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Magnetosphere

In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field.[1][2] It is created by a celestial body with an active interior dynamo.

A rendering of the magnetic field lines of the magnetosphere of the Earth.

In the space environment close to a planetary body with a dipole magnetic field such as Earth, the field lines resemble a simple magnetic dipole. Farther out, field lines can be significantly distorted by the flow of electrically conducting plasma, as emitted from the Sun (i.e., the solar wind) or a nearby star.[3][4] Planets having active magnetospheres, like the Earth, are capable of mitigating or blocking the effects of solar radiation or cosmic radiation. Interactions of particles and atmospheres with magnetospheres are studied under the specialized scientific subjects of plasma physics, space physics, and aeronomy.

History

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Main article: Magnetosphere chronology

Study of Earth's magnetosphere began in 1600, when William Gilbert discovered that the magnetic field on the surface of Earth resembled that of a terrella, a small, magnetized sphere. In the 1940s, Walter M. Elsasser proposed the model of dynamo theory, which attributes Earth's magnetic field to the motion of Earth's iron outer core. Through the use of magnetometers, scientists were able to study the variations in Earth's magnetic field as functions of both time and latitude and longitude.

Beginning in the late 1940s, rockets were used to study cosmic rays. In 1958, Explorer 1, the first of the Explorer series of space missions, was launched to study the intensity of cosmic rays above the atmosphere and measure the fluctuations in this activity. This mission observed the existence of the Van Allen radiation belt (located in the inner region of Earth's magnetosphere), with the follow-up Explorer 3 later that year definitively proving its existence. Also during 1958, Eugene Parker proposed the idea of the solar wind, with the term 'magnetosphere' being proposed by Thomas Gold in 1959 to explain how the solar wind interacted with the Earth's magnetic field. The later mission of Explorer 12 in 1961 led by the Cahill and Amazeen observation in 1963 of a sudden decrease in magnetic field strength near the noon-time meridian, later was named the magnetopause. By 1983, the International Cometary Explorer observed the magnetotail, or the distant magnetic field.[4]

Structure and behavior

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Magnetospheres are dependent on several variables: the type of astronomical object, the nature of sources of plasma and momentum, the period of the object's spin, the nature of the axis about which the object spins, the axis of the magnetic dipole, and the magnitude and direction of the flow of solar wind.

The planetary distance where the magnetosphere can withstand the solar wind pressure is called the Chapman–Ferraro distance. This is usefully modeled by the formula wherein   represents the radius of the planet,   represents the magnetic field on the surface of the planet at the equator, and   represents the velocity of the solar wind:

 

A magnetosphere is classified as "intrinsic" when  , or when the primary opposition to the flow of solar wind is the magnetic field of the object. Mercury, Earth, Jupiter, Ganymede, Saturn, Uranus, and Neptune, for example, exhibit intrinsic magnetospheres. A magnetosphere is classified as "induced" when  , or when the solar wind is not opposed by the object's magnetic field. In this case, the solar wind interacts with the atmosphere or ionosphere of the planet (or surface of the planet, if the planet has no atmosphere). Venus has an induced magnetic field, which means that because Venus appears to have no internal dynamo effect, the only magnetic field present is that formed by the solar wind's wrapping around the physical obstacle of Venus (see also Venus' induced magnetosphere). When  , the planet itself and its magnetic field both contribute. It is possible that Mars is of this type.[5]

Structure

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An artist's rendering of the structure of a magnetosphere: 1) Bow shock. 2) Magnetosheath. 3) Magnetopause. 4) Magnetosphere. 5) Northern tail lobe. 6) Southern tail lobe. 7) Plasmasphere.

Bow shock

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Infrared image and artist's concept of the bow shock around R Hydrae
Main article: Bow shock

The bow shock forms the outermost layer of the magnetosphere; the boundary between the magnetosphere and the ambient medium. For stars, this is usually the boundary between the stellar wind and interstellar medium; for planets, the speed of the solar wind there decreases as it approaches the magnetopause.[6] Due to interactions with the bow shock, the stellar wind plasma gains a substantial anisotropy, leading to various plasma instabilities upstream and downstream of the bow shock. [7]

Magnetosheath

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Main article: Magnetosheath

The magnetosheath is the region of the magnetosphere between the bow shock and the magnetopause. It is formed mainly from shocked solar wind, though it contains a small amount of plasma from the magnetosphere.[8] It is an area exhibiting high particle energy flux, where the direction and magnitude of the magnetic field varies erratically. This is caused by the collection of solar wind gas that has effectively undergone thermalization. It acts as a cushion that transmits the pressure from the flow of the solar wind and the barrier of the magnetic field from the object.[4]

Magnetopause

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Main article: Magnetopause

The magnetopause is the area of the magnetosphere wherein the pressure from the planetary magnetic field is balanced with the pressure from the solar wind.[3] It is the convergence of the shocked solar wind from the magnetosheath with the magnetic field of the object and plasma from the magnetosphere. Because both sides of this convergence contain magnetized plasma, the interactions between them are complex. The structure of the magnetopause depends upon the Mach number and beta of the plasma, as well as the magnetic field.[9] The magnetopause changes size and shape as the pressure from the solar wind fluctuates.[10]

Magnetotail

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Opposite the compressed magnetic field is the magnetotail, where the magnetosphere extends far beyond the astronomical object. It contains two lobes, referred to as the northern and southern tail lobes. Magnetic field lines in the northern tail lobe point towards the object while those in the southern tail lobe point away. The tail lobes are almost empty, with few charged particles opposing the flow of the solar wind. The two lobes are separated by a plasma sheet, an area where the magnetic field is weaker, and the density of charged particles is higher.[11]

Earth's magnetosphere

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See also: Earth's magnetic field § Magnetosphere, and Van Allen radiation belt
Further information: Plasmasphere
 
Artist's rendition of Earth's magnetosphere
 
Diagram of Earth's magnetosphere

Over Earth's equator, the magnetic field lines become almost horizontal, then return to reconnect at high latitudes. However, at high altitudes, the magnetic field is significantly distorted by the solar wind and its solar magnetic field. On the dayside of Earth, the magnetic field is significantly compressed by the solar wind to a distance of approximately 65,000 kilometers (40,000 mi). Earth's bow shock is about 17 kilometers (11 mi) thick[12] and located about 90,000 kilometers (56,000 mi) from Earth.[13] The magnetopause exists at a distance of several hundred kilometers above Earth's surface. Earth's magnetopause has been compared to a sieve because it allows solar wind particles to enter. Kelvin–Helmholtz instabilities occur when large swirls of plasma travel along the edge of the magnetosphere at a different velocity from the magnetosphere, causing the plasma to slip past. This results in magnetic reconnection, and as the magnetic field lines break and reconnect, solar wind particles are able to enter the magnetosphere.[14] On Earth's nightside, the magnetic field extends in the magnetotail, which lengthwise exceeds 6,300,000 kilometers (3,900,000 mi).[3] Earth's magnetotail is the primary source of the polar aurora.[11] Also, NASA scientists have suggested that Earth's magnetotail might cause "dust storms" on the Moon by creating a potential difference between the day side and the night side.[15]

Other objects

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Many astronomical objects generate and maintain magnetospheres. In the Solar System this includes the Sun, Mercury, Earth, Jupiter, Saturn, Uranus, Neptune,[16] and Ganymede. The magnetosphere of Jupiter is the largest planetary magnetosphere in the Solar System, extending up to 7,000,000 kilometers (4,300,000 mi) on the dayside and almost to the orbit of Saturn on the nightside.[17] Jupiter's magnetosphere is stronger than Earth's by an order of magnitude, and its magnetic moment is approximately 18,000 times larger.[18] Venus, Mars, and Pluto, on the other hand, have no magnetic field. This may have had significant effects on their geological history. It is theorized that Venus and Mars may have lost their primordial water to photodissociation and the solar wind. A strong magnetosphere greatly slows this process.[16][19]

 
Artist impression of the magnetic field around Tau Boötis b detected in 2020.

Magnetospheres generated by exoplanets are thought to be common, though the first discoveries did not come until the 2010s. In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet.[20][21] In 2019, the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[22][23] In 2020, a radio emission in the 14-30 MHz band was detected from the Tau Boötis system, likely associated with cyclotron radiation from the poles of Tau Boötis b a signature of a planetary magnetic field.[24][25] In 2021 a magnetic field generated by HAT-P-11b became the first to be confirmed.[26] The first unconfirmed detection of a magnetic field generated by a terrestrial exoplanet was found in 2023 on YZ Ceti b.[27][28][29][30]

See also

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References

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  1. ^ "Magnetospheres". NASA Science. NASA.
  2. ^ Ratcliffe, John Ashworth (1972). An Introduction to the Ionosphere and Magnetosphere. CUP Archive. ISBN 9780521083416.
  3. ^ a b c "Ionosphere and magnetosphere". Encyclopædia Britannica. Encyclopædia Britannica, Inc. 2012.
  4. ^ a b c Van Allen, James Alfred (2004). Origins of Magnetospheric Physics. Iowa City, Iowa USA: University of Iowa Press. ISBN 9780877459217. OCLC 646887856.
  5. ^ Blanc, M.; Kallenbach, R.; Erkaev, N.V. (2005). "Solar System Magnetospheres". Space Science Reviews. 116 (1–2): 227–298. Bibcode:2005SSRv..116..227B. doi:10.1007/s11214-005-1958-y. S2CID 122318569.
  6. ^ Sparavigna, A.C.; Marazzato, R. (10 May 2010). "Observing stellar bow shocks". arXiv:1005.1527 [physics.space-ph].
  7. ^ Pokhotelov, D.; von Alfthan, S.; Kempf, Y.; Vainio, R.; et al. (17 December 2013). "Ion distributions upstream and downstream of the Earth's bow shock: first results from Vlasiator". Annales Geophysicae. 31 (12): 2207–2212. Bibcode:2013AnGeo..31.2207P. doi:10.5194/angeo-31-2207-2013.
  8. ^ Paschmann, G.; Schwartz, S.J.; Escoubet, C.P.; Haaland, S., eds. (2005). Outer Magnetospheric Boundaries: Cluster Results (PDF). Space Sciences Series of ISSI. Vol. 118. Bibcode:2005ombc.book.....P. doi:10.1007/1-4020-4582-4. ISBN 978-1-4020-3488-6. {{cite book}}: |journal= ignored (help)
  9. ^ Russell, C.T. (1990). "The Magnetopause". In Russell, C.T.; Priest, E.R.; Lee, L.C. (eds.). Physics of magnetic flux ropes. American Geophysical Union. pp. 439–453. ISBN 9780875900261. Archived from the original on 2 February 1999.
  10. ^ Stern, David P.; Peredo, Mauricio (20 November 2003). "The Magnetopause". The Exploration of the Earth's Magnetosphere. NASA. Archived from the original on 19 August 2019. Retrieved 19 August 2019.
  11. ^ a b "The Tail of the Magnetosphere". NASA. Archived from the original on 7 February 2018. Retrieved 22 December 2012.
  12. ^ "Cluster reveals Earth's bow shock is remarkably thin". European Space Agency. 16 November 2011.
  13. ^ "Cluster reveals the reformation of Earth's bow shock". European Space Agency. 11 May 2011.
  14. ^ "Cluster observes a 'porous' magnetopause". European Space Agency. 24 October 2012.
  15. ^ http://www.nasa.gov/topics/moonmars/features/magnetotail_080416.html Archived 14 November 2021 at the Wayback Machine NASA, The Moon and the Magnetotail
  16. ^ a b "Planetary Shields: Magnetospheres". NASA. Retrieved 5 January 2020.
  17. ^ Khurana, K. K.; Kivelson, M. G.; et al. (2004). "The configuration of Jupiter's magnetosphere" (PDF). In Bagenal, Fran; Dowling, Timothy E.; McKinnon, William B. (eds.). Jupiter: The Planet, Satellites and Magnetosphere. Cambridge University Press. ISBN 978-0-521-81808-7.
  18. ^ Russell, C.T. (1993). "Planetary Magnetospheres". Reports on Progress in Physics. 56 (6): 687–732. Bibcode:1993RPPh...56..687R. doi:10.1088/0034-4885/56/6/001. S2CID 250897924.
  19. ^ NASA (14 September 2016). "X-ray Detection Sheds New Light on Pluto". nasa.gov. Retrieved 3 December 2016.
  20. ^ Charles Q. Choi (20 November 2014). "Unlocking the Secrets of an Alien World's Magnetic Field". Space.com. Retrieved 17 January 2022.
  21. ^ Kislyakova, K. G.; Holmstrom, M.; Lammer, H.; Odert, P.; Khodachenko, M. L. (2014). "Magnetic moment and plasma environment of HD 209458b as determined from Ly observations". Science. 346 (6212): 981–984. arXiv:1411.6875. Bibcode:2014Sci...346..981K. doi:10.1126/science.1257829. PMID 25414310. S2CID 206560188.
  22. ^ Passant Rabie (29 July 2019). "Magnetic Fields of 'Hot Jupiter' Exoplanets Are Much Stronger Than We Thought". Space.com. Retrieved 17 January 2022.
  23. ^ Cauley, P. Wilson; Shkolnik, Evgenya L.; Llama, Joe; Lanza, Antonino F. (December 2019). "Magnetic field strengths of hot Jupiters from signals of star-planet interactions". Nature Astronomy. 3 (12): 1128–1134. arXiv:1907.09068. Bibcode:2019NatAs...3.1128C. doi:10.1038/s41550-019-0840-x. ISSN 2397-3366. S2CID 198147426.
  24. ^ Turner, Jake D.; Zarka, Philippe; Grießmeier, Jean-Mathias; Lazio, Joseph; Cecconi, Baptiste; Emilio Enriquez, J.; Girard, Julien N.; Jayawardhana, Ray; Lamy, Laurent; Nichols, Jonathan D.; De Pater, Imke (2021), "The search for radio emission from the exoplanetary systems 55 Cancri, υ Andromedae, and τ Boötis using LOFAR beam-formed observations", Astronomy & Astrophysics, 645: A59, arXiv:2012.07926, Bibcode:2021A&A...645A..59T, doi:10.1051/0004-6361/201937201, S2CID 212883637
  25. ^ O'Callaghan, Jonathan (7 August 2023). "Exoplanets Could Help Us Learn How Planets Make Magnetism". Quanta Magazine. Retrieved 7 August 2023.
  26. ^ HAT-P-11 Spectral Energy Distribution Signatures of Strong Magnetization and Metal-poor Atmosphere for a Neptune-Size Exoplanet, Ben-Jaffel et al. 2021
  27. ^ Pineda, J. Sebastian; Villadsen, Jackie (April 2023). "Coherent radio bursts from known M-dwarf planet host YZ Ceti". Nature Astronomy. 7 (5): 569–578. arXiv:2304.00031. Bibcode:2023NatAs...7..569P. doi:10.1038/s41550-023-01914-0.
  28. ^ Trigilio, Corrado; Biswas, Ayan; et al. (May 2023). "Star-Planet Interaction at radio wavelengths in YZ Ceti: Inferring planetary magnetic field". arXiv:2305.00809 [astro-ph.EP].
  29. ^ "A magnetic field on a nearby Earth-sized exoplanet?". earthsky.org. 10 April 2023. Retrieved 7 August 2023.
  30. ^ O'Callaghan, Jonathan (7 August 2023). "Exoplanets Could Help Us Learn How Planets Make Magnetism". Quanta Magazine.

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