How do rotation measures work without dispersion measures

  1. Hi, I was wondering how the heck we can use rotation measures from extragalactic sources? I have been looking around for hours and I still don't understand how they can do it without an independent way to find the electron density along the path. Do they use a different method to find the electron density?

    well I understand how we could get an RM, but I don't understand how this could be used to find the magnetic fields without knowing something about the electron density.
  2. jcsd
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