The observed solar photospheric magnetic fields and Doppler velocities are frequently used to quantify the Poynting flux and helicity flux. Multiple methods have been developed for this purpose, but their estimates of the Poynting flux and helicity flux often differ from one another. Here we study the performance of three widely used methods on NOAA active region 12673: "poloidal-toroidal decomposition-Doppler-FLCT Ideal," "Differential Affine Velocity Estimator for Vector Magnetograms", and an extension of the latter with Doppler velocity constraint. We find that the values of the accumulated energy and helicity differ significantly between the three methods, even in sign. Using the Helmholtz-Hodge decomposition, we show that Doppler velocity can contribute significantly to the Poynting flux and helicity flux through the noninductive (curl-free) electric field. The different, ad hoc treatments of the Doppler and transverse velocities in the three methods are directly responsible for the discrepancies. We discuss the desired future observations that can better constrain these methods.

Determining the relationship between nanoflare energies and their delays is the key for understanding the physical mechanism of the events and the plasma response. Nanoflares analyzed in this study were generated self-consistently via prescribed photospheric motions in a 3D multi-strand simulation of a subset of active region magnetic flux. Energies and durations were quantified using three distinct methods. In this study, we investigated the correlation between nanoflare energies (E) and delays (τ_D) using two non-parametric, rank-based statistical tests. Across all methods, results consistently show little to no correlation. This is further supported by the distribution of the exponent α in the assumed relation E ∝ τ_D^α, which peaks near zero, and by broad delay distributions within fixed energy bins. These findings are irrespective of whether delays are correlated with the energy of the preceding or subsequent event. They also hold for a subset of high-energy nanoflares. The absence of correlation suggests that nanoflare onset is not solely determined by a critical value of magnetic stress and may involve triggering by other events, perhaps related to a locally complex topology.

We present a community effort to assess how open science can advance heliophysics and space weather modeling. Open science has the potential to enhance the quality and pace of scientific discovery, but its application to scientific modeling requires more careful consideration with respect to open data and open software guidelines, as complex scientific models are not ordinary software. We gathered feedback from modeling teams worldwide through a living survey and discussion sessions at the Open Science Workshop in College Park, USA, in 2024, and the COSPAR ISWAT Initiative Working Meeting in Cape Canaveral, USA, in 2025. We complement these findings with lessons learned from almost 25 years of experience at the Community Coordinated Modeling Center in enabling open use of models. We identify key roadblocks in current open science practices and guidelines and offer recommendations for future progress. Our findings are organized into four overlapping themes: open use of models and simulation results, open validation, open development, and open collaboration. An essential outcome of the discussion is the need for model developers and users to speak with a united voice and promote the role of models in future open science efforts. We introduce a new cross-domain community initiative called Heliophysics Open Modeling Environment (HOME), which will be integrated as an overarching activity within the COSPAR ISWAT Initiative. HOME will serve as a platform for modelers and model users to work together, facilitate community modeling, improve the scientific return on modeling investment, and advance innovation in heliophysics and space weather.

Coronal pseudostreamer flux systems have a specific magnetic configuration that influences the morphology and evolution of coronal mass ejections (CMEs) from these regions. Here we continue the analysis of the Wyper et al. magnetohydrodynamic simulation of a CME eruption from an idealized pseudostreamer configuration through the construction of synthetic remote-sensing and in situ observational signatures. We examine the pre-eruption and eruption signatures in extreme ultraviolet and white light from the low corona through the extended solar atmosphere. We calculate synthetic observations corresponding to several Parker Solar Probe–like trajectories at ∼10 R _⊙ to highlight the fine-scale structure of the CME eruption in synthetic WISPR imagery and the differences between the in situ plasma and field signatures of flank and central CME-encounter trajectories. Finally, we conclude with a discussion of several aspects of our simulation results in the context of interpretation and analysis of current and future Parker Solar Probe data.

Recent observations of the solar atmosphere in cool extreme-ultraviolet lines have reported the prevalence of coronal rain falling from coronal cloud filaments that are associated with the magnetic dips of coronal X-point structures. These filaments mysteriously appear as clouds of mass in the corona that subsequently shrink and disappear due to mass losses that drain as coronal rain along arced field lines. Using a two-and-a-half-dimensional magnetohydrodynamic model, we investigated evaporation-condensation as the formation mechanism of the subset of coronal cloud filaments that form above coronal X-points. Our simulation included the effects of field-aligned thermal conduction and optically thin radiation, and used the state-of-the-art transition region adaptive conduction (TRAC) method to model the formation, maintenance, and mass loss of a filament above a coronal X-point. This paper presents a physical model that demonstrates magnetic reconnection as a filament loss mechanism, producing hybrid filament/coronal rain via mass losses through the X-point. A detailed analysis of how the mass of the filament forces the field to reconnect is also presented, revealing three phases that characterize the evolution of the reconnecting current sheet and associated mass losses. We conclude that the formation of certain coronal cloud filaments and subsequent mass losses via coronal rain can be explained by the evaporation-condensation model combined with filament mass losses forced by magnetic reconnection. We also report that rebound shocks generated by the impact of coronal rain condensations on the chromosphere together with retractive upflows can cause upward-propagating condensations to form through a dynamic thermal runaway process.

We present a detailed analysis of a 3D MHD simulation of a subset of the magnetic flux in an active region. The simulation models the generation of nanoflares and response of the plasma to imposed photospheric motions. Our study focuses on characterizing the energy distribution and occurrence frequency of the nanoflares in the simulation that self-consistently heat the corona. This field line–based analysis reveals that the nanoflare energy distribution (energy per unit cross sectional area) follows a log-normal profile, where low-energy nanoflares are significantly more prevalent than those with high energy. When compared with the plasma cooling time, different energy nanoflares tend to repeat with different frequencies. Low-energy nanoflares repeat at high frequencies, while high-energy nanoflares repeat at low frequencies. However, the thermal evolution of plasma along the individual field lines is governed predominantly by the high-energy nanoflares. These findings provide critical insights into the role of small-scale magnetic reconnection events in heating the solar corona.

The energy that heats the magnetically closed solar corona originates in the complex motions of the massive photosphere. Turbulent photospheric convection slowly displaces the footpoints of coronal field lines, causing them to become twisted and tangled. Magnetic stresses gradually build until reaching a breaking point when the field reconnects and releases a sudden burst of energy. We simulate this basic picture of nanoflares using a high-fidelity, three-dimensional, multi-stranded magnetohydrodynamic simulation that starts with a fully stratified atmosphere. This simulation includes the effects of field-aligned thermal conduction and optically thin radiation and uses the state-of-the-art Transition Region Adaptive Conduction (TRAC) method to capture the response of the plasma to the nanoflare heating. We find that our physical model supports a unified explanation for both the diffuse emission observed in active regions and the bright coronal loops. Specifically, our results suggest that the diffuse emission originates from spatially and temporally uncorrelated nanoflares, whereas coherent clusters of nanoflares - nanoflare storms - are responsible for the formation of bright coronal loops. Quantitative comparisons between the simulated emission and observed characteristics of coronal loops show that key observed properties - such as loop widths, lifetimes and cross sections - are reasonably well reproduced by the model. The idea that avalanche spread naturally leads to circular cross sections in coronal loops is strongly supported. Our results also suggest that phase differences in heating and cooling events across neighboring magnetic flux strands are a plausible explanation for the anomalous cross-field motions of coronal loops that were recently reported in high-resolution observations.

Coronal mass ejections (CMEs) from pseudostreamers represent a significant fraction of large-scale eruptions from the Sun. In some cases, these CMEs take a narrow jet-like form reminiscent of coronal jets; in others, they have a much broader fan-shaped morphology like CMEs from helmet streamers. We present results from a magnetohydrodynamic simulation of a broad pseudostreamer CME. The early evolution of the eruption is initiated through a combination of breakout interchange reconnection at the overlying null point and ideal instability of the flux rope that forms within the pseudostreamer. This stage is characterized by a rolling motion and deflection of the flux rope toward the breakout current layer. The stretching out of the strapping field forms a flare current sheet below the flux rope; reconnection onset there forms low-lying flare arcade loops and the two-ribbon flare footprint. Once the CME flux rope breaches the rising breakout current layer, interchange reconnection with the external open field disconnects one leg from the Sun. This induces a whip-like rotation of the flux rope, generating the unstructured fan shape characteristic of pseudostreamer CMEs. Interchange reconnection behind the CME releases torsional Alfv & eacute;n waves and bursty dense outflows into the solar wind. Our results demonstrate that pseudostreamer CMEs follow the same overall magnetic evolution as coronal jets, although they present different morphologies of their ejecta. We conclude that pseudostreamer CMEs should be considered a class of eruptions that are distinct from helmet-streamer CMEs, in agreement with previous observational studies.

We present the first results of three-dimensional (3D) numerical magnetohydrodynamic (MHD) simulations of the onset of magnetic reconnection via the tearing instability in dynamically thinning current sheets in the solar corona. In all our simulations, the onset of the non-linear tearing instability, which leads to the break-up of the thinning current sheet, does not occur until after the instability growth time becomes faster than the dynamic thinning time. Furthermore, as in previous 3D MHD simulations of static current sheets in the corona, for some parameters, the amount of magnetic shear is a fundamental switch-on parameter, which has consequences for coronal heating models. These results open up the possibility of using observable quantities of coronal current sheets to predict when they will break-up and release magnetic energy to power various energetic phenomena and/or heat the atmosphere.

The thickness of current sheets is extremely important, especially as it relates to the onset of fast magnetic reconnection. Onset determines how much magnetic free energy can build up in a field before it is explosively released. This has implications for many phenomena on the Sun and throughout the universe, including the heating of the solar corona. Significant effort has been devoted to the question of whether equilibrium current sheets in realistic geometries have finite or zero thickness. Using a simple force balance analysis, we show why current sheets without a guide field (2D) and with a guide field that is invariant in the guide field direction (2.5D) cannot be in equilibrium if they have both finite thickness and finite length. We then estimate the conditions under which the tension of a curved line-tied guide field can facilitate equilibrium in 3D sheets that are finite in all dimensions. Finally, we argue that some quasi-statically evolving current sheets undergoing slow stressing (e.g., when the coronal magnetic field is subjected to photospheric boundary driving) may reach a critical shear, at which point they lose equilibrium, spontaneously collapse, and reconnect. The critical shear is generally consistent with the heating requirements of solar active regions.

We present a strategy for answering the question of how the magnetically closed corona is heated. Pursuing this strategy will lead to important new knowledge about fundamental physical processes such as magnetic reconnection and the role of coupling between disparate spatial scales. The strategy is focused on the goal of constructing physics-based models that can nowcast and eventually forecast the solar spectral irradiance at an accuracy that is useful for space weather operations. Such models will also be invaluable for studying the spectral irradiance from other stars and its effect on the development of life and the habitability of planets. We recommend investments that are needed for ultimate success, including a dedicated modeling program and a mission to observe the corona at ultra-high spatial resolution.

Results from 2.5D and 3D studies of the onset and development of the tearing instability are presented, using high fidelity resistive MHD simulations. A limited parameter study of the strength of the reconnecting field (or shear angle) was performed. An initially simple 1D equilibrium was used, consisting of a modified force-free current sheet, with periodic boundary conditions in all directions. In all cases, the linear and non-linear evolution led to a primary current sheet between two large flux ropes. The global reconnection rate during this later stage was analyzed in all simulations. It was found that in 2.5D the primary current sheet fragmented due to plasmoids, and as expected, the global reconnection rate, calculated using multiple methods, increases with the strength of the reconnecting field due to a stronger Alfvén speed. In 3D, the presence of interacting oblique modes of the tearing instability complicates the simple 2.5D picture, entangling the magnetic field of the inflow and introducing a negative effect on the reconnection rate. The two competing effects of stronger Alfvén speed and entangling, which both increase with the strength of the reconnecting field, resulted in a decrease in the reconnection rate with increasing reconnecting field. For all simulations, the 3D rates were less than in 2.5D, but suggest that as one goes to weak reconnecting field (or strong guide field), the system becomes more 2.5D like and the 2.5D and 3D rates converge. These results have relevance to situations like nano-flare heating and flare current sheets in the corona.

The connection between energy inputs and the generation of ion upflows and outflows is a topic of keen scientific interest and the subject of a number of empirical studies. Despite this interest, it remains uncertain how different ion species respond to energy input, what defines the upper and lower bounds of the ion flux, and what role solar illumination plays in regulating the relationship between energy input and ion upflows/outflows. This work simulates how ion flux scales with low and high altitude energization, and to a combination of both. Furthermore, we examine the influence of solar illumination on these relationships by considering how the scaling of ion flux with energy input changes over the solar cycle, comparing solar minimum and maximum, as well as how they change from day to night conditions. We find O+ flux tends to respond more strongly to energy inputs than H+ flux, with the O+ flux often exhibiting a lower activation energy and a greater dynamic range. The lower bound of the ion flux at 4,000 km is typically defined by the polar wind H+, although O+ upflows can dominate at low altitudes in the presence of significant frictional heating of the ion gas or soft electron precipitation. However, when significant soft electron precipitation and wave‐particle interactions are present simultaneously the lower bound of the ion flux at 4,000 km is defined by the O+. Finally, we find a difference between the steady state response of the outflow to energy input and the peak response.

Magnetic reconnection, a fundamentally important process in astrophysics, is believed to be initiated by the tearing instability of an electric current sheet, a region where magnetic field abruptly changes direction. Recent studies have suggested that the amount of magnetic shear in these structures is a critical parameter for the switch-on nature of magnetic reconnection in the solar atmosphere, at large spatial scales. We present results of visco-resistive magnetohydrodynamic simulations of magnetic reconnection in 3D current sheets with conditions appropriate to the solar corona. We follow the evolution of the linear and nonlinear 3D tearing instability. We find that, depending on the parameter space, magnetic shear can play a vital role in the onset of significant energy release and plasma heating. Two regimes in our study exist, dependent on whether the current sheet is longer or shorter than the wavelength of the fastest growing mode, thus determining whether subharmonics are present in the actual system. In one parameter regime, where the fastest growing parallel mode has subharmonics, the subsequent coalescence of 3D plasmoids dominates the nonlinear evolution, with magnetic shear playing only a weak role in the amount of energy released. In the second parameter regime, where the fastest growing parallel mode has no subharmonics, only strongly sheared current sheets, where 3D effects are strong enough, show any significant energy release. We expect both regimes to exist on the Sun, and so our results have important consequences for the question of reconnection onset in various solar physics applications.

Ionospheric heating experiments using high-frequency ordinary (O)-mode electromagnetic waves have shown the induced formation of magnetic field-aligned density striations in the ionospheric F region, in association with lower hybrid (LH) and upper hybrid (UH) turbulence. In recent experiments using high-power transmitters, the creation of new plasma regions and the formation of descending artificial ionospheric layers (DAILs) have been observed. These are attributed to suprathermal electrons ionizing the neutral gas, so that the O-mode reflection point and associated turbulence is moving to a progressively lower altitude. We present the results of two-dimensional (2-D) Vlasov simulations used to study the mode conversion of an O-mode pump wave to trapped UH waves in a small-scale density striation of circular cross section. Subsequent multiwave parametric decays lead to UH and LH turbulence and to the excitation of electron Bernstein (EB) waves. Large-amplitude EB waves result in rapid stochastic electron heating when the wave amplitude exceeds a threshold value. For typical experimental parameters, the electron temperature is observed to rise from 1,500 K to about 8,000 K in a fraction of a millisecond, much faster than Ohmic heating due to collisions which occurs on a timescale of an order of a second. This initial heating could then lead to further acceleration due to Langmuir turbulence near the critical layer. Stochastic electron heating therefore represents an important potential mechanism for the formation of DAILs.

Ionospheric modification experiments conducted using high frequency, L-O mode electromagnetic waves have shown the induced formation of magnetic field-aligned density striations in the ionospheric F-region 1 , 2 . These striations are observed in association with lower and upper-hybrid turbulence and significant electron heating within the striation. The initial electron heating to temperatures of $>4000\mathrm {K}$ is believed to be a prerequisite for the formation of suprathermal electron tails by strong Langmuir turbulence 3 . Such tails can result in the ionisation of neutrals and the formation of descending artificial ionospheric layers (DAILS) 4 . In the current context, we present the results of 1D and 2D numerical simulations conducted using a VlasovMaxwell code 5 to study the mode-conversion / coupling of an L-O mode pump wave to trapped upper hybrid waves within a density striation. Subsequent multi-wave parametric decay is observed leading to lower-hybrid turbulence and high amplitude electron Bernstein waves which (once exceeding the threshold amplitude for stochasticity) can result in significant electron heating. The electron temperatures observed of $>5000\mathrm {K}$ in simulation are sufficient to initialise the formation of suprathermal electron tails as a precursor to generating descending artificial ionospheric layers (DAILS).

This study presents a theoretical approach to calculate the inductive electric field, and it is further applied to global MHD simulations of the magnetosphere. The contribution of the inductive component to the total electric field is found by decomposing the motional electric field into a superposition of an irrotational and a solenoidal vector and assuming that the time‐varying magnetic field vanishes on the boundary. We find that a localized change in the magnetic field generates an inductive electric field whose effect extends over all space, meaning that the effect of the inductive electric field is global even if the changes in the magnetic field are localized. Application of this formalism to disturbed times provides strong evidence that during periods of increased activity the electric field induced by the localized change in magnetic field can be comparable to (or larger than) the potential electric fields in certain regions. This induced field exhibits significant spatial and temporal variations, which means that particles that drift into different regions of space are being exposed to different means of acceleration. These results suggest that the inductive electric field could have a substantial contribution to particle energization in the near‐Earth region even though the changes in the magnetic fields occur at distances of several tens of Earth radii. This finding is particularly important for ring current modeling which in many cases excludes inductive contributions to the total particle drift.