The Milky Way has an associated magnetic field. But how big is it influence in the solar system and can we measure it? I mean, when measuring, how can we make the distinction between the field of the sun and that of the Milky Way? It seems reasonable to assume that both are in the same direction. If it adds a constant field to that of the sun how could we tell?

Would two measurements suffice? Say one at a certain distance and the other at twice the distance?


1 Answer 1


There is no galactic magnetic field inside the (inner) solar system. The solar wind is a near-perfect plasma and according to the frozen-in theorem it carries the magnetic field of the Sun with it extremely well.

At some stage in the outer solar system this assumption will break down, especially at the transition region from the solar wind to the interstellar medium. The Sun's magnetosphere ends about at the distance of the voyager probes. Interactions magnetospheric boundaries, thus of different plasma environments with different magnetic fields, can be highly dynamic- and are not trivial. They are subject to ongoing research and depend on the relative field strenths, and flux densities and their temporal variations etc.

We measure the interestellar and galactic magnetic field via spectroscopy like observing Zeeman effect etc on gas in the interstellar space.

  • $\begingroup$ So the galactic field is canceled by the suns field? $\endgroup$ Jun 23, 2021 at 8:34
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    $\begingroup$ It's not cancelled. It's not present here. The interplanetary field cannot penetrate into the magnetosphere of the Sun. $\endgroup$ Jun 23, 2021 at 8:42
  • $\begingroup$ Is it pushed out? Why it cant penetrate if it has the same direction? $\endgroup$ Jun 23, 2021 at 8:45
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    $\begingroup$ The solar wind plasma is the carrier of the Sun's magnetic field as it's a fast-flowing near-perfect conductor. The diffucion timescale for the interstellar magnetic field to penetrate into the solar wind is extremely large as the magnetic Reynolds number is much bigger than unity. Thus the outward motion of the plasma is faster than anything of the galactic magnetic field can diffuse inward - up to a point where the solar wind has slowed down enough; thus there the solar magnetosphere's boundary is found. en.wikipedia.org/wiki/Magnetic_diffusion $\endgroup$ Jun 23, 2021 at 12:09
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    $\begingroup$ The galactic magnetic field is carried by plasma. But that plasma can't flow into the Solar System because of the outward flow of the solar wind. It's a bit like how the salty water of the ocean doesn't flow up a river because the fresh water of the river is flowing down into the ocean. Of course, some fast interstellar particles do manage to penetrate into the Solar System, even into the inner system, but the solar wind keeps a lot of stuff out. See en.wikipedia.org/wiki/Heliosphere $\endgroup$
    – PM 2Ring
    Jun 23, 2021 at 12:11

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