A Faraday Rotation Study of the Stellar Bubble and HII Region Associated
  with the W4 Complex
  We utilized the Very Large Array to make multifrequency polarization
measurements of 20 radio sources viewed through the IC 1805 HII region and
"Superbubble", as well as in the immediate vicinity. The measurements at
frequencies between 4.33 and 7.76 GHz yield Faraday rotation measures along 27
lines of sight to these sources (some sources have more than one component).
The Faraday rotation measures (RM) are used to probe the plasma structure of
the IC 1805 HII region and to test the degree to which the Galactic magnetic
field is heavily modified (amplified) by the dynamics of the HII region. We
find that similar to the Rosette Nebula (Savage et al. 2013, Costa et al. 2016)
and the Cygnus OB1 association (Whiting et al. 2009), IC 1805 constitutes a
"Faraday rotation anomaly", or a region of increased RM relative to the general
Galactic background value. Although the RM observed on lines of sight through
the region vary substantially, the |RM| due to the nebula is commonly 600 --
800 rad m^-2. In spite of this, the observed RMs are not as large as simple,
analytic models of magnetic field amplification in HII regions might indicate.
This suggests that the Galactic field is not increased by a substantial factor
within the ionized gas in an HII region. We also find that with one exception,
the sign of the RM for all sources is that expected for the polarity of the
Galactic field in this direction. The same behavior was found for the Rosette
Nebula, and qualitatively indicates that turbulent fluctuations in the Galactic
field on spatial scales of $\sim 10$ pc are smaller than the mean Galactic
field. Finally, our results show intriguing indications that some of the
largest values of |RM| occur for lines of sight that pass outside the fully
ionized shell of the IC 1805 HII region, but pass through the Photodissociation
Region (PDR) associated with IC 1805.
