting in the log T = 5.1 − 5.5 temperature range, particularly during the catastrophic cooling stage prior to rain appearance during flares. These methods are in principle applicable to any passband from any instrument suffering from similar cool and hot emission ambiguity as long as there is good coverage of the hot temperature range.

A major coronal heating theory based on magnetic reconnection relies on the existence of braided magnetic field structures in the corona. In this small-angle reconnection scenario, numerical simulations indicate that the reconnected magnetic field lines are driven sideways by magnetic tension and can overshoot from their new rest position, thereby leading to low-amplitude transverse MHD waves. This provides an efficient mechanism for transverse MHD wave generation, and the direct causality also constitutes substantial evidence of reconnection from braiding. However, this wave-generation mechanism has never been directly observed. Recently, the telltale signature of small-angle reconnection in a sheared coronal structure has been identified through nanojets, which are small, short-lived, and fast jetlike bursts in the nanoflare range transverse to the guide field. We present for the first time Interface Region Imaging Spectrograph and Solar Dynamics Observatory observations of transverse MHD waves in a coronal loop that directly result from braiding-induced reconnection. The reconnection is identified by the presence of nanojets at the loop apex that release nanoflare-range energy. We find that the oscillations have an energy flux on the order of 10 ^6 –10 ^8 erg cm ^−2 s ^−1 , which is within the budget to power active region loops. The estimated kinetic and thermal energy from the nanojets is also sufficient to power the transverse waves and sustain the observed heating at the loop apex. This discovery provides major support to (a) existing theories that transverse MHD waves can be a signature of reconnection, (b) the existence of braiding in coronal structures, and (c) the coronal reconnection scenario identified by nanojets.

Context. Traditional models of coronal oscillations rely on a modelling of the coronal structures that support them as compact cylindrical waveguides. An alternative model of the structure of the corona has recently been proposed, in which the thin strand-like coronal loops, that are observed in the extreme-UV (EUV) emission are the result of the line-of-sight integration of warps in more complex coronal structures. This is referred to as the coronal veil model. Aims. We extend the implications of the coronal veil model of the solar corona to models of coronal oscillations. Methods. Using convection-zone-to-corona simulations with the radiation-magnetohydrodynamics (rMHD) code Bifrost, we analysed the structure of the self-consistently formed simulated corona. We focused on the spatial variability of the volumetric emissivity of the Fe IX 171.073 & Aring; EUV line and on the variability of the Alfv & eacute;n speed, which captures the density and magnetic structuring of the simulated corona. We traced features associated with large magnitudes of the Alfv & eacute;n speed gradient, which trap MHD waves and act as coronal waveguides. We searched for the correspondence with emitting regions, which appear as strand-like loops in the line-of-sight-integrated EUV emission. Results. We find that the cross sections of the waveguides bounded by large Alfv & eacute;n speed gradients become less circular and more distorted with increasing height in the solar atmosphere. The waveguide filling factors corresponding to the fraction of the waveguides filled with plasma that emits in the given EUV wavelength range from 0.09-0.44. This suggests that we can only observe a small fraction of the waveguide. Similarly, the projected waveguide widths in the plane of the sky are several times larger than the widths of the apparent loops that are observed in the EUV. Conclusions. We conclude that the coronal veil structure is independent of the model. As a result, we find a lack of straightforward correspondence between peaks in the integrated emission profile that constitute apparent coronal loops and regions of plasma bound by a large Alfv & eacute;n speed gradient that act as waveguides. Coronal waveguides cannot be reliably identified based on emission in a single EUV wavelength is not reliable in the simulated corona formed in convection-zone-to-corona models.

The synthesis of porous, lattice, or microstructure geometries has captured the attention of many researchers in recent years. Implicit forms, such as triply periodic minimal surfaces (TPMS) has captured a significant attention, recently, as tiles in lattices, partially because implicit forms have the potential for synthesizing with ease more complex topologies of tiles, compared to parametric forms. In this work, we show how variable offsets of implicit forms could be used in lattice design as well as lattice analysis, while graded wall and edge thicknesses could be fully controlled in the lattice and even vary within a single tile. As a result, (geometrically) heterogeneous lattices could be created and adapted to follow analysis results while maintaining continuity between adjacent tiles. We demonstrate this ability on several 3D models, including TPMS.

It is cold (relatively), dense, and it moves engimatically through the solar corona. Turn your solar umbrella upside down to take in the coronal rain... Abstract It is cold (relatively), dense, and it moves engimatically through the solar corona. Turn your solar umbrella upside down to take in the coronal rain...