The central message of the collection is that coronal rain and prominences, long treated as separate phenomena, are in fact closely related expressions of the same underlying physics: coronal condensation. Whether a cooling clump of plasma falls back to the solar surface as rain or hangs suspended for days as a prominence depends largely on the magnetic topology in which it forms. Condensations that form in closed, steeply curved loops tend to drain rapidly along field lines as rain, while those that settle into magnetic dips can remain buoyantly supported as prominences. Their different lifetimes and appearances are therefore not arbitrary but are dictated by the interplay of radiative losses, heating, gravity, flows, and magnetic forces, all acting on plasma that spans several orders of magnitude in temperature and density.

The theoretical foundation is revisited by Thomas Waters and Andrew Stricklan, who re-examine the radiative cooling of optically thin plasma and identify a catastrophic cooling mode distinct from the classical thermal instability. This mode is governed by the isochoric instability criterion, which applies to plasma compressed at constant volume, and crucially it cannot be stabilized by thermal conduction. That means condensations can form under conditions where conventional thermal-instability arguments would predict a stable equilibrium. The result widens the theoretical window within which rapid cooling in hot, low-density plasmas should be interpreted, and it may explain observations of abrupt, near-catastrophic temperature drops in coronal loops that standard theory has struggled to accommodate.

A sweeping theoretical and numerical perspective comes from Rony Keppens, Yuhang Zhou, and Chengcai Xia, whose review places prominences and coronal rain within a unified framework of multiphase coronal plasma. They trace how gravity, flows, heating prescriptions, and magnetic topology shape both the linear growth of instabilities and their full nonlinear magnetohydrodynamic evolution. Their assessment is candid about what remains unsolved: the fine structure of prominences, their internal dynamics, and their complete life cycle from birth to eruption remain major challenges for numerical studies. Complementing this, Valeriia Liakh and Jack Jenkins review a decade and a half of prominence and coronal rain modeling with the open-source MPI-AMRVAC code, following the progression from simplified one-dimensional setups to multidimensional simulations that self-consistently produce cool condensations, and charting a path toward three-dimensional models that include partial ionization, realistic radiative transfer, and the full mass cycle between chromosphere and corona.

Radiation becomes a formidable complication once plasma cools enough to become optically thick, because photons can then be absorbed as well as emitted. Petr Heinzel and colleagues demonstrate through non-local thermodynamic equilibrium, or non-LTE, radiative transfer calculations that the optically thin loss functions commonly used in simulations break down for cool, dense condensations. Realistic net radiative rates must include both losses and gains from the absorption of incident radiation, and their models quantify how condensations relax toward radiative equilibrium on timescales that depend on plasma pressure and geometric thickness. In a companion review, Heinzel and Stanislav Gunár survey five decades of non-LTE prominence modeling, from one-dimensional slabs to two-dimensional and emerging three-dimensional multi-thread structures, and argue that coupling dynamic MHD models with multidimensional radiative transfer is now essential for interpreting rapidly moving eruptive prominences captured by modern instruments.

On the observational side, disentangling emission from plasma at wildly different temperatures along the same line of sight is the central challenge. Paolo Antolin and colleagues extend a technique called Response Fitting to separate cool, warm, and hot contributions within several passbands of the Atmospheric Imaging Assembly on the Solar Dynamics Observatory and the slit-jaw imager of the Interface Region Imaging Spectrograph. The method improves the decomposition of the AIA 94 and 304 angstrom channels and allows the hot Fe XXI flare emission contaminating the IRIS 1330 and 1400 angstrom passbands to be estimated. Applied to a joint AIA-IRIS observation, it reveals an approximately seven-fold increase in cool plasma associated with flare-driven coronal rain, and because it is computationally efficient it opens the door to near-real-time thermal diagnostics of the solar atmosphere.

That diagnostic power is already yielding surprises. Seray Şahin and Paolo Antolin used IRIS and SDO/AIA observations of coronal-rain showers neighboring a modest C7.5 flare to test whether a flare can influence loops that are not themselves flaring. They compared the amount, intensity, and velocity of rain across the pre-flare, impulsive, and gradual phases, finding that the number of detected rain events rose by roughly 27 percent from the pre-flare to the impulsive phase, while average intensity and downflow velocity increased by about 17 and 18 percent respectively by the gradual phase. The implication is striking: a flare can alter the thermodynamic conditions of neighboring, apparently quiescent loops, and the tidy distinction between quiescent and flare-driven coronal rain is not always clear-cut.

Thermal non-equilibrium, in which plasma on closed loops undergoes repeated cycles of heating, cooling, and condensation, has also now been caught operating far beyond its usual habitat. Clara Froment and Sophie Masson report long-period extreme-ultraviolet pulsations and recurring coronal-rain showers in a non-eruptive pseudo-streamer observed over roughly two and a half days with SDO/AIA. The ordering of the EUV emission peaks and the appearance of rain near the end of each cooling cycle provide strong evidence for thermal non-equilibrium cycles occurring both beneath the pseudo-streamer dome and in regions dominated by open or large-scale magnetic fields. Continuous interchange reconnection accompanied the evolution, suggesting that the interaction between reconnection and thermal non-equilibrium may govern how condensations are released and transported near open-closed magnetic boundaries, with possible consequences for the solar wind itself.

Two further studies round out the physical picture. Pengfei Chen introduces the evocatively named concept of solar filament physiognomy, the practice of inferring magnetic properties from the appearance and fine structure of filaments in imaging observations, a valuable complement in an era when direct measurements of the coronal magnetic field remain difficult. Lorenzo Melis and Roberto Soler analyze the Kelvin-Helmholtz instability at the interface between partially ionized prominence plasma and the fully ionized corona, showing that compressibility and acoustic effects matter at observed flow speeds and that ambipolar diffusion generally destabilizes the interface, lowering the threshold velocity and broadening the unstable parameter range. And Yuhong Fan’s review of MHD simulations of prominence-forming flux ropes demonstrates that prominence material is no passive tracer: its weight modifies the equilibrium, stability, and eruptive evolution of the flux rope that hosts it, ultimately shaping coronal mass ejections.

Taken together, the collection sketches a field in transition, where observers, theorists, and modelers converge on a single coupled system. Future models must move toward fully three-dimensional treatments that combine MHD with non-LTE radiative transfer, non-equilibrium ionization, and multi-fluid effects, while synthetic observables will be essential for meaningful comparison with the rapidly improving data from the Daniel K. Inouye Solar Telescope, Solar Orbiter, and Proba-3. The ultimate goal is to follow condensations continuously, from their thermodynamic formation through their drainage, suspension, or eruption, and thereby assemble a comprehensive picture of the coronal mass and energy cycle. In that picture, the rain that falls through the Sun’s corona and the great crimson prominences that arch above its limb are not curiosities but signposts pointing to the same fundamental physics.

Subject of Research: Coronal cooling and the formation, evolution, and eruption of cool plasma in the solar corona

Article Title: Coronal Cooling: Rain, Prominences, and Eruptions – Editorial

Article References: Şahin, S., Druett, M. K., Liakh, V., Popescu Braileanu, B., & Rees-Crockford, T. (2026). Coronal Cooling: Rain, Prominences, and Eruptions – Editorial. Solar Physics, 301(10), Article 151. https://doi.org/10.1007/s11207-026-02746-7

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02746-7

Keywords: solar physics, coronal rain, prominences, solar filaments, thermal instability, radiative transfer, MHD simulations, coronal mass ejections, SDO/AIA, IRIS, thermal non-equilibrium, solar corona

The scale of the computational challenge is staggering. Temperature contrasts in astrophysical multiphase systems span roughly ten orders of magnitude, and the cold gas, while dense, can be scattered in tiny structures across enormous volumes. Observations suggest cold clouds in the circumgalactic medium may be as small as tens of parsecs. To fully resolve such structures across a Milky Way-sized halo would require more than a quadrillion resolution elements, a number so vast that no supercomputer on Earth comes close. As a result, cold gas quantities in typical cosmological simulations remain unconverged, and researchers have had to resort to a hierarchy of idealized experiments, from turbulent mixing layers to cloud-crushing simulations, each isolating one piece of the physics.

One of the central results the review highlights concerns the survival of cold clouds blasted by hot winds, the classic ‘cloud crushing’ problem. In the absence of cooling, a cold cloud embedded in a supersonic wind is shredded by Kelvin-Helmholtz and Rayleigh-Taylor instabilities on a characteristic crushing time. Worse, the drag time needed to accelerate the cloud is far longer than the destruction time, giving rise to the long-standing ‘entrainment problem’: how can cold gas be flung to hundreds or even thousands of kilometers per second in galactic winds if it should be destroyed before it can accelerate? The answer, it turns out, lies in radiative cooling. When mixed gas at the interface cools faster than the cloud is disrupted, hot gas condenses onto the cold phase, allowing clouds not only to survive but to grow in mass as they are carried along.

This insight has been crystallized into a simple survival criterion: clouds endure if the cooling time of the mixed gas is shorter than their destruction time, which can be recast as a minimum cloud size of roughly a few parsecs under typical wind conditions. Simulations show that surviving clouds grow continuously through cooling-driven mass transfer in their turbulent wakes, and that this same mixing efficiently transfers momentum, accelerating even dense molecular clouds entrained in galactic outflows. Magnetic fields, once heralded as a potential savior of the entrainment problem, help but are not sufficient on their own for the high density contrasts typical of real astrophysical clouds; combined with cooling, however, they shift the survival threshold by orders of magnitude.

The review also delves into thermal instability, the classic mechanism by which a hot medium can spontaneously fragment into a cold, clumpy phase. When radiative cooling increases as temperature drops, small density perturbations run away into dense clumps. In stratified halo atmospheres, precipitation occurs when the ratio of cooling time to free-fall time falls below a critical value of order ten, a criterion modified by turbulence, halo rotation, magnetic fields and cosmic rays. A related and still contentious question is whether cooling clouds ‘shatter’ into a characteristic scale of tiny fragments or instead undergo a violent pulsation dubbed ‘splattering’ before fragmenting, with recent three-dimensional simulations tending to favor the latter picture.

Scaling up, the review surveys supernova-driven bubbles, stratified ‘tall box’ simulations of galaxy disks, and fully global models of dwarf and Milky Way-mass galaxies. A consistent picture emerges: most outflowing mass travels in the warm phase at around 10,000 Kelvin, while most of the energy is carried by the hot, million-degree gas. Hot gas mass loading factors hover near 0.1 across a wide range of star formation rates, and warm outflows in massive galaxies tend to fall back as fountain flows rather than escaping. Including cosmic rays transforms these results, converting fountains into steady, cooler, denser winds that can double the outflowing mass and substantially reshape the circumgalactic medium.

At the largest scales, the review examines how simulations handle the circumgalactic and intracluster media. In cluster cores, jets from supermassive black holes stir turbulence that triggers local thermal instability, producing ‘chaotic cold accretion’ in which cold filaments rain onto the central galaxy and feed the black hole in a self-regulating cycle. In cosmological zoom-in simulations, a recent revolution has come from ‘super-Lagrangian’ refinement schemes that boost resolution specifically in the halo, reaching below 100 parsecs in the circumgalactic medium. These enhanced-resolution models consistently show more and smaller cool clouds, higher covering fractions of cool gas, and non-converged cloud mass functions, confirming that cold gas structure in halo simulations is far from fully resolved.

What emerges most clearly from this comprehensive synthesis is that the diverse simulation approaches, from idealized mixing layers to full cosmological models, are not competitors but complementary layers of a single framework. Small-scale experiments provide the physical intuition, survival criteria and subgrid prescriptions that large-scale simulations need; large-scale simulations in turn supply the realistic boundary conditions, pressures and turbulence levels under which the small-scale physics operates. The ultimate arbiter, the authors stress, is observation, and connecting simulations to real spectra, emission maps and absorption measurements through radiative transfer remains one of the field’s most demanding tasks.

The challenges ahead are formidable: achieving numerical convergence in multiphase diagnostics, capturing the interplay of magnetic fields, conduction, viscosity and cosmic rays, and resolving the critical scales that govern whether cold gas survives, grows or shatters. But the trajectory is clear. GPU-accelerated codes, adaptive refinement targeted at cooling lengths, and a maturing theoretical framework are converging on a unified picture of the multiphase Universe, one simulation at a time.

Subject of Research: Numerical simulations of multiphase gas dynamics in the interstellar, circumgalactic and intracluster media

Article Title: Simulations of multi-phase gas in and around galaxies

Article References: Gronke, M., & Schneider, E. E. (2026). Simulations of multi-phase gas in and around galaxies. Living Reviews in Computational Astrophysics, 12(1), Article 2. https://doi.org/10.1007/s41115-026-00025-7

Image Credits: AI Generated

DOI: 10.1007/s41115-026-00025-7

Keywords: multiphase gas, galaxy simulations, circumgalactic medium, thermal instability, galactic winds, cloud crushing, turbulent mixing layers, computational astrophysics, radiative cooling, cosmic rays, supernova feedback, Kelvin-Helmholtz instability