Magnetosphere of Saturn
UV | |
Total power | 0.5 TW |
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Radio emission frequencies | 10–1300 kHz |
The magnetosphere of
Saturn's magnetosphere is filled with
The interaction between Saturn's magnetosphere and the solar wind generates bright oval
In 1980–1981 the magnetosphere of Saturn was studied by the Voyager spacecraft. Up until September 2017 it was a subject of ongoing investigation by Cassini mission, which arrived in 2004 and spent over 13 years observing the planet.
Discovery
Immediately after the discovery of Jupiter's
These medium wave emissions were modulated with a period of about 10 h 30 min, which was interpreted as Saturn's
Structure
Internal field
Like
The magnetic field strength at Saturn's equator is about 21
Size and shape
Saturn's internal magnetic field deflects the solar wind, a stream of ionized particles emitted by the Sun, away from its surface, preventing it from interacting directly with its atmosphere and instead creating its own region, called a magnetosphere, composed of a plasma very different from that of the solar wind.[12] The magnetosphere of Saturn is the second–largest magnetosphere in the Solar System after that of Jupiter.[3]
As with Earth's magnetosphere, the boundary separating the solar wind's plasma from that within Saturn's magnetosphere is called the magnetopause.[2] The magnetopause distance from the planet's center at the subsolar point[note 1] varies widely from 16 to 27 Rs (Rs=60,330 km is the equatorial radius of Saturn).[14][15] The magnetopause's position depends on the pressure exerted by the solar wind, which in turn depends on solar activity. The average magnetopause standoff distance is about 22 Rs.[6] In front of the magnetopause (at the distance of about 27 Rs from the planet)[6] lies the bow shock, a wake-like disturbance in the solar wind caused by its collision with the magnetosphere. The region between the bow shock and magnetopause is called the magnetosheath.[16]
At the opposite side of the planet, the solar wind stretches Saturn's magnetic field lines into a long, trailing
Magnetospheric regions
Saturn's magnetosphere is often divided into four regions.[18] The innermost region co-located with Saturn's planetary rings, inside approximately 3 Rs, has a strictly dipolar magnetic field. It is largely devoid of plasma, which is absorbed by ring particles, although the radiation belts of Saturn are located in this innermost region just inside and outside the rings.[18] The second region between 3 and 6 Rs contains the cold plasma torus and is called the inner magnetosphere. It contains the densest plasma in the saturnian system. The plasma in the torus originates from the inner icy moons and particularly from Enceladus.[18] The magnetic field in this region is also mostly dipolar.[19] The third region lies between 6 and 12–14 Rs and is called the dynamic and extended plasma sheet. The magnetic field in this region is stretched and non-dipolar,[18] whereas the plasma is confined to a thin equatorial plasma sheet.[19] The fourth outermost region is located beyond 15 Rs at high latitudes and continues up to magnetopause boundary. It is characterized by a low plasma density and a variable, non-dipolar magnetic field strongly influenced by the Solar wind.[18]
In the outer parts of Saturn's magnetosphere beyond approximately 15–20 Rs[20] the magnetic field near the equatorial plane is highly stretched and forms a disk-like structure called magnetodisk.[21] The disk continues up to the magnetopause on the dayside and transitions into the magnetotail on the nightside.[22] Near the dayside it can be absent when the magnetosphere is compressed by the Solar wind, which usually happens when the magnetopause distance is smaller than 23 Rs. On the nightside and flanks of the magnetosphere the magnetodisk is always present.[21] The Saturnian magnetodisk is a much smaller analog of the Jovian magnetodisk.[17]
The plasma sheet in Saturn's magnetosphere has a bowl-like shape not found in any other known magnetosphere. When Cassini arrived in 2004, there was a winter in the northern hemisphere. The measurements of the magnetic field and plasma density revealed that the plasma sheet was warped and lay to the north of the equatorial plane, looking like a giant bowl. Such a shape was unexpected.[21]
Dynamics
The processes driving Saturn's magnetosphere are similar to those driving Earth's and Jupiter's.
Another distinguishing feature of Saturn's magnetosphere is high abundance of neutral gas around the planet. As revealed by ultraviolet observation of Cassini, the planet is enshrouded in a large cloud of hydrogen, water vapor and their dissociative products like hydroxyl, extending as far as 45 Rs from Saturn. In the inner magnetosphere the ratio of neutrals to ions is around 60 and it increases in the outer magnetosphere, which means that the entire magnetospheric volume is filled with relatively dense weakly ionized gas. This is different, for instance, from Jupiter or Earth, where ions dominate over neutral gas, and has consequences for the magnetospheric dynamics.[24]
Sources and transport of plasma
The plasma composition in Saturn's inner magnetosphere is dominated by the water group ions: O+, H2O+, OH+ and others,
In the outer parts of the magnetosphere the dominant ions are protons, which originate either from the Solar wind or Saturn's ionosphere.[29] Titan, which orbits close to the magnetopause boundary at 20 Rs, is not a significant source of plasma.[29][30]
The relatively cold plasma in the innermost region of Saturn's magnetosphere, inside 3 Rs (near the rings) consists mainly of O+ and O2+ ions.[25] There ions together with electrons form an ionosphere surrounding the saturnian rings.[31]
For both Jupiter and Saturn, transport of plasma from the inner to the outer parts of the magnetosphere is thought to be related to interchange instability.
In the magnetodisk region, beyond 6 Rs, the plasma within the co–rotating sheet exerts a significant centrifugal force on the magnetic field, causing it to stretch.[34][note 3] This interaction creates a current in the equatorial plane flowing azimuthally with rotation and extending as far as 20 Rs from the planet.[35] The total strength of this current varies from 8 to 17 MA.[34][35] The ring current in the saturnian magnetosphere is highly variable and depends on the solar wind pressure, being stronger when the pressure is weaker.[35] The magnetic moment associated with this current slightly (by about 10 nT) depresses the magnetic field in the inner magnetosphere,[36] although it increases the total magnetic moment of the planet and causing the size of the magnetosphere to become larger.[35]
Aurorae
Saturn has bright polar aurorae, which have been observed in the
Unlike Jupiter's, Saturn's main auroral ovals are not related to the breakdown of the co–rotation of the plasma in the outer parts of the planet's magnetosphere.
The aurorae of Saturn are highly variable.
Saturn kilometric radiation
Saturn is the source of rather strong low frequency radio emissions called Saturn kilometric radiation (SKR). The frequency of SKR lies in the range 10–1300 kHz (wavelength of a few kilometers) with the maximum around 400 kHz.[7] The power of these emissions is strongly modulated by the rotation of the planet and is correlated with changes in the solar wind pressure. For instance, when Saturn was immersed into the giant magnetotail of Jupiter during Voyager 2 flyby in 1981, the SKR power decreased greatly or even ceased completely.[7][44] The kilometric radiation is thought to be generated by the Cyclotron Maser Instability of the electrons moving along magnetic field lines related to the auroral regions of Saturn.[44] Thus the SKR is related to the auroras around the poles of the planet. The radiation itself comprises spectrally diffuse emissions as well as narrowband tones with bandwidths as narrow as 200 Hz. In the frequency–time plane, arc-like features are often observed, much like in the case of the Jovian kilometric radiation.[44] The total power of the SKR is around 1 GW.[7]
The modulation of the radio emissions by planetary rotation is traditionally used to determine the rotation period of the interiors of fluid giant planets.
Radiation belts
Saturn has relatively weak radiation belts, because energetic particles are absorbed by the moons and particulate material orbiting the planet.
The innermost region of the magnetosphere near the rings is generally devoid of energetic ions and electrons because they are absorbed by ring particles.
The saturnian radiation belts are generally much weaker than those of Jupiter and do not emit much
Interaction with rings and moons
The abundant population of solid bodies orbiting Saturn including moons as well as ring particles exerts a strong influence on the magnetosphere of Saturn. The plasma in the magnetosphere co-rotates with the planet, continuously impinging on the trailing hemispheres of slowly moving moons.[51] While ring particles and the majority of moons only passively absorb plasma and energetic charged particles, three moons – Enceladus, Dione and Titan – are significant sources of new plasma.[52][53] The absorption of energetic electrons and ions reveals itself by noticeable gaps in the radiation belts of Saturn near the moon's orbits, while the dense rings of Saturn eliminate all energetic electrons and ions closer than 2.2 RS, creating a low radiation zone in the vicinity of the planet.[48] The absorption of the co-rotating plasma by a moon disturbs the magnetic field in its empty wake—the field is pulled towards a moon, creating a region of a stronger magnetic field in the near wake.[51]
The three moons mentioned above add new plasma into the magnetosphere. By far the strongest source is Enceladus, which ejects a fountain of water vapor, carbon dioxide and nitrogen through cracks in its south pole region.[27] A fraction of this gas is ionized by the hot electrons and solar ultraviolet radiation and is added to the co-rotational plasma flow.[52] Titan once was thought to be the principal source of plasma in Saturn's magnetosphere, especially of nitrogen. The new data obtained by Cassini in 2004–2008 established that it is not a significant source of nitrogen after all,[29] although it may still provide significant amounts of hydrogen (due to dissociation of methane).[54] Dione is the third moon producing more new plasma than it absorbs. The mass of plasma created in the vicinity of it (about 6 g/s) is about 1/300 as much as near Enceladus.[53] However, even this low value can not be explained only by sputtering of its icy surface by energetic particles, which may indicate that Dione is endogenously active like Enceladus. The moons that create new plasma slow the motion of the co-rotating plasma in their vicinity, which leads to the pile-up of the magnetic field lines in front of them and weakening of the field in their wakes—the field drapes around them.[55] This is the opposite to what is observed for the plasma-absorbing moons.
The plasma and energetic particles present in the magnetosphere of Saturn, when absorbed by ring particles and moons, cause
Exploration
As of 2014 the magnetosphere of Saturn has been directly explored by four spacecraft. The first mission to study the magnetosphere was
In the 1990s, the Ulysses spacecraft conducted extensive measurements of the Saturnian kilometric radiation (SKR),[7] which is unobservable from Earth due to the absorption in the ionosphere.[58] The SKR is powerful enough to be detected from a spacecraft at the distance of several astronomical units from the planet. Ulysses discovered that the period of the SKR varies by as much as 1%, and therefore is not directly related to the rotation period of the interior of Saturn.[7]
Notes
- ^ The subsolar point is a point on a planet, never fixed, at which the Sun appears directly overhead.
- ^ On the dayside a noticeable magnetodisk only forms when the Solar wind pressure is low, and the magnetosphere has a size larger than about 23 Rs. However, when the magnetosphere is compressed by the Solar wind the dayside magnetodisk is quite small. On the other hand, in the dawn sector of the magnetosphere the disk-like configuration is present permanently.[21]
- ^ a b The contribution of the plasma thermal pressure gradient force may also be significant.[35] In addition, an important contribution to the ring current is provided by energetic ions with energy of more than about 10 keV.[35]
- ^ The difference between the southern and northern aurorae is related to the shift of the internal magnetic dipole to the northern hemisphere—the magnetic field in the northern hemisphere is slightly stronger than in the southern one.[39][40]
References
- ^ a b c Russel, 1993, p. 694
- ^ a b c d e f g Belenkaya, 2006, pp. 1145–46
- ^ a b Blanc, 2005, p. 238
- ^ a b c Sittler, 2008, pp. 4, 16–17
- ^ a b c Tokar, 2006
- ^ a b c Gombosi, 2009, p. 206, Table 9.1
- ^ a b c d e f Zarka, 2005, pp. 378–379
- ^ a b c d Bhardwaj, 2000, pp. 328–333
- ^ Smith, 1959
- ^ Brown, 1975
- ^ Kivelson, 2005, p. 2077
- ^ a b c d e f Russel, 1993, pp. 717–718
- ^ a b c d e Kivelson, 2005, pp. 303–313
- ^ Russel, 1993, p. 709, Table 4
- ^ Gombosi, 2009, p. 247
- ^ a b Russel, 1993, pp. 690–692
- ^ a b c Gombosi, 2009, pp. 206–209
- ^ a b c d e f Andre, 2008, pp. 10–15
- ^ a b Andre, 2008, pp. 6–9
- ^ Mauk, 2009, pp. 317–318
- ^ a b c d Gombosi, 2009, pp. 211–212
- ^ Gombosi, 2009, pp. 231–234
- ^ Blanc, 2005, pp. 264–273
- ^ Mauk, 2009, pp. 282–283
- ^ a b c d Young, 2005
- ^ Smith, 2008
- ^ a b c Gombosi, 2009, pp. 216–219
- ^ Smith, 2008, pp. 1–2
- ^ a b c Gombosi, 2009, pp. 219–220
- ^ a b Russell, 2008, p. 1
- ^ Gombosi, 2009, pp. 206, 215–216
- ^ a b c Gombosi, 2009, pp. 237–240
- ^ Sontag, 2021
- ^ a b Bunce, 2008, pp. 1–2
- ^ a b c d e f Gombosi, 2009, pp. 225–231
- ^ Bunce, 2008, p. 20
- ^ Kurth, 2009, pp. 334–342
- ^ a b c d e Clark, 2005
- ^ a b Nichols, 2009
- ^ Gombosi, 2009, pp. 209–211
- ^ a b Kurth, 2009, pp. 335–336
- ^ "Hubble observes energetic lightshow at Saturn's north pole". www.spacetelescope.org. Retrieved 30 August 2018.
- ^ a b Cowley, 2008, pp. 2627–2628
- ^ a b c Kurth, 2009, pp. 341–348
- ^ a b c Zarka, 2007
- ^ Gurnett, 2005, p. 1256
- ^ a b Andre, 2008, pp. 11–12
- ^ a b c d e f g Gombosi, 2009, pp. 221–225
- ^ a b c Paranicas, 2008
- ^ Zarka, 2005, pp. 384–385
- ^ a b Mauk, 2009, pp. 290–293
- ^ a b Mauk, 2009, pp. 286–289
- ^ a b Leisner, 2007
- ^ Mauk, 2009, pp. 283–284, 286–287
- ^ Mauk, 2009, pp. 293–296
- ^ a b c d Mauk, 2009, pp. 285–286
- ^ Johnson, 2008, pp. 393–394
- ^ Zarka, 2005, p. 372
Bibliography
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- Belenkaya, E.S.; Alexeev, I.I.; Kalagaev, V.V.; Blohhina, M.S. (2006). "Definition of Saturn's magnetospheric model parameters for the Pioneer 11 flyby" (PDF). Annales Geophysicae. 24 (3): 1145–56. .
- Bhardwaj, Anil; Gladstone, G. Randall (2000). "Auroral emissions of the giant planets". Reviews of Geophysics. 38 (3): 295–353. .
- Blanc, M.; Kallenbach, R.; Erkaev, N.V. (2005). "Solar System Magnetospheres". Space Science Reviews. 116 (1–2): 227–298. S2CID 122318569.
- Brown, Larry W. (1975). "Saturn radio emission near 1 MHz". Journal of Geophysical Research. 112: L89–L92. S2CID 123085550.
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- Clark, J.T.; Gerard, J.-C.; Grodent D.; et al. (2005). "Morphological differences between Saturn's ultraviolet aurorae and those of Earth and Jupiter" (PDF). Nature. 433 (7027): 717–719. S2CID 4379846. Archived from the original(PDF) on 2011-07-16.
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- Gombosi, Tamas I.; Armstrong, Thomas P.; Arridge, Christopher S.; et al. (2009). "Saturn's Magnetospheric Configuration". Saturn from Cassini–Huygens. Springer Netherlands. pp. 203–255. ISBN 978-1-4020-9217-6.
- Gurnett, D.A.; Kurth, W.S.; Hospodarsky, G.B.; et al. (2005). "Radio and Plasma Wave Observations at Saturn from Cassini's Approach and First Orbit". Science. 307 (5713): 1255–59. S2CID 19400012.
- Johnson, R.E.; Luhmann, J.G.; Tokar, R.L.; et al. (2008). "Production, ionization and redistribution of O2 in Saturn's ring atmosphere" (PDF). .
- Kivelson, Margaret Galland (2005). "The current systems of the Jovian magnetosphere and ionosphere and predictions for Saturn" (PDF). Space Science Reviews. 116 (1–2): 299–318. S2CID 17740545.
- Kivelson, M.G. (2005). "Transport and acceleration of plasma in the magnetospheres of Earth and Jupiter and expectations for Saturn" (PDF). Advances in Space Research. 36 (11): 2077–89. .
- Kurth, W.S.; ISBN 978-1-4020-9217-6.
- Leisner, S.; Khurana, K.K.; Russell, C.T.; et al. (2007). "Observations of Enceladus and Dione as Sources for Saturn's Neutral Cloud". Lunar and Planetary Science. XXXVIII (1338): 1425. Bibcode:2007LPI....38.1425L.
- Mauk, B.H.; Hamilton, D.C.; Hill, T.W.; et al. (2009). "Fundamental Plasma Processes in Saturn's Magnetosphere". Saturn from Cassini–Huygens. Springer Netherlands. pp. 281–331. ISBN 978-1-4020-9217-6.
- Nichols, J.D.; Badman, S.V.; Bunce, E.J.; et al. (2009). "Saturn's equinoctial auroras" (PDF). Geophysical Research Letters. 36 (24): L24102:1–5. hdl:2027.42/95061.
- Paranicas, C.; Mitchell, D.G.; Krimigis, S.M.; et al. (2007). "Sources and losses of energetic protons in Saturn's magnetosphere" (PDF). .
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- Russell, C.T.; Jackman, C.M.; Wei, H.Y.; et al. (2008). "Titan's influence on Saturnian substorm occurrence". Geophysical Research Letters. 35 (12): L12105. hdl:11336/20684.
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Further reading
- Arridge, C.S.; Russell, C.T.; Khurana, K.K.; et al. (2007). "Mass of Saturn's magnetodisc: Cassini observations". Geophysical Research Letters. 34 (9): L09108. .
- Burger, M.H.; Sittler, E.C.; Johnson, R.E.; et al. (2007). "Understanding the escape of water from Enceladus". .
- Hill, T.W.; doi:10.1029/2007JA012626. Archived from the original(PDF) on 2012-02-26. Retrieved 2010-05-05.
- Krimigis, S.M.; Sergis, N.; Mitchell, D.G.; et al. (2007). "A dynamic, rotating ring current around Saturn" (PDF). Nature. 450 (7172): 1050–53. S2CID 590002.
- Martens, Hilary R.; Reisenfeld, Daniel B.; Williams, John D.; et al. (2008). "Observations of molecular oxygen ions in Saturn's inner magnetosphere" (PDF). Geophysical Research Letters. 35 (20): L20103. .
- Russell, C.T.; Khurana, K.K.; Arridge, C.S.; Dougherty, M.K. (2008). "The magnetospheres of Jupiter and Saturn and their lessons for the Earth" (PDF). Advances in Space Research. 41 (8): 1310–18. doi:10.1016/j.asr.2007.07.037. Archived from the original(PDF) on 2012-02-15. Retrieved 2009-05-14.
- Smith, H.T.; Johnson, R.E.; Sittler, E.C. (2007). "Enceladus: The likely dominant nitrogen source in Saturn's magnetosphere" (PDF). .
- Southwood, D.J.; Kivelson, M.G. (2007). "Saturnian magnetospheric dynamics: Elucidation of a camshaft model" (PDF). .
- Stallard, Tom; Miller, Steve; Melin, Henrik; et al. (2008). "Jovian-like aurorae on Saturn". Nature. 453 (7198): 1083–85. S2CID 4413780.
- Saturn Sends Mixed Signals
External links
- NASA site about the emissions Archived 2023-03-28 at the Wayback Machine