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Coronal hole
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When observed in extreme ultraviolet, coronal holes appear as relatively dark patches in the Sun's corona. Here, there is a big coronal hole in the northern hemisphere.

A coronal hole is a region of the Sun'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 the solar wind. They are composed of relatively cool and tenuous plasma permeated by magnetic fields that are open to interplanetary 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 1973 Skylab mission, although eclipse photographs had hinted at polar dark regions earlier in the twentieth century.[3] Routine mapping now combines full-disk EUV imagers with ground-based synoptic 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 produce corotating interaction regions (CIRs). These regions can interact with Earth's magnetosphere to produce geomagnetic storms of minor to moderate intensity. During solar minima, CIRs are the main cause of geomagnetic storms.

History

[edit]
When the Sun's disk is obscured during a total solar eclipse or by a coronagraph (pictured), coronal structures not otherwise visible can be observed above the limb.[5]

Early observations of coronal holes date back to total solar eclipses between 1901 and 1954, when astronomers noticed polar darkenings adjacent to bright helmet streamers. These dim regions were later identified as magnetically open areas through detailed analysis.[6] The first quantitative observations of coronal holes were made by Max 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 by sounding rockets and in radio wavelength observations from the Sydney Chris Cross radio telescope. However, their nature remained unclear at the time. The true understanding of coronal holes emerged in the 1970s when X-ray telescopes aboard the Skylab mission operated above Earth's atmosphere, revealing detailed coronal structure.[4][7]

The advent of continuous extreme ultraviolet coverage from SOHO/EIT and SDO/AIA enabled automated detection of coronal holes and systematic analysis of their area, latitude, and magnetic flux throughout Solar cycles 23–25 (1996–2019).[8]

Characteristics

[edit]

A coronal hole refers to regions of the corona with low emission and predominantly open magnetic flux. Polar coronal holes are large, stable features that dominate during sunspot minima and persist for months to years at the Sun's poles, serving as the primary source of ambient fast solar wind. In contrast, mid-latitude and equatorial holes emerge and decay throughout the solar cycle and are smaller, more transient features. A satellite 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 for space 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 global magnetohydrodynamic simulations demonstrate that magnetic fields rooted inside coronal holes remain open and extend radially outward beyond approximately 2.5 R☉ solar radii. However, measurements of the heliospheric 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 1.0 megakelvin (MK) within 1.1 R☉, significantly cooler than the roughly 1.4 MK temperatures found in adjacent helmet 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-wind plasma before it escapes into interplanetary space.[12]

Formation and solar cycle

[edit]
Further information: Solar cycle
A coronal hole at the Sun's north pole observed in soft X-ray

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. During solar 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 decaying active regions of one polarity becomes dominant over the opposite polarity in a given area. This imbalanced magnetic flux then reconnects with the heliosphere, 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]

Solar wind

[edit]
Space weather effects
Further information: Solar wind and Space weather

Coronal holes are the primary source of fast solar 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 and Alfvén-wave pressure accelerate plasma along the weakly diverging flux tubes rooted in coronal-hole interiors, producing 650-800 km/s flow speeds near 1 astronomical unit (AU).[17][18] The solar wind exists primarily in two alternating states referred to as the slow solar wind and the fast 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]

Space-weather impacts

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CIRs can interact with Earth's magnetosphere, creating minor- to moderate-intensity geomagnetic 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 by coronal 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 of coronal mass ejections.[25] These geomagnetic disturbances cause Joule heating that expands the upper atmosphere, increasing atmospheric drag on satellites. Additionally, the compression regions within corotating interaction regions enhance relativistic electron populations in Earth's outer radiation belt and place additional strain on power 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]

Forecasting and monitoring

[edit]

Forecasters use persistence techniques that project measured coronal-hole boundaries forward in time, while multi-viewpoint EUV imaging reduces the longitudinal uncertainty that would otherwise accumulate as the Sun rotates.[28]

The Wang–Sheeley–Arge model converts synoptic magnetograms into solar-wind boundary conditions for the three-dimensional Enlil heliospheric model, enabling forecasters to predict when high-speed streams will arrive at Earth and estimate their peak velocities.[29] Modern convolutional 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 for geomagnetic storms.[30]

Parker Solar Probe

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The Parker 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 and 9.9 R☉solar radii in 2024 and 2025, the probe detected widespread switchbacks and signatures of interchange reconnection within the coronal hole's Alfvén-critical surface. These observations link the turbulent activity to newly opened magnetic flux tubes.[31]

Complementary observations from the Solar Orbiter mission using extreme ultraviolet imaging have revealed numerous small-scale picoflare 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 slower Alfvénic component of the solar wind.[32] During 2024–2025, a series of equatorial coronal holes extending 30° in longitude generated recurring G2-level geomagnetic storms that affected terrestrial power grids over multiple solar rotations.[33]

See also

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References

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Further reading

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Coronal hole

View on Grokipedia
from Grokipedia
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 (EUV) and soft imagery, and featuring open magnetic field lines that extend freely into interplanetary space. These structures were first observed as dark regions in the corona through imaging from satellites like in the 1970s, which pierced Earth's atmosphere to image the corona directly and revealed their nature as magnetically open regions. Coronal holes typically exhibit plasma temperatures around 1 million —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 that fan outward without reconnecting. They serve as primary sources of the fast , accelerating charged particles to speeds of 700–800 km/s, in contrast to the slower wind from closed-field areas. Their size and location vary with the 11-year : at , 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. When Earth-facing, coronal holes drive high-speed streams that compress the , triggering moderate geomagnetic storms (G1–G2 levels), recurrent auroral displays, and disruptions to operations, radio communications, and power grids.

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 and wavelengths. These areas appear as dark patches in (EUV) and soft images because the sparse plasma emits less radiation, facilitating the escape of solar material along open pathways. 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. 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 . These features are associated with regions of open lines that allow plasma to stream outward, contributing to the generation of the fast solar wind.

Physical Properties

Coronal holes exhibit significantly lower plasma densities than the surrounding quiet corona, typically on the order of 10810^8 particles cm−3^{-3} or less at the base, compared to approximately 10910^9 cm−3^{-3} in closed-field quiet regions. This reduced density arises from the open configuration that permits plasma expansion and escape. profiles decrease radially outward, with values around 8×1078 \times 10^7 cm−3^{-3} inferred from line ratios in polar holes. The plasma temperature in coronal holes ranges from approximately 0.8 to 1.5 million , somewhat cooler than the 1.5–2 million often found in the quiet corona's closed structures. This temperature regime contributes to the overall lower of coronal holes, as the emission efficiency drops for lines formed at higher temperatures. Diagnostic studies using (EUV) lines confirm this range, with values around 0.9–1.0 million near the base derived from density-sensitive ratios. 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. Soft 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. These profiles contrast sharply with the enhanced emission from dense coronal loops, highlighting the holes' unconfined nature. 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 . Equatorial coronal holes, by contrast, are smaller—typically spanning 10–20° in latitude—and display irregular, transient shapes, sometimes associated with remnants. These structures can persist from several days to several months, with polar examples showing greater longevity due to persistent open flux. 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^{-1}. 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.

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. The magnetic fields at the base of coronal holes in the are predominantly unipolar, meaning they exhibit a dominant single polarity with minimal opposite-polarity , typically ranging from 1 to 10 Gauss in strength. This unipolarity arises from the concentration of magnetic 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 , forming an structure in the known as the Parker spiral, with field lines winding at angles of about 45 degrees at Earth's distance. The total open magnetic flux threading through coronal holes is estimated to be on the order of 102110^{21} to 102210^{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.

Formation Mechanisms

Coronal holes form primarily through processes that open previously closed 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. 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. Flux emergence from the plays a crucial role in initiating and maintaining coronal holes by introducing new that interacts with existing fields. Photospheric transports poleward through supergranular flows, concentrating opposite-polarity fields toward the poles and forming large unipolar regions where the net flux imbalance favors openness. Emerging flux tubes, often twisted, undergo cancellation and reconnection upon piercing the surface, contributing to the expansion of unipolar patches that anchor coronal holes. 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 progression, driven by and flux transport, while polar holes can fragment into smaller structures due to interactions with emerging active regions. 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. Theoretical models emphasize 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 dissipation contributes more to heating within established open regions but not to their formation. Recent observations from 2025 highlight picoflare jets in inter-plume regions— the darker, less structured parts of coronal holes—as key progenitors of open . 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 streams.

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 . For instance, during the 1868 eclipse, French astronomer 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. 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 and , 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. By the 1960s, ultraviolet spectroheliograms from rocket flights further highlighted these dim areas, but their connection to broader solar phenomena remained unclear. The breakthrough in identifying coronal holes came in the early 1970s with space-based and observations. The 7 (OSO-7), launched in 1971, detected high-speed streams in 1972 that were traced back to large, dark patches on the solar disk, suggesting links to open regions. This was followed by the mission (1973–1974), where the S082A provided the first clear full-disk images of these features, revealing them as extensive, low-emission voids, particularly at the poles. The term "coronal holes" was formalized in around this time, notably in work by Richard Munro and colleagues, who analyzed their physical properties using OSO-4 and 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 , a concept observationally confirmed through the identification of coronal holes in the . These discoveries marked a pivotal shift in understanding the Sun's outer atmosphere and its influence on the .

Modern Observation Techniques

Modern observations of coronal holes rely heavily on space-based EUV and imagers, which provide high-resolution images of the solar atmosphere to map these dark regions daily. The (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. Complementing this, the (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. 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 sources. 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° and resolutions down to 17 arcseconds near perihelion, capturing dynamic features like plumes and jets emerging from coronal holes in Thomson-scattered light. Spectroscopic instruments further characterize plasma dynamics within coronal holes by measuring line shifts and intensities. The Hinode mission's EUV Spectrometer (EIS), active since 2006, resolves Doppler shifts in emission lines (e.g., Fe XII at 195 ) to detect upflows and downflows at scales of ~2 arcseconds, revealing small-scale flows in hole boundaries. In the 2020s, Solar Orbiter's Imager (EUI) has provided unprecedented high-resolution off-limb views, with the 174 channel imaging polar coronal holes at ~1 arcsecond resolution from up to 0.28 AU, uncovering fine structures and linking them to acceleration. Recent advancements in the 2024–2025 period include persistent monitoring of long-lived coronal holes using SDO/AIA. 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 . 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. These maps are often combined with heliospheric imagers from the mission, such as COR2, to connect coronal hole outflows to interplanetary structures, enabling stereoscopic views of evolving plasma streams.

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. 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. 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. Over the course of a , 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. This migration aligns with observations from instruments like the (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. In , 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. This reversal, completed in the 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.

Generation of Solar Wind

Coronal holes are the primary source of the fast , which originates from regions of open magnetic field lines and reaches speeds of 500– km/s, in contrast to the slower of 250–450 km/s emanating from the streamer belt regions. This fast is characterized by its steady flow and lower density, emerging directly from the low-density plasma within coronal holes. The acceleration of the in coronal holes occurs primarily through the dissipation of Alfvén waves and along the open lines, providing the necessary energy to drive the outflow from subsonic to supersonic speeds. 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. The plasma composition in this fast wind features a higher 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 . These outflows form high-speed (HSS) due to the rigid of coronal holes, which co-rotates with the Sun's surface and imparts a consistent to the escaping plasma, leading to structured streams that interact with slower wind ahead. Recent observations from 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 through intermittent events that generate Alfvénic fluctuations and supply the required energy.

Impacts on Space Weather

Effects on Earth's Magnetosphere

High-speed solar wind streams originating from coronal holes propagate through interplanetary space and reach after approximately 2-4 days, depending on their velocity of 500-800 km/s. Upon arrival, these streams exert on 's , causing it to compress and the dayside to shift inward by several radii. This compression alters the configuration of the , enhancing the coupling between the and 's magnetic field. As high-speed streams from coronal holes overtake slower ahead of them, they form corotating interaction regions (CIRs) at their leading edges, where plasma compression generates forward and reverse shocks. These shocks intensify the interplanetary within CIRs, often resulting in periods of southward Bz orientation that facilitate at the . The enhanced compression from CIRs further squeezes the , amplifying the overall pressure imbalance and contributing to prolonged geomagnetic disturbances. The response of the to CIR-induced currents includes plasma erosion through dayside reconnection, which injects energy into the magnetotail and triggers substorms. 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 . Such dynamics can persist for multiple days due to the recurrent nature of CIRs tied to persistent coronal holes. The locations of coronal holes on the Sun influence the warping of the (HCS), a large-scale structure that separates opposite magnetic polarities in the . This warping modulates the geometry of high-speed stream arrivals at , determining whether streams encounter the or arrive at higher latitudes, thereby affecting the orientation and impact of the interplanetary . In situ monitoring of solar wind parameters linked to coronal holes is conducted by satellites such as and DSCOVR, positioned at the Sun-Earth L1 . These missions measure key variables including plasma speed, , , and the interplanetary components, enabling the identification of high-speed streams and CIRs originating from coronal holes up to an hour before their impact at .

Geomagnetic Storms and Auroras

Coronal holes generate high-speed streams that interact with slower ambient 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. These storms typically exhibit a gradual onset over several hours, contrasting with the abrupt commencements associated with coronal mass ejections (CMEs). 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 . 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. In 2025, a persistent equatorial coronal hole directed high-speed toward , triggering a geomagnetic storm watch and widespread auroral visibility across northern U.S. states and on June 14. Similarly, in October 2025, a large coronal hole-induced high-speed stream prompted NOAA alerts and led to -G3 storms, enhancing auroras observable as far south as 40° in the . 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.

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