Solar ProminencesEdit
Solar prominences are among the most striking and enduring features of the Sun’s dynamic atmosphere. These large, bright structures consist of cool, dense plasma suspended in the hot outer solar atmosphere, or corona, by magnetic fields. They arc above the solar surface for days or weeks, and some erupt violently, releasing material into interplanetary space as coronal mass ejections. Because they trace the Sun’s magnetic scaffolding, prominences provide crucial clues about solar magnetism, solar activity cycles, and space weather impacts on Earth. When observed against the solar disk, prominences appear as dark filaments; when seen at the limb, they glow as luminous arches against the dark space beyond the Sun.
Prominences occur in a variety of sizes and shapes, from small, ephemeral knots to sprawling, long-lived structures that outline magnetic loop systems. They are intimately linked to the Sun’s magnetic field, forming along polarity inversion lines where the magnetic field changes direction. The plasma within prominences is cooler (roughly 8,000–20,000 kelvin) and denser than the surrounding corona, yet it remains suspended in the gravitational field of the Sun by magnetic forces. The apparent stability of a prominence is therefore a balance between magnetic support and gravitational weight, along with thermal and magnetic pressure within the surrounding corona.
Anatomy and magnetic structure
Magnetic support and topology
Prominences are guided and held in place by the Sun’s magnetic field. They typically reside in regions where magnetic field lines form closed, sheared, or twisted configurations. One common mental model is a magnetic flux rope—a bundle of field lines that encircles and supports the cool plasma against solar gravity. The prominence material collects in dips of the magnetic field, where the gravitational force is counteracted by magnetic tension. This arrangement explains why prominences are found near the edges of active regions or along the quiet-Sun magnetic network, particularly along the boundaries between regions of opposite magnetic polarity.
Temperature and composition
The plasma in prominences is substantially cooler than the surrounding corona. Despite the cooler temperatures, the material is highly opaque in certain spectral lines, notably the hydrogen alpha (Hα) transition. This makes prominences bright in Hα images when the material is viewed at the limb, and dark when seen as filaments against the brighter solar disk. The composition is primarily hydrogen, with helium and trace heavier elements, and the densities can be several orders of magnitude higher than the surrounding coronal plasma.
Dynamic behavior
Prominences are not static; they exhibit flows and oscillations on timescales from minutes to hours, and they can reorganize themselves as the coronal magnetic field evolves. Small-scale changes in the magnetic field can trigger motion along field lines, while larger rearrangements can lead to partial or full eruptions. Observations across wavelengths—from optical to ultraviolet and extreme ultraviolet—reveal a multi-thermal, multi-component structure where cool plasma sits amid hotter surrounding material.
Formation and evolution
Formation mechanisms
The two leading explanations for prominence formation emphasize the role of magnetic fields in both shaping and feeding the cool plasma into the corona. In one view, thermal non-equilibrium and condensation of coronal plasma along dipped magnetic field lines rapidly generates cool, dense material that collects in promontories. In another view, material is drawn up from the chromosphere along magnetic field lines into the coronal portion of a loop system, where it becomes trapped in magnetic dips.
Lifetimes and stability
Prominences can persist for days to weeks, but their stability is contingent on the large-scale magnetic configuration. Changes in the photospheric magnetic field (and the associated coronal magnetic field) can destabilize a prominence, leading to partial erosion, a complete eruption, or a reconfiguration into a different loop system. The eruption phase often coincides with the release of mass and magnetic energy into the heliosphere as a coronal mass ejection, which can interact with planetary environments.
Eruptive events and space weather
A prominent aspect of prominences is their connection to eruptive events. When a prominence becomes unstable, it may erupt outward, sometimes with the surrounding coronal field, producing a coronal mass ejection. Such events are a central component of space weather forecasting because the ejected plasma and magnetic field can interact with Earth’s magnetosphere, potentially affecting satellites, power grids, and communication systems. The timing, speed, and magnetic orientation of eruptions are active areas of solar physics research.
Observations and terminology
Wavelengths and imaging
Prominences are observed most clearly in the hydrogen Hα line (a specific red line of neutral hydrogen) when viewed at the solar limb. In space-based ultraviolet and extreme ultraviolet instruments, prominences reveal themselves through absorption and emission features in lines such as Lyman-α and lines from ionized metals. Across different wavelengths, prominences display a resonant mix of cool dense cores and surrounding hot plasma, making them valuable probes of magnetic structure and thermodynamics.
Filaments on the solar disk
When prominences are projected against the solar disk, their cool plasma appears dark as filaments against the brighter background of the solar surface. Filaments trace the same magnetic structures as off-limb prominences, and tracking their evolution provides insight into magnetic field changes and the onset of eruptions.
Instrumentation and major observatories
Long-running solar missions and telescopes have cataloged prominences during multiple solar cycles. Ground-based networks, as well as space missions, have delivered high-resolution imagery and spectroscopic data that support modeling of magnetic fields, plasma flows, and energy balance. Notable facilities and missions include instruments designed for multi-wavelength solar observations, as well as specialized spectrographs for analyzing line profiles and Doppler shifts.
Types, lifecycle, and phenomenology
Quiescent and active-region prominences
Prominences are often categorized by their typical environments. Quiescent prominences lie in relatively calm regions of the solar surface and can endure for extended periods, whereas active-region prominences form in magnetically energized locales near sunspots and typically have shorter lifetimes and more dynamic behavior.
Eruptive prominences and their consequences
Eruptive prominences are the subset that become unstable and expel material into interplanetary space. Their eruptions are frequently linked to coronal mass ejections, which are key drivers of space weather at Earth and other planets. The precise mechanisms that trigger eruptions—whether through magnetic reconnection, photospheric motion, or accumulation of mass and twist—remain active research topics with implications for forecasting solar activity.
Observational signatures
Prominences can show a range of observational signatures, including helical motion along field lines, Doppler-shifted components indicating flows, and changes in brightness associated with heating or cooling processes. The spectral profiles of prominences help scientists infer temperature, density, and velocity structure.
Prominence and the solar cycle
Prominence activity is modulated by the overall solar magnetic cycle, which governs the distribution of magnetic polarity across the solar surface and the frequency of energetic events. During solar maximum, the Sun tends to produce more complex magnetic configurations and more frequent prominence eruptions. During solar minimum, prominences still form and evolve, but the activity level tends to be lower. The cycle-wide statistics of prominences provide a complementary perspective on the changing magnetic landscape of the Sun.