Rotation Measure to Dispersion Ratios and Line-of-Sight Magnetic Fields

This paper computes density-weighted line-of-sight magnetic fields from published rotation measure and dispersion measure pairs for two repeating fast radio bursts and three Galactic pulsars, quantifies the dynamic range of environments the ratios reveal, and converts between field strengths and the rotation measures future surveys must be able to recover. The headline results are as follows. Mean line-of-sight field toward FRB 121102: 221 microgauss; Mean line-of-sight field toward FRB 180916: 0.402 microgauss; Mean line-of-sight field toward the Vela pulsar: 0.57 microgauss; Ratio of the FRB 121102 field to the Vela pulsar field: 388; Field implied if a rotation measure of 10 rad m-2 accompanied a dispersion of 500: 0.0246 microgauss; Rotation measure a 3 microgauss field would produce over a dispersion of 500: 1.22e+03 rad m-2. All values are computed from the stated assumptions and published inputs; the study is simulated and computed, not measured.