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Coronal loop
Coronal loop
Coronal loop
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Typical coronal loops observed by TRACE
Dynamics of coronal loops observed by SDO

In solar physics, a coronal loop is a well-defined arch-like structure in the Sun's atmosphere made up of relatively dense plasma confined and isolated from the surrounding medium by magnetic flux tubes. Coronal loops begin and end at two footpoints on the photosphere and project into the transition region and lower corona. They typically form and dissipate over periods of seconds to days[1] and may span anywhere from 1 to 1,000 megametres (621 to 621,000 mi) in length.[2]

Coronal loops are often associated with the strong magnetic fields located within active regions and sunspots. The number of coronal loops varies with the 11 year solar cycle.

Origin and physical features

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Due to a natural process called the solar dynamo driven by heat produced in the Sun's core, convective motion of the electrically conductive plasma which makes up the Sun creates electric currents, which in turn create powerful magnetic fields in the Sun's interior. These magnetic fields are in the form of closed loops of magnetic flux, which are twisted and tangled by solar differential rotation (the different rotation rates of the plasma at different latitudes of the solar sphere). A coronal loop occurs when a curved arc of the magnetic field projects through the visible surface of the Sun, the photosphere, protruding into the solar atmosphere.

Within a coronal loop, the paths of the moving electrically charged particles which make up its plasma—electrons and ions—are sharply bent by the Lorentz force when moving transverse to the loop's magnetic field. As a result, they can only move freely parallel to the magnetic field lines, tending to spiral around these lines. Thus, the plasma within a coronal loop cannot escape sideways out of the loop and can only flow along its length. This is known as the frozen-in condition.[3]

The strong interaction of the magnetic field with the dense plasma on and below the Sun's surface tends to tie the magnetic field lines to the motion of the Sun's plasma; thus, the two footpoints (the location where the loop enters the photosphere) are anchored to and rotate with the Sun's surface. Within each footpoint, the strong magnetic flux tends to inhibit the convection currents which carry hot plasma from the Sun's interior to the surface, so the footpoints are often (but not always) cooler than the surrounding photosphere. These appear as dark spots on the Sun's surface, known as sunspots. Thus, sunspots tend to occur under coronal loops, and tend to come in pairs of opposite magnetic polarity; a point where the magnetic field loop emerges from the photosphere is a North magnetic pole, and the other where the loop enters the surface again is a South magnetic pole.

Coronal loops form in a wide range of sizes, from the minimum observable scale (< 100 km) to 10,000 km. There is currently no accepted theory of what defines the edge of a loop, which is embedded in a general corona that is itself strongly magnetized. Coronal loops have a wide variety of temperatures along their lengths. Loops at temperatures below 1 megakelvin (MK) are generally known as cool loops; those existing at around 1 MK are known as warm loops; and those beyond 1 MK are known as hot loops. Naturally, these different categories radiate at different wavelengths.[4]

A related phenomenon is the open flux tube, in which magnetic fields extend from the surface far into the corona and heliosphere; these are the source of the Sun's large scale magnetic field (magnetosphere) and the solar wind.

  • A diagram showing the evolution of the solar magnetic flux over one solar cycle
    A diagram showing the evolution of the solar magnetic flux over one solar cycle
  • Diagram of the low corona and transition region, where many scales of coronal loops can be observed
    Diagram of the low corona and transition region, where many scales of coronal loops can be observed
  • A modelled example of a quiescent coronal loop (energy contributions)
    A modelled example of a quiescent coronal loop (energy contributions)

Location

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Coronal loops have been shown on both active and quiet regions of the solar surface. Active regions on the solar surface take up small areas but produce the majority of activity and are often the source of flares and coronal mass ejections due to the intense magnetic field present. Active regions produce 82% of the total coronal heating energy.[5][6]

Dynamic flows

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Many solar observation missions have observed strong plasma flows and highly dynamic processes in coronal loops. For example, SUMER observations suggest flow velocities of 5–16 km/s in the solar disk, and other joint SUMER/TRACE observations detect flows of 15–40 km/s.[7][8] Very high plasma velocities (in the range of 40–60 km/s) have been detected by the Flat Crystal Spectrometer (FCS) on board the Solar Maximum Mission.

History of observations

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For broader coverage of this topic, see Solar observation.

Before 1991

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Despite progress made by ground-based telescopes and eclipse observations of the corona, space-based observations became necessary to escape the obscuring effect of the Earth's atmosphere. Rocket missions such as the Aerobee flights and Skylark rockets successfully measured solar extreme ultraviolet (EUV) and X-ray emissions. However, these rocket missions were limited in lifetime and payload. Later, satellites such as the Orbiting Solar Observatory series (OSO-1 to OSO-8), Skylab, and the Solar Maximum Mission (the first observatory to last the majority of a solar cycle: from 1980 to 1989) were able to gain far more data across a much wider range of emission.[9][10]

1991–present day

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Full-disk mosaic of the million-degree corona by TRACE

In August 1991, the solar observatory spacecraft Yohkoh launched from the Kagoshima Space Center. During its 10 years of operation, it revolutionized X-ray observations. Yohkoh carried four instruments; of particular interest is the SXT instrument, which observed X-ray-emitting coronal loops. This instrument observed X-rays in the 0.25–4.0 keV range, resolving solar features to 2.5 arc seconds with a temporal resolution of 0.5–2 seconds. SXT was sensitive to plasma in the 2–4 MK temperature range, making its data ideal for comparison with data later collected by TRACE of coronal loops radiating in the extra ultraviolet (EUV) wavelengths.[11]

The next major step in solar physics came in December 1995, with the launch of the Solar and Heliospheric Observatory (SOHO) from Cape Canaveral Air Force Station. SOHO originally had an operational lifetime of two years. The mission was extended to March 2007 due to its resounding success, allowing SOHO to observe a complete 11-year solar cycle. SOHO has 12 instruments on board, all of which are used to study the transition region and corona. In particular, the Extreme ultraviolet Imaging Telescope (EIT) instrument is used extensively in coronal loop observations. EIT images the transition region through to the inner corona by using four band passes—171 Å FeIX, 195 Å FeXII, 284 Å FeXV, and 304 Å HeII, each corresponding to different EUV temperatures—to probe the chromospheric network to the lower corona.

In April 1998, the Transition Region and Coronal Explorer (TRACE) was launched from Vandenberg Air Force Base. Its observations of the transition region and lower corona, made in conjunction with SOHO, give an unprecedented view of the solar environment during the rising phase of the solar maximum, an active phase in the solar cycle. Due to the high spatial (1 arc second) and temporal resolution (1–5 seconds), TRACE has been able to capture highly detailed images of coronal structures, whilst SOHO provides the global (lower resolution) picture of the Sun. This campaign demonstrates the observatory's ability to track the evolution of steady-state (or 'quiescent') coronal loops. TRACE uses filters sensitive to various types of electromagnetic radiation; in particular, the 171 Å, 195 Å, and 284 Å band passes are sensitive to the radiation emitted by quiescent coronal loops.

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Coronal loop

View on Grokipedia
from Grokipedia
A coronal loop is an arch-shaped structure in the Sun's corona consisting of relatively dense, hot plasma confined within closed lines, appearing as bright features in (EUV) and emissions. These loops typically span lengths of 10 to 200 megameters (Mm), with cross-sections around 100 to 1000 kilometers wide, and contain plasma at temperatures ranging from about 1 million (MK) to over 10 MK, and densities of 10^8 to 10^11 particles per cubic centimeter. Coronal loops form the fundamental building blocks of the X-ray-bright solar corona, particularly in active regions near sunspots, where they trace the underlying architecture by channeling plasma along flux tubes without significant cross-field diffusion. They evolve dynamically over timescales from minutes to weeks, driven by continuous energy input that maintains their overdense state against gravitational stratification, with scale heights of about 50 Mm at 1 MK. Observations from missions such as the (SDO), Hinode, TRACE, and reveal fine-scale structuring into multi-stranded components, oscillations with periods of a few minutes, and plasma flows including upflows exceeding 100 km/s and redshifts around 10 km/s. These structures play a pivotal role in by facilitating the study of coronal heating mechanisms, such as nanoflares and magnetohydrodynamic (MHD) waves, which convert into to sustain the corona's million-degree temperatures despite the Sun's surface being only about 6000 . Coronal loops also link the to the corona, enabling mass and energy transport that contributes to phenomena like solar flares and coronal mass ejections, while their three-dimensional geometry—often reconstructed using stereoscopic data from —highlights their semicircular or curved profiles connecting regions like sunspot penumbrae to plages.

Fundamentals

Definition and overview

Coronal loops are bright, arch-like structures in the Sun's corona, consisting of dense plasma confined and guided along closed lines that connect regions of opposite polarity on the solar surface. These loops typically form above active regions and sunspots, serving as the fundamental building blocks of the and (EUV) bright corona by channeling and isolating hot plasma from the surrounding environment. In terms of scale, coronal loops exhibit a wide range of dimensions, with heights or semi-lengths typically spanning 5,000 to 100,000 km, widths of 1,000 to 10,000 km, and lifetimes from minutes to several days, depending on their association with solar activity such as flares. Their visibility in EUV and wavelengths arises from the high temperatures of the confined plasma, which range from 1 to 20 million —far exceeding the roughly 6,000 of the underlying —allowing them to emit strongly through thermal bremsstrahlung and line radiation. The plasma within coronal loops is primarily composed of fully ionized and , behaving as an ideal, low-beta gas that closely follows the lines due to the dominance of magnetic forces over gas . This confinement maintains the loops' structure and enables efficient energy transport along the field, highlighting their central role in the dynamic equilibrium of the solar atmosphere.

Physical characteristics

Coronal loops typically exhibit a semicircular or fan-like , particularly within active regions, where multiple loops may form arcades fanning out from magnetic concentrations. These structures are anchored at their footpoints in the or , with lengths ranging from approximately 10,000 to 200,000 km and cross-sections that remain roughly constant along their extent, deviating from perfect circularity by less than 30%. The structure of coronal loops often features a monotonic increase from cooler footpoints to a hotter apex, reflecting and heating patterns. In typical loops, footpoint temperatures are around 1 MK, rising to apex values of 3–10 MK for hot loops, though many observed loops show nearly isothermal profiles with minimal gradients along their length. Loops are frequently multi-thermal, comprising strands or components with a broad distribution spanning 1–20 MK, as revealed by differential emission measure analyses, indicating unresolved finer structures or dynamic evolution. Electron density profiles in coronal loops decrease with height from the footpoints, typically ranging from 10910^9 to 101010^{10} cm−3^{-3} in bright quiescent loops, with higher values up to 101110^{11} cm−3^{-3} during flares. This variation is quantified using emission measure, defined as the integral of the square of the along the , which peaks in the 2–10 MK range for loops and helps map density distributions non-uniformly across multi-thermal structures. Brightness in coronal loops arises from thermal emission, with significant variations observed in (for hot plasma >2 MK) and (EUV) wavelengths (for warmer plasma ~1 MK). Cooler loops, with temperatures below 1 MK, are prominent in the transition region and appear in EUV bands, contributing to the overall multi-thermal emission profile and highlighting enhancements in lower-temperature components.

Formation and dynamics

Magnetic origins

Coronal loops originate from the emergence of magnetic flux from the solar interior, where convective motions buoy up twisted magnetic flux tubes through the convection zone to the photosphere, forming bipolar magnetic regions in active regions. This process, first described by magnetic buoyancy principles, drives the initial structuring of loops as arched field lines piercing the surface. Subsequent magnetic reconnection between emerging flux and pre-existing coronal fields refines the loop topology, connecting opposite polarity footpoints and confining plasma along closed field lines. The strength in coronal loops typically ranges from 10 to 100 Gauss at the photospheric footpoints, where intense concentrations anchor the structures, and weakens to 1 to 10 Gauss along the coronal apex due to expansion. Twisted tubes play a crucial role in this configuration, providing the helicity that stabilizes loops against rapid disruption while enabling gradual evolution through and shearing. Coronal loops are most prevalent during , when enhanced emergence populates s with sunspots and surrounding plages, amplifying the density of bipolar structures. These features diminish toward as formation wanes, linking loop abundance directly to the 11-year dynamics. Topologically, coronal loops represent closed lines that trap plasma in the hot corona, contrasting with open field lines in where escapes freely. In prominences, loops often manifest as arcade structures, with multiple aligned arches supporting cool filamentary material against the overarching field.

Plasma flows and heating

Siphon flows in coronal loops arise from differences between the two footpoints, resulting in asymmetric plasma motion along the loop from the higher- end to the lower- end. These flows, guided by the lines, can reach speeds up to 100 km/s and are typically subsonic or supersonic depending on the loop geometry and heating asymmetry. The high temperatures in coronal loops, often exceeding 10^6 K, require continuous heating to balance energy losses. One prominent mechanism is nanoflare heating, involving numerous small-scale events that release energy impulsively. Each nanoflare deposits approximately 10^{24} to 10^{26} erg, collectively maintaining the loop's thermal structure through repeated occurrences. An alternative is wave heating driven by Alfvén waves, which propagate along the loop and dissipate energy via or resonant absorption, contributing to the overall energy input. The energy balance in coronal loops is governed by the equation dEdt=H−L−T,\frac{dE}{dt} = H - L - T, where EE is the density, HH is the volumetric heating rate, LL represents radiative losses given by ne2Λ(T)n_e^2 \Lambda(T) with Λ(T)≈10−22\Lambda(T) \approx 10^{-22} erg cm3^3 s−1^{-1} at coronal temperatures around 10^6 , and TT denotes conductive losses described by the flux Fc=−κ∇TF_c = -\kappa \nabla T with κ∝T5/2\kappa \propto T^{5/2}. This balance ensures quasi-static equilibrium, with heating countering the dominant losses from and conduction along the loop. Heating events in coronal loops trigger chromospheric evaporation, where intense energy deposition heats chromospheric plasma, driving upflows that fill the loop with hot material. These upflows, often observed at speeds of 10–20 km/s near the footpoints and decreasing with height, replenish the coronal density and sustain the loop's structure.

Observations

Early detections

Early observations of the solar corona, primarily conducted from the ground during total solar , began revealing structured, loop-like features in white light as early as the 1940s. These photographs captured arch-like extensions and streamers emanating from the solar limb, interpreted as plasma confined along lines in the corona. , developed by Bernard Lyot in and deployed at observatories like Pic du Midi in the 1940s and 1950s, enabled routine imaging of the corona without waiting for , further highlighting these elongated, curved structures amid the fainter diffuse emission. By the , and data had established that such features varied with solar activity, often appearing brighter near active regions. Pioneering space-based detections in the came from suborbital flights equipped with grazing-incidence telescopes, which first imaged the corona in soft s. A landmark flight on June 8, 1968, captured high-resolution photographs during a , revealing bright, arched structures interconnecting active regions—early evidence of hot coronal loops emitting at temperatures exceeding 10 million . Subsequent missions in the late and early confirmed these loop-like emissions as persistent features of the quiescent corona, not just flares, with brightness concentrated above magnetically complex areas. Theoretical groundwork for understanding these structures emerged in the late , when Eugene Parker highlighted the "coronal heating problem"—the enigma of how the corona reaches million-degree temperatures despite cooling by expansion into the . Parker argued that localized structures like loops could facilitate energy transport from the , channeling magnetic energy to heat confined plasma volumes. This framework underscored the need for loop observations to resolve the energy balance. The Skylab mission (1973–1974) marked a breakthrough with its (), delivering the first prolonged imaging of the corona via instruments like the S-054 , which produced over 32,000 images. These revealed intricate networks of hot, bright loops spanning active regions, with temperatures up to 3–5 million and lengths of hundreds of thousands of kilometers. Skylab also discovered "loop prominences," cool, dense plasma threads suspended along these hot loops, providing initial insights into multi-temperature plasma dynamics within coronal structures.

Modern imaging and data

The Yohkoh mission, operational from 1991 to 2001, marked a significant advancement in coronal loop observations through its (SXT), which achieved a resolution of approximately 2.5 arcseconds, enabling the first detailed imaging of loop in soft X-rays. This instrument revealed the arch-like morphology and internal threading of loops within active regions, distinguishing between bright, hot plasma confined in magnetic flux tubes and surrounding diffuse emission. A key discovery from SXT data was the identification of cooling flows in coronal loops, where plasma cools radiatively while draining along field lines, providing evidence of dynamic thermal evolution in quasi-steady structures. The Transition Region and Coronal Explorer (TRACE), launched in 1998 and operational until 2010, introduced high-resolution (EUV) imaging at 1 arcsecond resolution, targeting cooler loops formed at temperatures around 1 million . TRACE's observations highlighted the intricate, filamentary substructure of these loops, often resolving individual threads within larger arches and capturing their evolution over timescales of minutes to hours. Complementing this, the Hinode mission, launched in 2006 and ongoing, employs the Extreme-ultraviolet Imaging Spectrometer (EIS) to measure Doppler shifts in emission lines, quantifying plasma flows along loops with velocities up to 100 km/s. EIS data have demonstrated bidirectional flows in loop legs, with blue shifts indicating upflows and red shifts downflows, offering direct spectroscopic evidence of mass circulation driven by heating imbalances. More recent missions have further enhanced multi-wavelength coverage and proximity measurements. The (SDO), launched in 2010 and still active, uses the Atmospheric Imaging Assembly (AIA) to produce time-series movies across seven EUV and two UV channels, resolving loop dynamics at 0.6 arcsecond resolution and 12-second cadence. These observations capture the multi-thermal nature of loops, showing how plasma at different temperatures (0.5–20 million ) threads the same magnetic structure, and enable tracking of eruptions and reconnections in real time. The , launched in 2018 and continuing operations, provides the first in-situ measurements within 20 solar radii (and as low as ~8.5 solar radii as of 2024), sampling plasma and in the inner corona associated with coronal magnetic structures including those near loop tops and open field regions. Its data reveal high-beta plasma environments consistent with coronal conditions, including switchbacks and energetic particles indicative of reconnection processes. The mission, launched in 2020 and operational as of 2025, complements these efforts with its Imager (EUI), providing high-resolution EUV images at 1 arcsecond resolution or better, enabling stereoscopic observations of coronal loops in conjunction with SDO. These observations have revealed nearly circular cross-sections of loops, temporally coherent intensity variations, and persistent in medium-sized loops, enhancing understanding of their three-dimensional morphology and dynamics. Key findings from these missions include evidence of nanoflares as a heating mechanism, with 2012 SDO/AIA observations detecting impulsive brightenings in loop footpoints that release energy equivalent to 10^24–10^25 ergs, sufficient to maintain coronal temperatures without large-scale flares. Additionally, TRACE data from the late 1990s provided the first detections of loop oscillations, observing transverse kink modes with periods of 2–5 minutes and damping times of about 10 minutes, interpreted as magnetohydrodynamic waves propagating along loop waveguides.

Theoretical models

Equilibrium and stability

Coronal loops achieve when the downward gravitational force on the plasma is balanced by the upward along the loop's curved structure. In static models, this balance is coupled with energy considerations, where heating is offset by and . The seminal Rosner-Tucker-Vaiana (RTV) scaling laws, derived from steady-state solutions assuming uniform heating and neglecting flows, relate the maximum temperature Tmax⁡T_{\max} to the base pressure pp and loop semi-length LL via Tmax⁡≈1.4×103(pL)1/3T_{\max} \approx 1.4 \times 10^3 (p L)^{1/3} (in cgs units), while the heating rate scales as H∝p7/6L−5/6H \propto p^{7/6} L^{-5/6}. These relations highlight how conduction dominates energy transport in hot loops, with radiation more significant in cooler segments, providing a foundational framework for understanding loop energetics. Magnetohydrodynamic (MHD) equilibrium in coronal loops requires the Lorentz force to balance plasma pressure gradients and gravity, often approximated under low-β\beta conditions where magnetic tension dominates. Force-free fields, satisfying ∇×B=αB\nabla \times \mathbf{B} = \alpha \mathbf{B}, represent ideal configurations where the current is parallel to the magnetic field, minimizing magnetic stress while supporting loop topology. For linear force-free models with uniform α\alpha, analytical solutions in cylindrical geometry describe twisted flux tubes that align with observed loop shears, enabling extrapolation from photospheric magnetograms to coronal heights. These models capture the helical structure essential for loop stability, with α\alpha quantifying the twist level. Stability analyses reveal thresholds beyond which loops become susceptible to resistive MHD instabilities. The kink mode, an ideal or resistive instability driven by excessive magnetic twist, sets in when the twist angle exceeds approximately 2.5π2.5\pi for line-tied loops, leading to helical deformations that can trigger reconnection and energy release. Similarly, the ballooning instability arises from pressure-driven perturbations in curved field lines, with a critical loop length Lc≈2πR/βL_c \approx 2\pi R / \sqrt{\beta}
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