Faraday Rotation in Magnetized Plasma

What you are seeing: a linearly polarized radio wave entering a column of magnetized plasma. The polarization angle rotates by $\chi = RM\, \lambda^2$ over the path, where $RM = 8.12 \times 10^5 \int n_e B_\parallel\, \mathrm{d}z$ . Multi-wavelength view shows three colors rotating by different amounts (the $\lambda^2$ signature)

Figure 1. Polarization rotation through magnetized plasma; chi(L) = RM lambda^2. Multi-wavelength helices show the wavelength scaling. Method: closed-form RM = 8.12e5 n_e B_par L.

B_par (uG)3.0

n_e (cm^-3)0.030

L (pc)1000

lambda (cm)21

presetP

WHAT TO TRY

Raise the parallel field B_par or the electron density n_e: the rotation measure RM climbs, and the polarization angle winds through more turns along the column. RM probes the magnetized plasma.
Increase the wavelength: the rotation grows as chi = RM lambda squared, so long radio waves twist far more than short ones. Measuring chi at two wavelengths isolates RM.
Switch presets from a galactic pulsar to the Sgr A* foreground: the enormous RM there twists the angle through many turns, mapping the magnetic field near the galactic centre.