Acoronal hole is a region of theSun's corona that appears dark in extreme-ultraviolet (EUV) and soft-X-ray images because its plasma is cooler and more rarefied than the surrounding corona.[1] Despite its name, a coronal hole is not an actual physical hole or void in the Sun's corona. The darkness reveals open magnetic field lines that guide plasma directly into interplanetary space, producing the fast component of thesolar wind. They are composed of relatively cool and tenuousplasma permeated bymagnetic fields that are open tointerplanetary space.[2] This results in decreased temperature and density of the plasma at the site of a coronal hole, as well as an increased speed in the average solar wind measured in interplanetary space.
Coronal holes were first identified unambiguously in soft-X-ray images from the 1973Skylab mission, although eclipse photographs had hinted at polar dark regions earlier in the twentieth century.[3] Routine mapping now combinesfull-disk EUV imagers with ground-basedsynoptic magnetographs to track hole evolution and feed space-weather forecasts.[4]
Streams of fast solar wind originating from coronal holes can interact with slow solar wind streams to producecorotating interaction regions (CIRs). These regions can interact withEarth's magnetosphere to producegeomagnetic storms of minor to moderate intensity. Duringsolar minima, CIRs are the main cause of geomagnetic storms.
Early observations of coronal holes date back tototal solar eclipses between 1901 and 1954, when astronomers noticed polar darkenings adjacent to brighthelmet streamers. These dim regions were later identified as magnetically open areas through detailed analysis.[6] The first quantitative observations of coronal holes were made byMax Waldmeier in 1956 and 1957, who used coronagraphic images of the green emission line at 5303 Å to identify these features.[5]
During the 1960s, coronal holes became visible in X-ray images captured bysounding rockets and in radio wavelength observations from theSydney Chris Cross radio telescope. However, their nature remained unclear at the time. The true understanding of coronal holes emerged in the 1970s whenX-ray telescopes aboard theSkylab mission operated above Earth's atmosphere, revealing detailed coronal structure.[4][7]
The advent of continuousextreme ultraviolet coverage fromSOHO/EIT andSDO/AIA enabled automated detection of coronal holes and systematic analysis of their area, latitude, andmagnetic flux throughoutSolar cycles 23–25 (1996–2019).[8]
Acoronal hole refers to regions of the corona with low emission and predominantly openmagnetic flux. Polar coronal holes are large, stable features that dominate duringsunspot minima and persist for months to years at the Sun's poles, serving as the primary source of ambient fastsolar wind. In contrast, mid-latitude and equatorial holes emerge and decay throughout the solar cycle and are smaller, more transient features. Asatellite hole is a low-latitude coronal hole that maintains a magnetic connection to a polar hole through a narrow corridor of open magnetic field lines.[9] This distinction is important forspace weather forecasting, as satellite holes can produce variable fast solar wind streams that sweep across Earth's orbital plane more frequently than the steady polar wind.
Computer models using potential-field source-surface extrapolations and globalmagnetohydrodynamic simulations demonstrate thatmagnetic fields rooted inside coronal holes remain open and extend radially outward beyond approximately2.5 R☉solar radii. However, measurements of theheliospheric magnetic field at 1 AU consistently indicate more open magnetic flux than most models predict, a discrepancy known as the open-flux problem.[10] Proposed solutions to this problem include incomplete coverage of polar magnetic fields in observations and narrow open corridors along coronal hole boundaries that remain unresolved in low-resolution magnetic field maps.[1]
Electron temperatures in polar coronal holes range from 0.7 to 1.0megakelvin (MK) within1.1 R☉, significantly cooler than the roughly 1.4 MK temperatures found in adjacenthelmet streamers.[11] Electron densities at similar heights are approximately half those found in quiet-Sun regions.Ultraviolet spectroscopic observations reveal blueshifted emission lines in magnetic network lanes, indicating nascent plasma outflows.[11] Chemical composition analyses show low ionization states and only mild enhancements of elements with low first-ionization potential, characteristics that reflect the brief coronal residence time of fast-windplasma before it escapes into interplanetary space.[12]
Coronal holes are closely tied to the solar cycle because their size, number, and location change dramatically as the Sun's magnetic field evolves through its 11-year cycle, with holes being most prominent and extensive during solar minimum periods. Duringsolar maximum, the Sun's polar magnetic fields reverse, closing existing open magnetic field lines and generating new flux of opposite polarity. This process reforms polar coronal holes during the declining phase of the solar cycle and at solar minimum.[7][13] During solar maxima, the number of coronal holes decreases until the magnetic fields on the Sun reverse. Afterwards, fresh coronal holes appear near the new poles. The coronal holes then increase in size and number, extending further from the poles as the Sun moves toward a solar minimum again.[14]
Mid-latitude coronal holes typically form when magnetic flux from decayingactive regions of one polarity becomes dominant over the opposite polarity in a given area. This imbalanced magnetic flux then reconnects with theheliosphere, creating an open field region.[15]
Along the boundaries of coronal holes,interchange reconnection occurs between open and closed magnetic field lines. This process transports open magnetic flux across the solar surface and generates slow solar wind streams near the edges of coronal holes.[16]
Coronal holes are the primary source of fastsolar wind streams, which escape more readily through their open magnetic field lines compared with the closed loops that confine plasma elsewhere in the corona.
Wave-driven turbulent heating andAlfvén-wave pressure accelerate plasma along the weakly diverging flux tubes rooted in coronal-hole interiors, producing 650-800 km/s flow speeds near 1astronomical unit (AU).[17][18] The solar wind exists primarily in two alternating states referred to as theslow solar wind and thefast solar wind. Fast streams originate inside coronal holes, whereas the slow component at 350-450 km/s often emerges from open-closed boundaries, active-region outflows, and pseudostreamer tops.[19][20][18]
Fast streams overtake slower wind ahead of them, creating stream interaction regions that corotate with the Sun and can steepen into forward and reverse shocks beyond 2 AU.[21][22][23]
CIRs can interact withEarth's magnetosphere, creating minor- to moderate-intensitygeomagnetic storms. The majority of moderate-intensity geomagnetic storms originate from CIRs. Geomagnetic storms originating from CIRs typically have a gradual commencement over hours and are not as severe as storms caused bycoronal mass ejections (CMEs), which usually have a sudden onset.
G1 and G2 geomagnetic storms represent minor and moderate levels of geomagnetic activity on the NOAA Space Weather Scale. G1 storms produce weak fluctuations in power grids and minor satellite operational anomalies, while G2 storms can cause voltage alarms in high-latitude power systems and affect satellite orbital drag calculations.[24]
High-speed solar wind streams from persistent coronal holes cause recurring geomagnetic activity in the G1–G2 range, producing sustained disturbances rather than the sudden, intense spikes characteristic ofcoronal mass ejections.[25] These geomagnetic disturbances causeJoule heating that expands the upperatmosphere, increasing atmospheric drag onsatellites. Additionally, the compression regions within corotating interaction regions enhance relativistic electron populations in Earth's outerradiation belt and place additional strain onpower grid systems.[26]
Since coronal holes and associated CIRs can last for several months over multiple solar rotations,[22][23] predicting the recurrence of this type of disturbance is often possible significantly further in advance than for CME-related disturbances.[4][27][5]
Forecasters use persistence techniques that project measured coronal-hole boundaries forward in time, while multi-viewpointEUV imaging reduces the longitudinal uncertainty that would otherwise accumulate as the Sun rotates.[28]
TheWang–Sheeley–Arge model converts synopticmagnetograms into solar-wind boundary conditions for the three-dimensionalEnlil heliospheric model, enabling forecasters to predict when high-speed streams will arrive atEarth and estimate their peak velocities.[29] Modernconvolutional neural networks can automatically identify and map coronal holes in EUV images while providing uncertainty estimates for their boundaries, leading to improved ensemble forecasts of solar wind conditions and more reliable probabilistic warnings forgeomagnetic storms.[30]
TheParker Solar Probe passes through coronal-hole interiors during each close approach to the Sun, providing the first direct measurements of plasma conditions in regions where fast solar wind originates. The spacecraft's instruments measure particle distributions, magnetic fields, and wave activity that help validate theoretical models of solar wind acceleration.
During its closest approaches at distances of 13.4 and9.9 R☉solar radii in 2024 and 2025, the probe detected widespreadswitchbacks and signatures ofinterchange reconnection within the coronal hole'sAlfvén-critical surface. These observations link the turbulent activity to newly opened magnetic flux tubes.[31]
Complementary observations from theSolar Orbiter mission usingextreme ultraviolet imaging have revealed numerous small-scalepicoflare jets within polar coronal holes. These findings support theoretical models proposing that small-scale magnetic reconnection events contribute to both fast solar wind and the slowerAlfvénic component of the solar wind.[32] During 2024–2025, a series of equatorial coronal holes extending 30° in longitude generated recurringG2-level geomagnetic storms that affected terrestrialpower grids over multiple solar rotations.[33]
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