How do we measure objects' magnetic fields?

[ogxela]

I frequently hear on AstronomyCast and in other places about how the Sun, Jupiter, Magnetars, and various other bodies have magnetic fields of various strengths, and I was wondering how we get those measurements, especially for objects that are very very far away.

[cjameshuff]

I frequently hear on AstronomyCast and in other places about how the Sun, Jupiter, Magnetars, and various other bodies have magnetic fields of various strengths, and I was wondering how we get those measurements, especially for objects that are very very far away.

For planets in the solar system, we've got direct measurements from probes that have made flybys, or in some cases, have stayed in orbit.

For more indirect measurements, there's things like cyclotron radiation. Magnetism does not affect charged particles in the same way everyday magnets affect each other. The force a magnetic field exerts on a charged particle is at right angles to the direction of the magnetic field and the direction of movement of the particle relative to the field. This leads to things like ions and electrons following spiral paths. Since accelerating charged particles radiate electromagnetic radiation, and the particles are mainly electrons and protons (hydrogen nuclei), and thus of known mass, we can figure out the strength of the magnetic field from the frequency of that radiation.

Another method is to use an optical effect called Faraday rotation...interstellar gases in a magnetic field will rotate the polarization of light passing through them.

[ngc3314]

Plus an effect known as Zeeman splitting. The electron clouds of some kinds of atoms change shape, and thus allowed energy levels, depending on their orientation in a magnetic field, which quantum effects limit to discrete directions. Spectral lines from these so-called magnetically-sensitive states show wavelength splittings which are proportional to the field strength. Most famously, this was used to show the magnetic fields in sunspots, and has been used to look at magnetic properties integrated across distant stars as well. Geoff Marcy's thesis work dealt with magnetic fields in M dwarfs, before he found more interesting topics.

[Nereid]

Just to add some details of the astronomical techniques:

* the "spiralling around magnetic field lines" signal is "synchrotron emission" and it has a very characteristic signature, directly observed in the microwave and radio parts of the EM spectrum

* Faraday rotation is more commonly observed in the radio (and microwave) bands

* "Origin of Galactic and Extragalactic Magnetic Fields", a recent arXiv preprint, contains a short summary of these methods (plus Zeeman splitting), as applied to galactic and extragalactic objects, as well as a summary of the main results.