Coronal magnetic field

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The coronal magnetic field is the magnetic field that permeates the Solar corona, the Sun's hot and tenuous outer atmosphere. This field structures plasma into loops and open flux configurations, governs magnetic energy storage and release, and determines the conditions for the solar wind and eruptive space weather events. Most coronal processes occur in a low-beta regime where magnetic pressure dominates gas pressure, making the magnetic field the primary driver of plasma dynamics and energetics. [1]

Contents

The coronal magnetic field forms closed loops above active regions and in streamers, while opening through coronal holes to create heliospheric flux that carries the solar wind. Magnetic free energy accumulates through flux emergence and photospheric stressing, then releases through magnetic reconnection in solar flares and coronal mass ejections. The coronal topology includes null points, separatrices, and quasi-separatrix layers, which concentrate electric currents and favor reconnection processes. [2] [3]

Measurement techniques

This animation shows what happens as a sunspot (or starspot) forms and the magnetic field increases in strength. The light emerging from the spot starts to demonstrate the Zeeman effect. The dark spectra lines in the spectrum of the emitted light split into three components and the strength of the circular polarisation in parts of the spectrum increases significantly. This polarization effect is a powerful tool for astronomers to detect and measure stellar magnetic fields.

Direct measurement of the coronal magnetic field presents significant challenges because coronal emission lines are faint and broad, the medium is optically thin, and line-of-sight integration creates geometric ambiguities that complicate inversions. American solar physicist Paul G. Judge noted concerns over "angular ambiguities and line-of-sight" effects in forbidden line spectropolarimetry, while also outlining solutions to mitigate these issues. [4] Consequently, researchers use multiple complementary diagnostics that are sensitive to different heights and field regimes. [5]

Early detections of the coronal Zeeman effect in the near-infrared Fe XIII 1074.7 nm line measured line-of-sight field strengths of a few to tens of gauss above active regions, demonstrating the feasibility of routine coronal magnetometry with high-sensitivity coronagraphy. [6] [7] The COronal Multi-channel Polarimeter (CoMP) extended this approach to synoptic linear and circular polarization mapping in Fe XIII, enabling wave and seismology studies in the low corona. [8] The Daniel K. Inouye Solar Telescope now maps full-Stokes coronal spectra in Fe XIII and other lines, providing plane-of-sky and line-of-sight constraints from the Zeeman effect over active regions and streamers. [9]

The Hanle effect, which involves magnetic modification of resonance-scattering polarization, diagnoses weak to moderate coronal fields and resolves mixed-polarity regimes that are invisible to Zeeman circular polarization alone. [10] [11] However, interpretation must account for the Van Vleck 54.7° ambiguity and line-of-sight integration effects, which can be mitigated through multi-line inversions and tomographic constraints. [4]

Radio techniques provide powerful constraints on coronal magnetic fields from the chromosphere to many solar radii. Microwave gyroresonance and gyrosynchrotron imaging spectroscopy over active regions and flares retrieve magnetic field strength and its evolution in three dimensions. [12] [13] [14] Faraday rotation of spacecraft radio beacons and background cosmic sources samples the line-of-sight field out to several solar radii and constrains the radial profile of the coronal and inner-heliospheric field. [15] [16] [17]

Oscillations and waves observed in EUV and infrared Doppler data provide indirect measurements through magnetohydrodynamic seismology. The first global detections of ubiquitous Alfvénic motions with CoMP showed that wave energy and phase speeds can constrain the Alfvén speed and hence magnetic field strength. [18] British solar physicist Valery M. Nakariakov describes this method as "seismological probing of coronal plasma", highlighting its ability to recover field strength and topology from observed periods, damping, and phase speeds. [19] Global maps derived from CoMP magnetoseismology report plane-of-sky field strengths of 1–4 gauss between 1.05 and 1.35 R☉ solar radii, with a topology that tracks streamers and coronal holes. [20]

Modeling and applications

The magnetic field in the Sun's corona is often approximated as a force-free field. Picturing the Sun's Magnetic Field (25513266790).jpg
The magnetic field in the Sun's corona is often approximated as a force-free field.

In the absence of comprehensive coronal measurements, models infer the three-dimensional coronal field from photospheric magnetograms under simplifying assumptions. The potential field [ disambiguation needed ] source-surface model, introduced in 1969 and refined in subsequent decades, approximates the large-scale field by imposing a spherical source surface where field lines become radial. [21] [22] Nonlinear force-free field extrapolations attempt to recover stressed fields in active regions, but results depend on boundary preparation, resolution, and method, and can disagree on free energy and magnetic helicity budgets. [23] [24] [25] Comprehensive reviews compare potential, magnetohydrostatic, force-free, and full magnetohydrodynamic approaches, emphasizing the need to assimilate diverse observations to reduce model uncertainty. [26]

Photospheric vector magnetograms from the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory supply lower boundary conditions for extrapolations and data-driven magnetohydrodynamic simulations, while the Atmospheric Imaging Assembly provides EUV context and loop morphology that can validate field models. [27] [28] [29] Ground-based coronal spectropolarimeters such as CoMP and the Daniel K. Inouye Solar Telescope provide direct constraints on the coronal field vector in visible and infrared lines. [8] [9]

The inner-heliospheric coronal field can be probed by spacecraft that venture below or near the Alfvén critical surface. Measurements from the Parker Solar Probe demonstrate crossings into a magnetically dominated regime and provide evidence for interchange reconnection as a source of fast wind and magnetic switchbacks, linking coronal magnetic structure and dynamics to observed wind streams. [30] [31]

Operational solar wind models rely on the large-scale coronal magnetic field to set inner boundary conditions. The Wang–Sheeley–Arge framework combines potential-field source-surface topology with empirical relations between expansion factors, coronal hole boundaries, and wind speed to forecast the ambient solar wind at inner heliospheric distances. [32] [33] Assimilating new coronal magnetic field measurements improves constraints on open-flux regions, streamer positions, and current sheet geometry, which in turn refines forecasts of high-speed streams and background conditions for coronal mass ejection propagation. [26]

The modern understanding of the coronal magnetic field developed from eclipse polarimetry, coronagraphy, and radio astronomy, followed by infrared spectropolarimetry and space-based EUV imaging. Continuous magnetograms from space and global high-cadence EUV images enabled routine extrapolations and coronal seismology. Recent infrared spectropolarimetry with the Daniel K. Inouye Solar Telescope has produced the first detailed coronal magnetic field maps based on the Zeeman effect, opening a new era of direct coronal magnetometry. [6] [18] [9] Researchers distinguish between closed and open coronal fields, between potential and non-potential configurations, and between plane-of-sky and line-of-sight components. Measurements and models often report heights in solar radii and field strengths in gauss, with careful specification of the sensitivities and ambiguities of each diagnostic technique, as different methods sample complementary field components and height ranges. [5] [4]

See also

References

  1. ↑ Wiegelmann, Thomas; Sakurai, Tetsuya (2014), "The magnetic field in the solar atmosphere", The Astronomy and Astrophysics Review, 22 78, arXiv: 1410.4214 , Bibcode:2014A&ARv..22...78W, doi:10.1007/s00159-014-0078-7
  2. ↑ Démoulin, Pascal; Hénoux, Jean-Claude; Priest, Eric R.; Mandrini, Cristina H. (1996), "Quasi-separatrix layers in solar flares. I. Method", Astronomy & Astrophysics, 308: 643–655, Bibcode:1996A&A...308..643D
  3. ↑ Titov, V. S.; Démoulin, Pascal (2002), "Theory of magnetic connectivity in the solar corona", Journal of Geophysical Research: Space Physics, 107 (A8): 1164, Bibcode:2002JGRA..107.1164T, doi:10.1029/2001JA000278
  4. 1 2 3 Judge, Philip G. (2013), "From Forbidden Coronal Lines to Meaningful Magnetic-Field Measurements", Solar Physics, 288: 467–480, arXiv: 1304.3863 , doi:10.1007/s11207-013-0309-5
  5. 1 2 Landi, Enrico (2016), "Coronal plasma diagnostics from ground-based observations", Journal of Geophysical Research: Space Physics, 121 (11): 8238–8256, Bibcode:2016JGRA..121.8237L, doi:10.1002/2016JA022598, hdl: 2027.42/134492
  6. 1 2 Lin, Haosheng; Penn, Matthew J.; Tomczyk, Steven (2000), "A New Precise Measurement of the Coronal Magnetic Field Strength", The Astrophysical Journal Letters, 541 (2): L83 –L86, Bibcode:2000ApJ...541L..83L, doi:10.1086/312900
  7. ↑ Lin, Haosheng; Kuhn, Jeffrey R.; Coulter, Roy (2004), "Coronal Magnetic Field Measurements", The Astrophysical Journal Letters, 613 (2): L177 –L180, Bibcode:2004ApJ...613L.177L, doi:10.1086/425217
  8. 1 2 Tomczyk, Steven; Card, Gary L.; Darnell, Timothy; Elmore, David F.; Lull, Randall; Nelson, Peter G.; Streander, Kurt; Burkepile, Joan; Casini, Roberto; Judge, Philip G. (2008), "An Instrument to Measure Coronal Emission Line Polarization", Solar Physics, 247 (2): 411–428, Bibcode:2008SoPh..247..411T, doi:10.1007/s11207-007-9103-6
  9. 1 2 3 Schad, Thomas A.; Petrie, Gordon J. D.; Kuhn, Jeffrey R.; Fehlmann, Andre; Rimmele, Thomas; Tritschler, Alex (2024), "Mapping the Sun's coronal magnetic field using the Zeeman effect", Science Advances, 10 (37): e adq1604, arXiv: 2410.21568 , Bibcode:2024SciA...10.1604S, doi:10.1126/sciadv.adq1604, PMC   11421591 , PMID   39259791
  10. ↑ Raouafi, Nasir E. (2016), "Diagnostics of Coronal Magnetic Fields through the Hanle Effect in UV and IR Lines", Frontiers in Astronomy and Space Sciences, 3 20, arXiv: 1606.08493 , Bibcode:2016FrASS...3...20R, doi: 10.3389/fspas.2016.00020
  11. ↑ Derouich, Mohamed; Sahal-Bréchot, Sylvie; Johns-Krull, Christopher; Ramelli, Rodolfo (2010), "Hanle signatures of the coronal magnetic field in the linear polarization of the O VI 103.2 nm line", Astronomy & Astrophysics, 511: A7, arXiv: 0909.4977 , doi:10.1051/0004-6361/200913160
  12. ↑ Alissandrakis, Constantine E.; Gary, Dale E.; Hurford, Gordon J. (2021), "Radio Measurements of the Magnetic Field in the Solar Corona", Frontiers in Astronomy and Space Sciences, 8 591075, doi: 10.3389/fspas.2020.591075
  13. ↑ Gary, Dale E.; Chen, Bin; Dennis, Brian R.; Fleishman, Gregory D. (2018), "Microwave and Hard X-Ray Observations of the 2017 September 10 Solar Limb Flare" (PDF), The Astrophysical Journal, 863 (1): 83, arXiv: 1807.02498 , Bibcode:2018ApJ...863...83G, doi: 10.3847/1538-4357/aad0ef
  14. ↑ Fleishman, Gregory D.; Nita, Gelu M.; Chen, Bin; Yu, Sijie; Krucker, Säm (2020), "Decay of the coronal magnetic field can release sufficient energy to power a solar flare", Science, 367 (6485): 278–281, Bibcode:2020Sci...367..278F, doi:10.1126/science.aax6874, PMID   31949076
  15. ↑ Jensen, Elizabeth A.; Bisi, Mario M.; Breen, Andrew R. (2013), "Measurements of Faraday Rotation Through the Solar Corona During the 2009 Solar Minimum with the MESSENGER Spacecraft" (PDF), Solar Physics, 285 (1–2): 83–95, Bibcode:2013SoPh..285...83J, doi:10.1007/s11207-012-0213-4
  16. ↑ Kooi, Jason E.; Fischer, Patrick D.; Buffo, Jacob J.; Spangler, Steven R. (2013), "Measurements of Coronal Faraday Rotation at 4.6 Solar Radii", The Astrophysical Journal, 779 (2): 109, arXiv: 1307.1727 , doi:10.1088/0004-637X/779/2/109
  17. ↑ Mancuso, Stefano; Bemporad, Alessandro (2013), "Coronal magnetic field strength from Type II radio emission", Astronomy & Astrophysics, 555: A91, arXiv: 1403.3436 , doi:10.1051/0004-6361/201321321
  18. 1 2 Tomczyk, Steven; McIntosh, Scott W.; Keil, S. L.; Judge, Philip G.; Schad, Thomas; Seeley, David; Edmondson, J. (2007), "Alfvén Waves in the Solar Corona" , Science, 317 (5842): 1192–1196, Bibcode:2007Sci...317.1192T, doi:10.1126/science.1143304, PMID   17761876
  19. ↑ Nakariakov, Valery M.; Kolotkov, Dmitrii Y. (2020), "Magnetohydrodynamic Waves in the Solar Corona" , Annual Review of Astronomy and Astrophysics, 58: 441–481, Bibcode:2020ARA&A..58..441N, doi:10.1146/annurev-astro-032320-042940
  20. ↑ Yang, Zihao; Bethge, Christian; Tian, Hui; Tomczyk, Steven; Morton, Richard; Del Zanna, Giulio (2020), "Global maps of the magnetic field in the solar corona", Science, 369 (6507): 694–697, arXiv: 2008.03136 , Bibcode:2020Sci...369..694Y, doi:10.1126/science.abb4462, PMID   32764071
  21. ↑ Schatten, Kenneth H.; Wilcox, John M.; Ness, Norman F. (1969), "A model of interplanetary and coronal magnetic fields", Solar Physics, 6 (3): 442–455, Bibcode:1969SoPh....6..442S, doi:10.1007/BF00146478
  22. ↑ Altschuler, Martin D.; Newkirk, Gordon (1969), "Magnetic fields and the structure of the solar corona. I. Methods of calculating coronal fields", Solar Physics, 9 (1): 131–149, Bibcode:1969SoPh....9..131A, doi:10.1007/BF00145734
  23. ↑ Wiegelmann, Thomas; Sakurai, Tetsuya (2012), "Solar Force-free Magnetic Fields", Living Reviews in Solar Physics, 9 (5), Bibcode:2012LRSP....9....5W, doi: 10.12942/lrsp-2012-5
  24. ↑ DeRosa, Marc L.; Schrijver, Carolus J.; Barnes, Graham (2009), "A Critical Assessment of Nonlinear Force-Free Field Modeling of the Solar Corona for Active Region 10953", The Astrophysical Journal, 696 (2): 1780–1791, arXiv: 0902.1007 , Bibcode:2009ApJ...696.1780D, doi:10.1088/0004-637X/696/2/1780
  25. ↑ Peter, Hardi (2015), "Limitations of force-free magnetic field extrapolations" (PDF), Astronomy & Astrophysics, 584: A68, doi:10.1051/0004-6361/201527057
  26. 1 2 Wiegelmann, Thomas; Thalmann, J. K.; Solanki, Samuel K. (2017), "Coronal Magnetic Field Models", Space Science Reviews, 210 (1–4): 249–274, Bibcode:2017SSRv..210..249W, doi: 10.1007/s11214-015-0178-3
  27. ↑ Scherrer, Philip H.; Schou, J.; Bush, R. I. (2012), "The Helioseismic and Magnetic Imager (HMI) Investigation for the Solar Dynamics Observatory", Solar Physics, 275 (1–2): 207–227, Bibcode:2012SoPh..275..207S, doi: 10.1007/s11207-011-9834-2
  28. ↑ Hoeksema, J. Todd; Liu, Yang; Hayashi, Keiji (2014), "The HMI Vector Magnetic Field Pipeline: Overview and Performance", Solar Physics, 289: 3483–3530, arXiv: 1404.1881 , doi:10.1007/s11207-014-0516-8
  29. ↑ Lemen, James R.; Title, Alan M.; Akin, David J. (2012), "The Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory", Solar Physics, 275 (1–2): 17–40, Bibcode:2012SoPh..275...17L, doi:10.1007/s11207-011-9776-8
  30. ↑ Kasper, Justin C.; Bale, Stuart D.; Maruca, Bradley A. (2021), "Parker Solar Probe Enters the Magnetically Dominated Solar Corona", Physical Review Letters, 127 (25) 255101, Bibcode:2021PhRvL.127y5101K, doi:10.1103/PhysRevLett.127.255101, hdl: 10150/663300 , PMID   35029449
  31. ↑ Cranmer, Steven R. (2023), "The Sun's Alfvén Surface: Recent Insights and Prospects for Future Observations", Solar Physics, 298: 37, doi: 10.1007/s11207-023-02218-2
  32. ↑ Potential-Field Source-Surface Models, National Solar Observatory, 2020
  33. ↑ Arge, C. N.; Pizzo, V. J. (2000), "Improvement in the prediction of solar wind conditions using near-real time solar magnetic field updates", Journal of Geophysical Research: Space Physics, 105 (A5): 10465–10480, Bibcode:2000JGR...10510465A, doi:10.1029/1999JA000262
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