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harlequin
2004-Mar-13, 03:42 AM
The magnetic fields of Uranus and Neptune have puzzled astronomers. But now an article in Nature has a proposed solution.

Enigma of Uranus solved at last (http://story.news.yahoo.com/news?tmpl=story&cid=1540&e=4&u=/afp/space_uranus_magnetism)

Ian Goddard
2004-Mar-13, 04:37 AM
Thanks harlequin! Here's the abstract:

Nature 428, 151 - 153 (11 March 2004) (http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v428/n6979/abs/nature02376_fs.html)

Convective-region geometry as the cause of Uranus' and Neptune's unusual magnetic fields

SABINE STANLEY AND JEREMY BLOXHAM

The discovery of Uranus' and Neptune's non-dipolar, non-axisymmetric magnetic fields destroyed the picture—established by Earth, Jupiter and Saturn—that planetary magnetic fields are dominated by axial dipoles. Although various explanations for these unusual fields have been proposed, the cause of such field morphologies remains unexplained. Planetary magnetic fields are generated by complex fluid motions in electrically conducting regions of the planets (a process known as dynamo action), and so are intimately linked to the structure and evolution of planetary interiors. Determining why Uranus and Neptune have different field morphologies is not only critical for studying the interiors of these planets, but also essential for understanding the dynamics of magnetic-field generation in all planets. Here we present three-dimensional numerical dynamo simulations that model the dynamo source region as a convecting thin shell surrounding a stably stratified fluid interior. We show that this convective-region geometry produces magnetic fields similar in morphology to those of Uranus and Neptune. The fields are non-dipolar and non-axisymmetric, and result from a combination of the stable fluid's response to electromagnetic stress and the small length scales imposed by the thin shell.

Espritch
2004-Mar-13, 05:26 AM
So what does it mean exactly to say that a magnetic field is non-dipolar, non-axisymmetric? :-?

Staiduk
2004-Mar-13, 06:22 AM
So what does it mean exactly to say that a magnetic field is non-dipolar, non-axisymmetric? :-?

The quote went over my head ten syllables into the first word. :D
Still; I'd read 'non-dipolar, axis-symmetric' to mean it doesn't have a definable north-south orientation at or near the geographic poles. I suppose it stands to reason; I remember reading that a planet's magnetic field is largely created by a rotating molten core - as the planet spins; the molten (largely iron) core acts like a huge generator; producing a whopping magnetic field. Since the rotation's axis is at the poles; the field lines up so. If a planet doesn't have a molten (or ferrous I wonder?) core; a magnetic field wouldn't be created - or at least, it wouldn't line up so preciseley; it'd act more like a a big lump of a rare-earth magnet - a compass needle would sort of drift around depending where one stood.
That's my guess - am I close? :)

Squink
2004-Mar-13, 06:42 PM
Pictures of the measured and simulated fields can be seen in the Physics Web (http://www.PHYSICSWEB.ORG/article/news/8/3/6) article.

Espritch
2004-Mar-14, 02:06 AM
Ok. Based on the illustration from the link, they are non-axisymetric because the magnetic poles don't line up with the spin axis of the planet. They are non-dipolar because they have two different north and south magnetic poles (but they are dipolar in the sense that each north pole has a matching south pole). Weird.

Thanks for the link.

Staiduk
2004-Mar-14, 03:35 AM
Well; I was close enough for a newbie... :lol:

Tobin Dax
2004-Mar-14, 07:21 AM
Espritch, I think non-axisymmetric refers to the field not being polar (like a bar magnet, to paraphrase the image). It's not symmetric around an axis, whether or not that axis is the spin axis of the planet. The magnetic fields of Earth and pulsars are axisymmetric, but neither case lines up with the spin axis of the object. All that is needed for axisymmetry is cylindrical symmetry around a line.

Diamond
2004-Mar-15, 08:32 AM
They have quadrupoles.... :o