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Abstract:
Pulsars can act as an excellent probe of the Milky Way magnetic
field. The average strength of the Galactic magnetic field component
parallel to the line of sight can be estimated as 1.232 RM/DM, where
RM and DM are the rotation and dispersion measure. However, this
assumes that the thermal electron density and magnetic field of the
interstellar medium are uncorrelated. Using numerical simulations and
observations, we test the validity of this assumption. Based on
magnetohydrodynamical simulations of driven turbulence, we show that
the correlation between the thermal electron density and the
small-scale magnetic field increases with increasing Mach number of
the turbulence. We find that the assumption of uncorrelated thermal
electron density and magnetic fields is valid only for subsonic and
transsonic flows, but for supersonic turbulence, the field strength
can be severely overestimated. We then correlate existing pulsar
observations from the Australia Telescope National Facility with
regions of enhanced thermal electron density probed by CO, Zeeman,
neutral hydrogen and Halpha observations. Using these observational
data, we show that the thermal electron density and magnetic fields
are largely uncorrelated over kpc scales. Thus, we conclude that the
relation provides a good estimate of the magnetic field on Galactic
scales, but might break down on sub-kpc scales.
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