The Formation of Solar Prominences: Plasma Origin and Mechanisms

TL;DR

This review addresses how cool, dense prominence plasma originates and is sustained in the hot solar corona. It surveys four mass-accumulation pathways—injection, levitation, evaporation--condensation, and in-situ condensation—grounding each in theory, simulations, and observations, and highlighting the evaporation--condensation route as particularly developed in the past decade. The paper emphasizes that prominences emerge from dynamic magnetic structures and that multiple mechanisms likely operate in concert, with type-dependent dominance (e.g., active regions favoring injection, quiescent filaments favoring condensation). Numerical modeling, from 1D to fully 3D MHD with partial ionization and advanced conduction treatments, reveals coherent mass-loading cycles, thread-like morphologies, and mass–field coupling that reproduce many observed features. The work outlines open questions and calls for integrated multi-scale modeling and high-resolution, multi-wavelength observations to constrain heating, mass budgets, and the coexistence of formation pathways, informing space-weather relevance and solar-terrestrial connections.

Abstract

Solar prominences, or solar filaments, are cool and dense plasma structures in the hot solar corona, whose formation mechanisms have remained a fundamental challenge in solar physics. This review provides a comprehensive overview of the current theoretical, numerical, and observational understanding of prominence formation, with a focus on the origin of the dense plasma component. We begin by summarizing the magnetic field configurations that enable prominence support, followed by a classification of four representative plasma formation mechanisms: injection, levitation, evaporation--condensation, and in-situ condensation. Each mechanism is analyzed in terms of its physical basis, numerical realizations, and observational diagnostics. A central focus is placed on the evaporation--condensation scenario, which has seen significant development over the past decade through numerical simulations. We also discuss recent progress in modeling in-situ condensation triggered by magnetic reconnection and levitation dynamics. Throughout, we emphasize the complementary nature of different mechanisms and their potential coexistence in forming and maintaining prominence mass. Observational constraints and recent high-resolution data are reviewed to assess the physical plausibility of each mechanism. We conclude by highlighting open questions and future directions in connecting multi-scale physical processes to the observed diversity of prominence behaviors.

Figures (8)

• Figure 1: (a) A prominence observed by the Atmospheric Imaging Assembly leme2012 onboard the Solar Dynamics Observatory pesn2012 in the 171 Å channel, adapted from suy2012. (b) A filament imaged in H$\alpha$ by the Chinese H$\alpha$ Solar Explorer lic2022, courtesy of the CHASE team. (c) Co-spatial prominence and filament structures observed by SDO/AIA in the 304 Å channel, adapted from pare2014. (d) A representative example illustrating the typical alignment of a filament with the polarity inversion line (PIL), adapted from yanx2013. The background shows an SDO/AIA 304 Å image, overlaid with white and black magnetic field contours outlining positive and negative photospheric polarities from the associated HMI magnetogram.
• Figure 2: (a) Fine-scale filament threads observed by liny2005 using SST, revealing the highly structured nature of prominence material; (b) The first observational evidence of counterstreaming flows along filament threads, reported by zirk1998, highlighting dynamic plasma motions within the prominence spine.
• Figure 3: Representative magnetic configurations that support solar prominences. (a) 2D schematic of the KS model, showing horizontal magnetic field lines forming a magnetic dip above the polarity inversion line (adapted from anze1985); (b) 2D schematic of the KR model, in which a suspended flux rope is separated from the underlying bipolar field by electric currents (adapted from anze1985); (c) 3D sheared arcade configuration (adapted from devo2000); (d) 3D magnetic flux rope structure carrying a filament (adapted from zhou2018).
• Figure 4: Example of filament formation driven by recurrent chromospheric jets, adapted from wang2018. Panels (a1)--(a4) show a sequence of SDO/AIA 304 Å images capturing collimated jet-like ejections from the western footpoint of a forming filament. The white arrows highlight jet events that contribute to plasma injection along the filament channel. Panels (b1)--(b4) present the corresponding GONG H$\alpha$ observations taken shortly after each jet episode, showing progressive accumulation of cool material within the filament spine. This event illustrates a typical observational scenario in which localized, repetitive jet activity supplies mass to a growing filament structure.
• Figure 5: Example of filament formation through coronal condensation, adapted from yang2021. Panels (a1)--(a6), (b1)--(b6), and (c1)--(c6) show the evolution of the event in SDO/AIA 211 Å, 335 Å, and 304 Å channels, respectively. The red arrow in panel (b1) marks the onset of localized condensation in the corona, which subsequently develops into a coherent filament structure as plasma cools and accumulates. This event represents a well-resolved observational case of the evaporation--condensation process, in which thermally unstable plasma condenses in situ without apparent chromospheric injection. The presence of evaporation is supported by Hinode/EIS spectroscopic measurements showing significant blueshifts in high-temperature coronal lines (Fe xv, Fe xvi) at the flare footpoints, indicative of upward chromospheric evaporation flows.