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 study, led by Garima Karki of Kumaun University in Nainital, India, together with Brigitte Schmieder of the Observatoire de Paris and KU Leuven, Ramesh Chandra, Pooja Devi, Pascal Démoulin and Stefaan Poedts, focused on a quiescent filament observed on September 28, 2023. Quiescent filaments are the long-lived, relatively stable cousins of the structures that sometimes erupt as the engines of coronal mass ejections. They consist of dense, cool plasma at chromospheric temperatures, roughly 10,000 kelvin, embedded within the million-degree corona and supported against gravity by magnetic fields. Understanding how plasma enters, circulates within, and drains out of these structures is a central problem in solar physics, because the mass budget of a filament over its lifetime cannot be explained by static conditions alone. Material must be continuously supplied and removed, and the mechanisms responsible remain actively debated.

The key instrument in this investigation was the Télescope Héliographique pour l’Étude du Magnétisme et des Instabilités Solaires, or THEMIS, located on the Canary Island of Tenerife. THEMIS observed the filament in the H-alpha spectral line, the workhorse diagnostic of cool solar plasma, using its Multi-channel Subtractive Double Pass spectrograph in a scanning mode that delivered both high spatial resolution and high spectral resolution. Crucially, the THEMIS field of view happened to be centred on the northern end of the filament at the precise moment when one of a series of repetitive plasma motions swept through it. That coincidence of timing and pointing allowed the team to capture spectra of an elongated cool blob as it fell, and those spectra revealed H-alpha line profiles that were dramatically broadened and shifted, the spectroscopic fingerprints of extremely fast motion.

To convert those distorted line profiles into a velocity, the researchers employed a technique known as the cloud model, a classical method in solar spectroscopy that treats an individual absorbing structure as a uniform slab of plasma with well-defined properties: a source function, a Doppler width, a line-of-sight velocity, and an optical thickness. By fitting synthetic profiles to the observed ones and minimising the root-mean-square difference between model and measurement, the team derived a downflow speed of 89.8 kilometres per second, with a remarkably tight standard deviation of plus or minus 0.25 kilometres per second. For context, the sound speed in the cool filament plasma is on the order of 10 kilometres per second, so the falling material was moving at roughly nine times the local sound speed, unambiguously supersonic. Such speeds are rarely measured with this degree of confidence in quiescent filament structures, and the precision of the cloud-model fit lends the result considerable weight.

The supersonic measurement alone would be noteworthy, but the broader context of the observation makes it genuinely intriguing. Using data from the Global Oscillation Network Group, or GONG, a worldwide network of instruments that monitors the Sun in H-alpha around the clock, the team tracked plasma moving along the filament body toward its northern end. These repetitive motions suggested a conveyor-bear-like flow of material along the filament’s internal magnetic structure. When the flowing plasma reached the region imaged by THEMIS, it appeared to be falling toward the chromosphere. The researchers first considered a ballistic interpretation: material launched or displaced along the filament’s magnetic dips simply falling under gravity along a curved trajectory. A ballistic path could, in principle, account for the observed acceleration and speed, painting a picture of cool plasma draining from the filament body down into the dense lower atmosphere.

However, the team could not rule out a second and arguably more exotic scenario: coronal rain. Coronal rain occurs when plasma trapped in coronal magnetic loops undergoes a thermal instability. Under certain conditions, the delicate balance between heating and radiative cooling in the corona breaks down, and localised pockets of plasma cool catastrophically from millions of kelvin to chromospheric temperatures in a matter of minutes. The resulting condensations become dense and heavy, lose magnetic support, and plummet along the curved field lines of the hosting loop system, often appearing as shimmering showers of dark blobs in H-alpha and other cool-line observations. Coronal rain is now recognised as a ubiquitous phenomenon in the solar corona, and numerical simulations with modern magnetohydrodynamic codes have reproduced its formation in striking detail, including its connection to cycles of heating and cooling in coronal loops.

A crucial clue linking the falling plasma to the hot corona came from the Atmospheric Imaging Assembly, AIA, aboard NASA’s Solar Dynamics Observatory. In AIA’s hot extreme-ultraviolet channels, which sample plasma at temperatures of a million kelvin and above, the team identified a coronal bright point at exactly the same location as the supersonic downflow, with the temperature there reaching approximately 6 million kelvin. Coronal bright points are small, compact regions of enhanced emission associated with magnetic bipoles and frequent small-scale reconnection events. The spatial coincidence between a 6-million-kelvin bright point and a stream of 10,000-kelvin plasma falling at 90 kilometres per second suggests a physical connection: heating and reconnection at the bright point may destabilise nearby coronal plasma, triggering the thermal condensation that then rains down, or alternatively may inject material that subsequently follows a ballistic descent along the filament’s magnetic architecture.

The THEMIS spectra also confirmed a phenomenon that has fascinated solar physicists for decades: counter-streaming flows. Along the fine internal strands of the filament, which the high-resolution observations resolved to widths of less than one arcsecond, plasma was observed flowing in opposite directions simultaneously. Counter-streaming was first documented in prominences in the late 1990s and has since been attributed to various mechanisms, including intermittent heating events at the filament’s footpoints, waves propagating along the strands, and networks of small-scale jets from the chromosphere below. The fact that THEMIS measured these bidirectional flows in individual sub-arcsecond strands, using both imaging and spectroscopy, provides some of the most direct evidence yet that the filament’s fine structure is a dynamically active web of plasma conduits rather than a static reservoir of gas.

What makes this study compelling is the way it stitches together observations across an enormous range of scales and temperatures. A single arcsecond-scale blob of cool plasma, falling at nine times the sound speed, is connected to a million-degree bright point, to repetitive flows threading a filament tens of thousands of kilometres long, and to the grand question of how the Sun’s atmosphere cycles mass between its coolest and hottest layers. Whether the supersonic downflow proves to be ballistic drainage or coronal rain, the measurement demonstrates that quiescent filaments are anything but quiet: they are sites of continuous, violent mass transport that ground-based spectrometers of the THEMIS class are now capable of quantifying with exquisite precision. As solar physicists refine the interplay between thermal instability, magnetic topology and reconnection in these structures, each such observation brings the field closer to understanding not only the life cycle of filaments themselves but also the precursors of the eruptions that can send space weather hurtling toward Earth.

Subject of Research: Supersonic downflows in a quiescent solar filament observed with THEMIS and their relation to a coronal bright point

Article Title: Supersonic Flows Observed by THEMIS Related to a Coronal Bright Point and Filament

Article References: Karki, G., Schmieder, B., Chandra, R., Devi, P., Démoulin, P., & Poedts, S. (2026). Supersonic Flows Observed by THEMIS Related to a Coronal Bright Point and Filament. Solar Physics, 301(9), Article 148. https://doi.org/10.1007/s11207-026-02744-9

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02744-9

Keywords: solar physics, solar filament, prominence, THEMIS, coronal bright point, coronal rain, H-alpha spectroscopy, cloud model, counter-streaming flows, SDO/AIA, GONG, supersonic downflows