A coronal hole is a region of the Sun's outer atmosphere, or corona, where the plasma is significantly cooler and less dense than surrounding areas, resulting in a dark appearance in extreme ultraviolet (EUV) and soft X-ray imagery, and featuring open magnetic field lines that extend freely into interplanetary space.[1][2][3]These structures were first observed as dark regions in the corona through X-ray imaging from satellites like Skylab in the 1970s, which pierced Earth's atmosphere to image the corona directly and revealed their nature as magnetically open regions.[4][5][2]Coronal holes typically exhibit plasma temperatures around 1 million Kelvin—about half that of active coronal regions—and electron densities 2 to 10 times lower, with unipolar magnetic fields of 1–5 gauss at the photosphere that fan outward without reconnecting.[3][6]They serve as primary sources of the fast solar wind, accelerating charged particles to speeds of 700–800 km/s, in contrast to the slower wind from closed-field areas.[7][1]Their size and location vary with the 11-year solar cycle: at solar minimum, large, stable polar coronal holes dominate, covering up to 20–30% of the solar surface, while at maximum, smaller, transient equatorial holes emerge and migrate.[8][1][9]When Earth-facing, coronal holes drive high-speed solar wind streams that compress the magnetosphere, triggering moderate geomagnetic storms (G1–G2 levels), recurrent auroral displays, and disruptions to satellite operations, radio communications, and power grids.[1][7][1]
Definition and Characteristics
Definition
A coronal hole is a region in the Sun's corona characterized by low-density plasma, cooler temperatures approximately 1 million K compared to about 2 million K in the surrounding corona, and correspondingly reduced emissions in ultraviolet and X-ray wavelengths.[10][11] These areas appear as dark patches in extreme ultraviolet (EUV) and soft X-ray images because the sparse plasma emits less radiation, facilitating the escape of solar material along open pathways.[1][12]Unlike prominences, which consist of relatively cool and dense plasma suspended in the corona, or solar flares, which involve sudden, intense bursts of energy and heated material, coronal holes represent persistent, low-emission zones that endure for weeks to months without eruptive activity.[3][1] They typically span up to about 6% of the solar surface per polar hole, with the largest examples being the polar coronal holes that dominate during solar minimum.[13][14]These features are associated with regions of open magnetic field lines that allow plasma to stream outward, contributing to the generation of the fast solar wind.[1]
Physical Properties
Coronal holes exhibit significantly lower plasma densities than the surrounding quiet corona, typically on the order of $10^8 particles cm^{-3} or less at the base, compared to approximately $10^9 cm^{-3} in closed-field quiet regions.[15] This reduced density arises from the open magnetic field configuration that permits plasma expansion and escape.[16]Electron density profiles decrease radially outward, with values around $8 \times 10^7 cm^{-3} inferred from ultraviolet line ratios in polar holes.[16]The plasma temperature in coronal holes ranges from approximately 0.8 to 1.5 million K, somewhat cooler than the 1.5–2 million K often found in the quiet corona's closed structures.[17] This temperature regime contributes to the overall lower brightness of coronal holes, as the emission efficiency drops for lines formed at higher temperatures.[3] Diagnostic studies using extreme ultraviolet (EUV) lines confirm this range, with values around 0.9–1.0 million K near the base derived from density-sensitive ratios.[18]Emission profiles in coronal holes show reduced intensities across multiple wavelengths due to the sparse plasma. In the EUV, lines such as He II at 304 Å and Fe IX/X at 171 Å exhibit notably lower brightness compared to surrounding regions, reflecting the diminished density and temperature.[19] Soft X-ray observations from the Yohkoh satellite reveal coronal holes as dark patches with fluxes reduced by factors of 3–10 relative to the quiet corona, particularly in lines formed around 1–2 million K.[20] These profiles contrast sharply with the enhanced emission from dense coronal loops, highlighting the holes' unconfined nature.[3]In terms of size and morphology, polar coronal holes often extend up to 50° in heliographic latitude, forming large, stable regions that dominate the high-latitude corona during solar minimum.[13] Equatorial coronal holes, by contrast, are smaller—typically spanning 10–20° in latitude—and display irregular, transient shapes, sometimes associated with active region remnants.[21] These structures can persist from several days to several months, with polar examples showing greater longevity due to persistent open flux.[13]Velocity fields within coronal holes are predominantly outward, with rare instances of inward flows; however, dynamic features such as spicules and small-scale jets are occasionally observed, reaching speeds of 10–100 km s^{-1}.[22] These jets, often filamentary and aligned with open field lines, contribute localized enhancements to the plasma motion but do not dominate the overall quiescent profile.[23]
Magnetic Structure and Formation
Role of Magnetic Fields
Coronal holes are characterized by regions of open magnetic field lines that extend from the solar photosphere outward into interplanetary space, without reconnecting to form closed loops as seen in active regions. These open configurations allow plasma to escape freely, contributing to the low density and cool temperatures observed in coronal holes. Unlike the tangled, closed magnetic loops prevalent in the surrounding corona, the open fields in coronal holes create a divergent topology that facilitates the acceleration of the solar wind.[24]The magnetic fields at the base of coronal holes in the photosphere are predominantly unipolar, meaning they exhibit a dominant single polarity with minimal opposite-polarityflux, typically ranging from 1 to 10 Gauss in strength. This unipolarity arises from the concentration of magnetic flux in the supergranular network, where fields diverge radially outward, expanding superradially to fill the coronal volume. As the Sun rotates, these open field lines are carried outward by the solar wind, forming an Archimedean spiral structure in the heliosphere known as the Parker spiral, with field lines winding at angles of about 45 degrees at Earth's distance.[25][26]The total open magnetic flux threading through coronal holes is estimated to be on the order of $10^{21} to $10^{22} Mx, which dominates the heliospheric magnetic field and accounts for a significant portion of the Sun's overall open flux. This flux primarily originates from polar and large low-latitude coronal holes during solar minimum. Coronal holes can be classified as unipolar or bipolar types; unipolar holes feature a single dominant polarity, while bipolar holes contain regions of opposite polarities separated by pseudostreamers—narrow, unipolar streamer-like structures that lack a current sheet in the outer corona, unlike traditional bipolar streamers. Pseudostreamers often demarcate boundaries between adjacent coronal holes of the same polarity, influencing the overall magnetic connectivity.[27]
Formation Mechanisms
Coronal holes form primarily through magnetic reconnection processes that open previously closed magnetic field lines, allowing plasma to escape and creating regions of low density and temperature. Interchange reconnection, occurring between open field lines and adjacent closed loops, expels cooler chromospheric plasma into the corona while facilitating the ejection of hotter coronal material along newly opened paths, thereby establishing the unipolar open-field topology characteristic of these structures.[28] This mechanism is supported by magnetohydrodynamic simulations showing that reconnection at the boundaries between open and closed fields drives the dynamic evolution of coronal hole boundaries.[29]Flux emergence from the photosphere plays a crucial role in initiating and maintaining coronal holes by introducing new magnetic flux that interacts with existing fields. Photospheric convection transports magnetic flux poleward through supergranular flows, concentrating opposite-polarity fields toward the poles and forming large unipolar regions where the net flux imbalance favors openness.[25] Emerging flux tubes, often twisted, undergo cancellation and reconnection upon piercing the surface, contributing to the expansion of unipolar patches that anchor coronal holes.[30]Over time, coronal holes evolve through migration, fragmentation, and eventual dissipation, influenced by ongoing surface dynamics. Low-latitude holes often migrate equatorward as part of the solar cycle progression, driven by differential rotation and flux transport, while polar holes can fragment into smaller structures due to interactions with emerging active regions.[31][32] Dissipation typically occurs when reconnection with newly emerged opposite-polarity flux closes open field lines, reducing the hole's area and reconnecting it to the surrounding closed-field corona.[33]Theoretical models emphasize magnetic reconnection as the primary driver for creating and sustaining open field lines in coronal holes, rather than wave-based heating mechanisms alone, which are insufficient to maintain the required field openness against closure tendencies. Simulations indicate that impulsive reconnection events provide the necessary energy input and topological changes, while Alfvén wave dissipation contributes more to heating within established open regions but not to their formation.[34]Recent observations from 2025 highlight picoflare jets in inter-plume regions— the darker, less structured parts of coronal holes—as key progenitors of open magnetic flux. These small-scale, intermittent jets, with energies in the picoflare range, emerge from base-level reconnection in unipolar areas, injecting plasma and opening field lines that contribute to both fast and Alfvénic slow solar wind streams.[35]
Observation and History
Historical Discovery
Early hints of dark regions in the solar corona, interpreted as areas of lower density, were noted during total solar eclipses in the 19th century. For instance, during the 1868 eclipse, French astronomer Pierre Janssen observed features in the corona that suggested variations in brightness and structure, though these were not yet understood as distinct low-emission zones. Such eclipse sightings provided initial qualitative evidence of coronal inhomogeneities, but lacked the resolution to identify systematic patterns.[24]In the mid-20th century, ground-based observations using coronagraphs and spectroheliograms began to reveal low-emission areas in the corona more consistently, though they were not immediately recognized as "holes." During the 1940s and 1950s, Swiss astronomer Max Waldmeier conducted the first quantitative measurements of these regions using the green coronal line at 5303 Å, identifying persistent low-intensity patches indicative of reduced plasma density.[24] By the 1960s, ultraviolet spectroheliograms from rocket flights further highlighted these dim areas, but their connection to broader solar phenomena remained unclear.[16]The breakthrough in identifying coronal holes came in the early 1970s with space-based X-ray and ultraviolet observations. The Orbiting Solar Observatory 7 (OSO-7), launched in 1971, detected high-speed solar wind streams in 1972 that were traced back to large, dark patches on the solar disk, suggesting links to open magnetic field regions. This was followed by the Skylab mission (1973–1974), where the S082A X-ray telescope provided the first clear full-disk images of these features, revealing them as extensive, low-emission voids, particularly at the poles.[36] The term "coronal holes" was formalized in scientific literature around this time, notably in work by Richard Munro and colleagues, who analyzed their physical properties using OSO-4 and Skylab data.Theoretical foundations for these observations were laid earlier by Eugene Parker, who in 1958 predicted the existence of open magnetic field lines in the corona as a source of the solar wind, a concept observationally confirmed through the identification of coronal holes in the 1970s. These discoveries marked a pivotal shift in understanding the Sun's outer atmosphere and its influence on the heliosphere.
Modern Observation Techniques
Modern observations of coronal holes rely heavily on space-based EUV and X-ray imagers, which provide high-resolution images of the solar atmosphere to map these dark regions daily. The Solar and Heliospheric Observatory (SOHO)'s Extreme Ultraviolet Imaging Telescope (EIT), operational since the 1990s, captures full-disk images in four EUV wavelengths (171 Å, 195 Å, 284 Å, and 304 Å), enabling the identification of coronal holes through their low-emission signatures in the transition region and inner corona.[37] Complementing this, the Solar Dynamics Observatory (SDO)'s Atmospheric Imaging Assembly (AIA), launched in 2010, offers continuous coverage with seven EUV channels and resolutions as fine as 1 arcsecond per pixel, facilitating routine synoptic mapping and detailed studies of hole evolution.[38]Coronagraphs extend these observations to the outer corona by blocking the bright solar disk, revealing white-light structures associated with coronal holes. SOHO's Large Angle and Spectrometric Coronagraph (LASCO) images the corona from 1.1 to 32 solar radii, providing views of streamer belts and open field lines tracing back to holes, which aids in monitoring solar wind sources.[39] Since 2018, the Parker Solar Probe's Wide-field Imager for Solar Probe (WISPR) has advanced inner corona imaging with a 13.5° to 108° field of view and resolutions down to 17 arcseconds near perihelion, capturing dynamic features like plumes and jets emerging from coronal holes in Thomson-scattered light.[40]Spectroscopic instruments further characterize plasma dynamics within coronal holes by measuring line shifts and intensities. The Hinode mission's EUV Imaging Spectrometer (EIS), active since 2006, resolves Doppler shifts in emission lines (e.g., Fe XII at 195 Å) to detect upflows and downflows at scales of ~2 arcseconds, revealing small-scale flows in hole boundaries.[41] In the 2020s, Solar Orbiter's Extreme Ultraviolet Imager (EUI) has provided unprecedented high-resolution off-limb views, with the 174 Å channel imaging polar coronal holes at ~1 arcsecond resolution from up to 0.28 AU, uncovering fine structures and linking them to solar wind acceleration.[42]Recent advancements in the 2024–2025 period include persistent monitoring of long-lived coronal holes using SDO/AIA. Magnetic field extrapolation via Potential Field Source Surface (PFSS) models, based on photospheric magnetograms, complements these by predicting open field regions defining hole boundaries, with source surfaces at ~2.5 solar radii for accurate global topology.[43]Ground-based efforts from the National Solar Observatory (NSO) produce synoptic maps in multiple wavelengths (e.g., 10830 Å He I for hole identification), integrating daily observations into Carrington rotation composites at 0.2° resolution to track hole migration.[44] These maps are often combined with heliospheric imagers from the STEREO mission, such as COR2, to connect coronal hole outflows to interplanetary structures, enabling stereoscopic views of evolving plasma streams.[32]
Relation to Solar Activity
Variation with Solar Cycle
Coronal holes exhibit significant variations in size, location, and coverage throughout the approximately 11-year solar cycle, driven by the evolution of the Sun's global magnetic field. During solar minimum, large polar coronal holes dominate, often covering up to 20-30% of the solar surface in historical deep minima, though recent cycles like 23/24 showed smaller polar extents of about 6% per hemisphere.[13] These polar holes feature opposite polarities in each hemisphere, with the northern and southern fields reversing around the cycle's maximum due to the migration and accumulation of magnetic flux.[45] In contrast, at solar maximum, polar holes diminish substantially as magnetic activity intensifies, giving way to smaller, transient low-latitude holes that form near active regions and typically span less than 5% of the surface collectively.[46]Over the course of a solar cycle, coronal holes migrate from equatorial latitudes toward the poles, reflecting the poleward transport of open magnetic flux. Non-polar holes, initially prominent at low latitudes during the rising phase, evolve and extend poleward, sometimes forming elongated structures known as "elephant trunk" extensions that connect equatorial regions to polar holes.[47][48] This migration aligns with observations from instruments like the Solar Dynamics Observatory (SDO), which track hole boundaries across latitudes. Overall, coronal hole coverage averages 10-20% of the solar surface across a cycle, peaking at minima (e.g., around 12.8% during the Cycle 23 minimum) and dropping to about 2.2% near maximum.[46][9]In Solar Cycle 25, which began in late 2019 and reached maximum around mid-2025, early observations indicate a continuation of these patterns with some peculiarities. As of early 2025, a small northern polar coronal hole has reformed with reversed polarity compared to 2021, while the southern polar hole remains underdeveloped and is expected to emerge later in the year.[49] This reversal, completed in the northern hemisphere by January 2024 and southern by September 2023, signals the onset of declining-phase polar hole growth, consistent with prior cycles but in a weaker overall magnetic context.[49]
Generation of Solar Wind
Coronal holes are the primary source of the fast solar wind, which originates from regions of open magnetic field lines and reaches speeds of 500–800 km/s, in contrast to the slower solar wind of 250–450 km/s emanating from the streamer belt regions.[50][51] This fast solar wind is characterized by its steady flow and lower density, emerging directly from the low-density plasma within coronal holes.[52]The acceleration of the solar wind in coronal holes occurs primarily through the dissipation of Alfvén waves and thermal conduction along the open magnetic field lines, providing the necessary energy to drive the outflow from subsonic to supersonic speeds.[53][54] A key transition happens at the critical point, typically located between 2 and 5 solar radii, where the flow speed equals the local sound speed, marking the onset of significant acceleration.[55] The plasma composition in this fast wind features a higher helium abundance of approximately 4–5%, reflecting the conditions in the coronal hole source regions, along with lower average charge states for heavy ions such as oxygen, where O⁵⁺ dominates over the higher O⁶⁺ prevalent in the slow solar wind.[56][57]These outflows form high-speed streams (HSS) due to the rigid rotation of coronal holes, which co-rotates with the Sun's surface and imparts a consistent angular velocity to the escaping plasma, leading to structured streams that interact with slower wind ahead.[58][59] Recent observations from Solar Orbiter in 2025 have revealed that picoflare jets within coronal holes act as progenitors for both fast and Alfvénic slow solar wind, "spraying" the wind outward like a fire hose through intermittent magnetic reconnection events that generate Alfvénic fluctuations and supply the required energy.[35][60]
Impacts on Space Weather
Effects on Earth's Magnetosphere
High-speed solar wind streams originating from coronal holes propagate through interplanetary space and reach Earth after approximately 2-4 days, depending on their velocity of 500-800 km/s.[61][62] Upon arrival, these streams exert dynamic pressure on Earth's magnetosphere, causing it to compress and the dayside magnetopause to shift inward by several Earth radii.[50] This compression alters the configuration of the magnetosphere, enhancing the coupling between the solar wind and Earth's magnetic field.[50]As high-speed streams from coronal holes overtake slower solar wind ahead of them, they form corotating interaction regions (CIRs) at their leading edges, where plasmacompression generates forward and reverse shocks.[1] These shocks intensify the interplanetary magnetic field within CIRs, often resulting in periods of southward Bz orientation that facilitate magnetic reconnection at the magnetopause.[63] The enhanced compression from CIRs further squeezes the magnetosphere, amplifying the overall pressure imbalance and contributing to prolonged geomagnetic disturbances.[64]The response of the magnetopause to CIR-induced currents includes plasma erosion through dayside reconnection, which injects energy into the magnetotail and triggers substorms.[65] These substorms lead to rapid enhancements in auroral activity and facilitate the intensification of the ring current, as energetic particles are accelerated and trapped in the inner magnetosphere.[66] Such dynamics can persist for multiple days due to the recurrent nature of CIRs tied to persistent coronal holes.[64]The locations of coronal holes on the Sun influence the warping of the heliospheric current sheet (HCS), a large-scale structure that separates opposite magnetic polarities in the solar wind.[67] This warping modulates the geometry of high-speed stream arrivals at Earth, determining whether streams encounter the subsolar point or arrive at higher latitudes, thereby affecting the orientation and impact of the interplanetary magnetic field.[67]In situ monitoring of solar wind parameters linked to coronal holes is conducted by satellites such as ACE and DSCOVR, positioned at the Sun-Earth L1 Lagrange point.[68][69] These missions measure key variables including plasma speed, density, temperature, and the interplanetary magnetic field components, enabling the identification of high-speed streams and CIRs originating from coronal holes up to an hour before their impact at Earth.[68][69]
Geomagnetic Storms and Auroras
Coronal holes generate high-speed solar wind streams that interact with slower ambient solar wind to form corotating interaction regions (CIRs), leading to minor-to-moderate geomagnetic storms classified as G1 (Kp=5) to G2 (Kp=6) on the NOAA scale.[1][70] These storms typically exhibit a gradual onset over several hours, contrasting with the abrupt commencements associated with coronal mass ejections (CMEs).[71]During passages of these high-speed streams, the planetary Kp index often reaches values of 5 to 7, indicating unsettled to active geomagnetic conditions, while the disturbance storm time (Dst) index commonly ranges from -50 to -150 nT, reflecting moderate ring current enhancements in Earth's magnetosphere.[71][62]The enhanced particle precipitation driven by these storms intensifies auroral displays at high latitudes, with the auroral oval expanding equatorward to as low as 40° geographic latitude during stronger events, making the northern lights visible from mid-latitude locations.[72][70]In June 2025, a persistent equatorial coronal hole directed high-speed solar wind toward Earth, triggering a G2 geomagnetic storm watch and widespread auroral visibility across northern U.S. states and Canada on June 14.[73] Similarly, in October 2025, a large coronal hole-induced high-speed stream prompted NOAA space weather alerts and led to G2-G3 storms, enhancing auroras observable as far south as 40° latitude in the northern hemisphere.[74][75]Due to the Sun's 27-day rotation period, coronal holes can produce recurrent high-speed streams, resulting in periodic geomagnetic storms every solar rotation that pose risks to satellites through increased atmospheric drag and to power grids via induced geomagnetically induced currents.[61][71]