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. Coronal Hole Front and Center.jpg
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.

Contents

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

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. Helmet streamers at min.jpg
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.

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

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

A coronal hole at the Sun's north pole observed in soft X-ray Sun in X-Ray.png
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

Space weather effects Space weather effects.jpg
Space weather effects

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

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

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

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

References

  1. 1 2 Cranmer, Steven R. (2009), "Coronal holes", Living Reviews in Solar Physics, 6 (1): 3, Bibcode:2009LRSP....6....3C, doi: 10.12942/lrsp-2009-3 , PMC   4841186 , PMID   27194961
  2. ↑ Freedman, Roger A., and William J. Kaufmann III. "Our Star, the Sun." Universe. 8th ed. New York: W.H. Freeman, 2008. 419–420. Print.
  3. ↑ Krieger, A. S.; Timothy, A. F.; Roelof, E. C. (1973), "A coronal hole and its identification as the source of a high velocity solar wind stream", Solar Physics, 29 (2): 505–525, Bibcode:1973SoPh...29..505K, doi:10.1007/BF00150828
  4. 1 2 3 Kennewell, John; McDonald, Andrew. "What is a Coronal Hole?". Australian Government Bureau of Meteorology. Archived from the original on 11 August 2015.
  5. 1 2 3 Cranmer, Steven R. (2009). "Coronal Holes". Living Reviews in Solar Physics. 6 (1): 3. arXiv: 0909.2847 . Bibcode:2009LRSP....6....3C. doi: 10.12942/lrsp-2009-3 . PMC   4841186 . PMID   27194961.
  6. ↑ Hayakawa, Hisashi (2021), "Graphical evidence for the solar coronal structure during the Maunder Minimum", Journal of Space Weather and Space Climate, 11: 6, doi:10.1051/swsc/2020075
  7. 1 2 "Massive Coronal Hole on the Sun". NASA. 24 June 2013. Archived from the original on 19 December 2020. Retrieved 31 October 2014.
  8. ↑ Lowder, Chris; Qiu, Jiong; Leamon, Robert (2016), "Coronal holes and open magnetic flux over Cycles 23 and 24", The Astrophysical Journal, 822 (1): 57, doi: 10.3847/0004-637X/822/1/57
  9. ↑ Antiochos, Spiro K.; Mikić, Zoran; Titov, Vadim S.; Lionello, Roberto; Linker, Jon A. (2011), "A Model for the Sources of the Slow Solar Wind", The Astrophysical Journal, 731 (2): 112, arXiv: 1102.3704 , Bibcode:2011ApJ...731..112A, doi:10.1088/0004-637X/731/2/112
  10. ↑ Riley, Pete (2019), "Can an unobserved concentration of magnetic flux above the solar poles resolve the open-flux problem?", The Astrophysical Journal, 884 (1): 18, doi: 10.3847/1538-4357/ab3a98
  11. 1 2 Wilhelm, Klaus (1998), "The solar corona above polar coronal holes as seen by SUMER on SOHO", The Astrophysical Journal, 500: 1023–1038, doi:10.1086/305768
  12. ↑ von Steiger, Rudolf; Geiss, Johannes (1995), "Abundance variations in the solar wind", Advances in Space Research, 15 (7): 3–12, Bibcode:1995AdSpR..15g...3V, doi:10.1016/0273-1177(94)00103-Q (inactive 11 November 2025){{citation}}: CS1 maint: DOI inactive as of November 2025 (link)
  13. ↑ Harvey, Karen L.; Recely, Frank (2002), "Polar coronal holes during Cycles 22 and 23", Solar Physics, 211 (1–2): 31–52, Bibcode:2002SoPh..211...31H, doi:10.1023/A:1022469023581
  14. ↑ Fox, Karen (19 July 2013). "Large Coronal Hole Near the Sun's North Pole". NASA. Archived from the original on 12 November 2020. Retrieved 31 October 2014.
  15. ↑ Heinemann, Stephan G. (2020), "Long-term evolution of coronal-hole properties", Astronomy & Astrophysics, 639: A88, doi:10.1051/0004-6361/202037613
  16. ↑ Bale, Stuart D. (2023), "Interchange reconnection as the source of the fast solar wind", Nature, 599: 400–405, doi:10.1038/s41586-023-05955-3
  17. ↑ Verdini, Andrea; Velli, Marco (2007), "Turbulence-driven model for heating and acceleration of the fast wind in coronal holes", The Astrophysical Journal Letters, 662: L111 –L114, doi:10.1086/519522
  18. 1 2 Cranmer, Steven R.; Gibson, Sarah E.; Riley, Pete (November 2017). "Origins of the Ambient Solar Wind: Implications for Space Weather". Space Science Reviews. 212 (3–4): 1345–1384. arXiv: 1708.07169 . Bibcode:2017SSRv..212.1345C. doi:10.1007/s11214-017-0416-y.
  19. ↑ Brooks, David H.; Warren, Harry P. (2011), "Connecting active-region outflows to the slow solar wind", The Astrophysical Journal Letters, 727 (1): L13, arXiv: 1009.4291 , doi:10.1088/2041-8205/727/1/L13
  20. ↑ Geiss, J.; Gloeckler, G.; Von Steiger, R. (April 1995). "Origin of the solar wind from composition data". Space Science Reviews. 72 (1–2): 49–60. Bibcode:1995SSRv...72...49G. doi:10.1007/BF00768753.
  21. ↑ Richardson, Ian G. (2018), "Solar wind stream interaction regions throughout the heliosphere", Living Reviews in Solar Physics, 15 (1) 1, Bibcode:2018LRSP...15....1R, doi:10.1007/s41116-017-0011-z, PMC   6390897 , PMID   30872980
  22. 1 2 Tsurutani, Bruce T.; Gonzalez, Walter D.; Gonzalez, Alicia L. C.; Guarnieri, Fernando L.; Gopalswamy, Nat; Grande, Manuel; Kamide, Yohsuke; Kasahara, Yoshiya; Lu, Gang; Mann, Ian; McPherron, Robert; Soraas, Finn; Vasyliunas, Vytenis (July 2006). "Corotating solar wind streams and recurrent geomagnetic activity: A review". Journal of Geophysical Research: Space Physics. 111 (A7) 2005JA011273. Bibcode:2006JGRA..111.7S01T. doi:10.1029/2005JA011273.
  23. 1 2 Temmer, Manuela (December 2021). "Space weather: the solar perspective: An update to Schwenn (2006)". Living Reviews in Solar Physics. 18 (1): 4. arXiv: 2104.04261 . Bibcode:2021LRSP...18....4T. doi:10.1007/s41116-021-00030-3.
  24. ↑ NOAA Space Weather Scales, NOAA Space Weather Prediction Center, 2019
  25. ↑ Tsurutani, Bruce T.; Gonzalez, Walter D. (2006), "Corotating solar-wind streams and recurrent geomagnetic activity", Journal of Geophysical Research: Space Physics, 111 (A7): A07S01, Bibcode:2006JGRA..111.7S01T, doi:10.1029/2005JA011273
  26. ↑ Emmert, John T. (2015), "Thermospheric mass density: a review", Advances in Space Research, 56 (5): 773–824, Bibcode:2015AdSpR..56..773E, doi:10.1016/j.asr.2015.05.038
  27. ↑ "Fast Solar Wind Causes Aurora Light Shows". NASA. 9 October 2015. Retrieved 11 April 2022.
  28. ↑ Rotter, Thomas (2012), "Relation between coronal-hole areas on the Sun and solar-wind parameters at 1 AU", Solar Physics, 281 (2): 793–813, Bibcode:2012SoPh..281..793R, doi:10.1007/s11207-012-0101-y
  29. ↑ Arge, Charles N.; Pizzo, Vincenzo J. (2000), "Improving prediction of solar-wind conditions using near-real-time magnetic field updates", Journal of Geophysical Research: Space Physics, 105 (A5): 10465–10479, doi:10.1029/1999JA000262
  30. ↑ Collin, Daniel (2025), "Forecasting high-speed solar-wind streams from coronal-hole features in EUV images", Space Weather, 23 e2024SW004125, doi:10.1029/2024SW004125
  31. ↑ Raouafi, Nour E. (2023), "Parker Solar Probe: four years of discoveries at solar minimum and the rising phase of Solar Cycle 25", Space Science Reviews, 219 (1): 11, doi:10.1007/s11214-023-00952-4
  32. ↑ Chitta, Lakshmi P. (2025), "Picoflare jets in polar coronal holes as progenitors of fast and Alfvénic slow solar wind", Astronomy & Astrophysics, 690: A12, doi:10.1051/0004-6361/202452737
  33. ↑ Podladchikova, Tatiana (2025), "Simulating high-speed solar-wind streams from coronal-hole properties and geometry", Scientific Reports, 15: 97246, doi:10.1038/s41598-025-97246-2, PMID   40234602

Further reading

  1. Gombosi, Tamas (1998). Physics of the Space Environment. New York: Cambridge University Press. ISBN   0-521-59264-X.
  2. Jiang, Y., Chen, H., Shen, Y., Yang, L., & Li, K. (2007, January). Hα dimming associated with the eruption of a coronal sigmoid in the quiet Sun. Solar Physics, 240(1), 77–87.
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