We determined Faraday rotation measures ( RMs ) towards 137 pulsars in the northern sky , using Low-Frequency Array ( LOFAR ) observations at 110–190 MHz .
This low-frequency RM catalogue , the largest to date , improves the precision of existing RM measurements on average by a factor of 20 – due to the low frequency and wide bandwidth of the data , aided by the RM synthesis method .
We report RMs towards 25 pulsars for the first time .
The RMs were corrected for ionospheric Faraday rotation to increase the accuracy of our catalogue to \approx 0.1 rad m ^ { -2 } .
The ionospheric RM correction is currently the largest contributor to the measurement uncertainty .
In addition , we find that the Faraday dispersion functions towards pulsars are extremely Faraday thin – mostly less than 0.001 rad m ^ { -2 } .
We use these new precise RM measurements ( in combination with existing RMs , dispersion measures , and distance estimates ) to estimate the scale height of the Galactic halo magnetic field : 2.0 \pm 0.3 kpc for Galactic quadrants I and II above and below the Galactic plane ( we also evaluate the scale height for these regions individually ) .
Overall , our initial low-frequency catalogue provides valuable information about the 3-D structure of the Galactic magnetic field .