Editing Ganymede (moon) (section)

===Magnetosphere===
[[Image:Ganymede magnetic field.svg#Summary|thumb|Magnetic field of the Jovian satellite Ganymede, which is embedded into the magnetosphere of Jupiter. Closed field lines are marked with green color.]]
The ''Galileo'' craft made six close flybys of Ganymede from 1995 to 2000 (G1, G2, G7, G8, G28 and G29)<ref name="Kivelson2002" /> and discovered that Ganymede has a permanent (intrinsic) [[magnetic moment]] independent of the Jovian magnetic field.<ref name="Kivelson1997">{{cite journal |last1=Kivelson |first1=M.G. |last2=Khurana |first2=K.K. |last3=Coroniti |first3=F.V. |last4=Joy |first4=S. |last5=Russell |first5=C. T. |last6=Walker |first6=R. J. |last7=Warnecke |first7=J. |last8=Bennett |first8=L. |last9=Polanskey |first9=C. |display-authors=2 |title=The magnetic field and magnetosphere of Ganymede |journal=Geophys. Res. Lett. |date=1997 |volume=24 |issue=17 |pages=2155–2158 |doi=10.1029/97GL02201 |url=http://www.igpp.ucla.edu/people/mkivelson/Publications/97GL02201.pdf |bibcode=1997GeoRL..24.2155K |doi-access=free |access-date=January 15, 2008 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050805/http://www.igpp.ucla.edu/people/mkivelson/Publications/97GL02201.pdf |url-status=live }}</ref> The value of the moment is about {{nowrap |1.3{{E-sp |13}} T·m<sup>3</sup>}},<ref name="Kivelson2002" /> which is three times larger than the [[Mercury's magnetic field|magnetic moment of Mercury]]. The magnetic dipole is tilted with respect to the rotational axis of Ganymede by 176°, which means that it is directed against the Jovian magnetic moment.<ref name="Kivelson2002" /> Its north pole lies below the [[orbital plane (astronomy)|orbital plane]]. The [[dipole|dipole magnetic field]] created by this permanent moment has a strength of 719 ± 2 [[Tesla (unit)|nT]] at Ganymede's equator,<ref name="Kivelson2002" /> which should be compared with the Jovian magnetic field at the distance of Ganymede—about 120 nT.<ref name="Kivelson1997" /> The equatorial field of Ganymede is directed against the Jovian field, meaning [[Magnetic reconnection|reconnection]] is possible. The intrinsic field strength at the poles is two times that at the equator—1440 nT.<ref name="Kivelson2002" />
[[Image:15-33i2-JupiterMoon-Ganymede-Aurora-20150312.png|thumb|right|Aurorae on Ganymede—auroral belt shifting may indicate a subsurface saline ocean.]]
The permanent magnetic moment carves a part of space around Ganymede, creating a tiny [[magnetosphere]] embedded inside [[Jupiter's magnetosphere|that of Jupiter]]; it is the only moon in the Solar System known to possess the feature.<ref name="Kivelson1997" /> Its diameter is 4–5 Ganymede radii.<ref name="Kivelson1998">{{cite journal |last1=Kivelson |first1=M.G. |last2=Warnecke |first2=J. |last3=Bennett |first3=L. |last4=Joy |first4=S. |last5=Khurana |first5=K. K. |last6=Linker |first6=J. A. |last7=Russell |first7=C. T. |last8=Walker |first8=R. J. |last9=Polanskey |first9=C. |display-authors=2 |title=Ganymede's magnetosphere: magnetometer overview |journal=J. Geophys. Res. |date=1998 |volume=103 |issue=E9 |pages=19,963–19,972 |doi=10.1029/98JE00227 |url=http://www.igpp.ucla.edu/people/mkivelson/Publications/98JE00227.pdf |bibcode=1998JGR...10319963K |doi-access=free |access-date=January 15, 2008 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050819/http://www.igpp.ucla.edu/people/mkivelson/Publications/98JE00227.pdf |url-status=live }}</ref> The Ganymedian magnetosphere has a region of closed [[field line]]s located below 30° latitude, where [[charged particle]]s ([[electron]]s and [[ion]]s) are trapped, creating a kind of [[radiation belt]].<ref name="Kivelson1998" /> The main ion species in the magnetosphere is single ionized oxygen<ref name="Eviatar2001" /> (O<sup>+</sup>) which fits well with Ganymede's tenuous oxygen [[atmosphere]]. In the polar cap regions, at latitudes higher than 30°, magnetic field lines are open, connecting Ganymede with Jupiter's ionosphere.<ref name="Kivelson1998" /> In these areas, the energetic (tens and hundreds of [[kiloelectronvolt]]) electrons and ions have been detected,<ref name="Paranicas1999" /> which may cause the auroras observed around the Ganymedian poles.<ref name="Feldman2000" /> In addition, heavy ions precipitate continuously on Ganymede's polar surface, [[sputtering]] and darkening the ice.<ref name="Paranicas1999" />

The interaction between the Ganymedian magnetosphere and Jovian [[plasma (physics)|plasma]] is in many respects similar to that of the [[solar wind]] and Earth's magnetosphere.<ref name="Kivelson1998" /><ref name="Volwerk1999">{{cite journal |last1=Volwerk |first1=M. |last2=Kivelson |first2=M.G. |last3=Khurana |first3=K.K. |last4=McPherron |first4=R.L. |title=Probing Ganymede's magnetosphere with field line resonances |journal=J. Geophys. Res. |date=1999 |volume=104 |issue=A7 |pages=14,729–14,738 |doi=10.1029/1999JA900161 |url=http://www.igpp.ucla.edu/people/mkivelson/Publications/1999JA900161.pdf |bibcode=1999JGR...10414729V |doi-access=free |access-date=January 15, 2008 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050807/http://www.igpp.ucla.edu/people/mkivelson/Publications/1999JA900161.pdf |url-status=live }}</ref> The plasma co-rotating with Jupiter impinges on the trailing side of the Ganymedian magnetosphere much like the solar wind impinges on the Earth's magnetosphere. The main difference is the speed of plasma flow—[[supersonic]] in the case of Earth and [[Speed of sound|subsonic]] in the case of Ganymede. Because of the subsonic flow, there is no [[bow shock]] off the trailing hemisphere of Ganymede.<ref name="Volwerk1999" />

In addition to the intrinsic magnetic moment, Ganymede has an induced dipole magnetic field.<ref name="Kivelson2002" /> Its existence is connected with the variation of the Jovian magnetic field near Ganymede. The induced moment is directed radially to or from Jupiter following the direction of the varying part of the planetary magnetic field. The induced magnetic moment is an order of magnitude weaker than the intrinsic one. The [[field strength]] of the induced field at the magnetic equator is about 60 nT—half of that of the ambient Jovian field.<ref name="Kivelson2002" /> The induced magnetic field of Ganymede is similar to those of Callisto and Europa, indicating that Ganymede also has a subsurface water ocean with a high electrical conductivity.<ref name="Kivelson2002" />

Given that Ganymede is completely differentiated and has a metallic core,<ref name="Showman1999" /><ref name="Hauk2006" /> its intrinsic magnetic field is probably generated in a similar fashion to the Earth's: as a result of conducting material moving in the interior.<ref name="Kivelson2002" /><ref name="Hauk2006" /> The magnetic field detected around Ganymede is likely to be caused by compositional convection in the core,<ref name="Hauk2006" /> if the magnetic field is the product of dynamo action, or magnetoconvection.<ref name="Kivelson2002" /><ref name="Hauck2002">{{cite journal |last1=Hauck |first1=Steven A. |last2=Dombard |first2=A. J. |last3=Solomon |first3=S. C. |last4=Aurnou |first4=J. M. |title=Internal structure and mechanism of core convection on Ganymede |journal=Lunar and Planetary Science |volume=XXXIII |date=2002 |page=1380 |url=http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1380.pdf |bibcode=2002LPI....33.1380H |access-date=October 21, 2007 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050805/http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1380.pdf |url-status=live }}</ref>

Despite the presence of an iron core, Ganymede's magnetosphere remains enigmatic, particularly given that similar bodies lack the feature.<ref name="Showman1999" /> Some research has suggested that, given its relatively small size, the core ought to have sufficiently cooled to the point where fluid motions, hence a magnetic field would not be sustained. One explanation is that the same orbital resonances proposed to have disrupted the surface also allowed the magnetic field to persist: with Ganymede's eccentricity pumped and tidal heating of the mantle increased during such resonances, reducing heat flow from the core, leaving it fluid and convective.<ref name="Bland2007" /> Another explanation is a remnant magnetization of silicate rocks in the mantle, which is possible if the satellite had a more significant dynamo-generated field in the past.<ref name="Showman1999" />