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Session 17 - ISM, Star Formation & Planets.
Oral session, Monday, June 09
North Main Hall C/D,

[17.03] Observations of the Magnetic Field in the Neutral ISM

E. M. Murphy (U. Virginia and NRAO), F. J. Lockman (NRAO)

We compare the strength of the magnetic field in the neutral ISM to that in the ionized ISM along eight lines of sight. The sight lines include six pulsars and two extragalactic objects. The field in the neutral ISM was obtained from new measurements of the Zeeman splitting of the 21 cm line with the NRAO 140-foot telescope. The field in the ionized medium was taken from published rotation and dispersion measures. We find that the average field in the neutral medium is 7.3 \muG, while the field in the ionized ISM along these same sight lines is only 1.0 \muG. This implies that the magnetic field pressure is a factor of 50 stronger in the neutral ISM than in the ionized ISM. The excess magnetic field pressure in the neutral medium may be sufficient to balance the excess thermal pressure in the ionized medium. We also find that the strength and orientation of the magnetic field in the neutral and ionized ISM are uncorrelated. As part of the 21 cm Zeeman work, we have obtained extensive measurements of the polarization properties of the NRAO 140 Foot Telescope. We find that the interaction of the nearby (<1\fdg5) circularly polarized sidelobes and the surrounding HI emission can account for less than 10% of the Zeeman effect along these lines of sight. We show that more distant sidelobes are likely to be even less important.

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