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Sources of magnetic fields in space near Earth
I've got a question about earth's magnetic field. Diagrams show earth's magnetosphere stretching out opposite the sun from the incoming solar wind. I think the Nova episode "magnetic storm" gives me the impression that the Earth's magnetic field lines move like a flag flapping in the wind (which doesn't mesh with my impression of what field lines are.)
Is the deformation a result of the vector sum of the magnetic field from the solar wind itself and earth's magnetic field? Otherwise I can't get my head around how the magnetic field is deformed?
Perhaps my understanding of fields is limited, but I think of the magnetic field around a magnet is a map of the force the magnet applies. I can understand how a secondary field might add to this to make a new field (vector sum) but it seems to be portrayed as if the earth's field is changed--which I can't understand without changes in the currents which give rise to the field in the first place.
Thanks for your help-
(A chemistry/physics teacher with more background in chemistry.)
REPLY
The magnetic field observed in space is the sum of several contributions. There is the field of the Earth's core, of course, and the field carried by the solar wind. But in addition, there exist several other sources, due to currents generated in space.
Space around Earth contains conducting plasma and so can carry electric currents, and these currents also contribute. Those currents are mostly generated by motions of the plasma, often motions generated by mechanical forces, which mimic many phenomena which we expect in neutral gases, in the familiar environment. For instance, the solar wind encounters the Earth's magnetic field and is forced around it, just as ordinary wind is forced to flow around (say) a flagpole.
This deflection involves additional currents. It should be added that those currents are loss-free, no ohmic resistance impedes them. They are just means by which the plasma re-arranges itself.
For better understanding I recommend to you the file
http://www.phy6.org/Education/wimfproj.html
a copy of which is also in
http://www.phy6.org/ stargaze/Simfproj.htm
That sections introduces (but does not prove--that would take to much math) a basic property of plasma, which often helps in deriving the secondary fields and in visualizing the process which creates them. Namely
If two or more ions start out located on the same field line, they will always share the same field line.
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Solar wind ions move from the Sun, attached to solar field lines. Before leaving the Sun, they had nothing to do with terrestrial field lines, so one expects that they won't do so in the future, either.
For those ions to share a field line with ions of the Earth's magnetosphere, would require some splicing of solar and terrestrial field lines: such splicing ("magnetic reconnection") apparently does exist and is important in creating storms and substorms in the magnetosphere, driven by energy of the solar wind. But it is a phenomenon limited to regions where the magnetic field is very weak, and most of the solar wind just flows around the magnetosphere, leaving a well defined boundary ("magnetopause") between it and the solar wind.
That separation can be viewed as a mechanical process--the solar wind hits and obstacle and flows around it. In magnetic terms, however, it is achieved by electric currents on the magnetopause, due to the encounter between the magnetic fields of the Sun and the Earth.
Those currents create an extra magnetic field: inside the magnetosphere, it cancels ("shields out") most of the effects of the solar wind. Outside the magnetosphere it "shields out" the Earth's field, excluding it from interplanetary space.
The solar wind also exerts a pressure (like ordinary wind!), and this helps shape the boundary. It is closest to us at noon, while on the night side, where the compressed plasma encounters no opposing force, it stretches into a long tail. And because magnetic field lines stay attached to plasma ions which form that tail, they too get deformed.
That tail creates currents of its own, where two bundles of field lines lie next to each other--a bundle coming out of the south polar region of Earth and an opposites bundle flowing into the north polar region. Even though both come from Earth, they must be kept separate, and an electric current flowing between them (from dawn to dusk, in the "plasma sheet") maintains that separation. And yes, sometimes the bundles flap a bit, though less so than most flags.
So what we observe in space is the sum of several current systems in space, plus the Earth's field. I have not yet mentioned here the currents associated with the polar aurora ("Birkeland currents"). It's all fairly complex, fairly variable, and we are still trying to understand many of the features.
Thanks for asking! You will find much more in "Exploration of the Earth's Magnetosphere,"
