We present first results for Faraday rotation of compact polarized sources ( 1 to 2 GHz continuum ) in The HI/OH/Recombination line ( THOR ) survey of the inner Galaxy .
In the Galactic longitude range 39 \degr < \ell < 52 \degr , we find rotation measures in the range -310 { rad m ^ { -2 } } \leq RM \leq + 4219 rad m ^ { -2 } , with the highest values concentrated within a degree of \ell = 48 \degr at the Sagittarius arm tangent .
Most of the high RM s arise in diffuse plasma , along lines of sight that do not intersect H II regions .
For \ell > 49 \degr , RM drops off rapidly , while at \ell < 47 \degr , the mean RM is higher with a larger standard deviation than at \ell > 49 \degr .
We attribute the RM structure to the compressed diffuse Warm Ionized Medium in the spiral arm , upstream of the major star formation regions .
The Sagittarius arm acts as a significant Faraday screen inside the Galaxy .
This has implications for models of the Galactic magnetic field and the expected amount of Faraday rotation of Fast Radio Bursts from their host galaxies .
We emphasize the importance of sensitivity to high Faraday depth in future polarization surveys .