Pulsars as probes of the interstellar medium

Pulsar observations offer an opportunity to determine the distributions of the magnetic field, electron density, and neutral atomic hydrogen density in the interstellar space. Pulse polarization and time of arrival measurements indicate a relatively uniform magnetic field of about 3 µG, whereas new distance measurements through 21-cm line absorption indicate an electron density of about 0.03 cm^-3which is uniform when averaged over large distances (a kiloparsec or more).