We describe a Faraday-rotation structure in the Interstellar Medium detected through polarimetric imaging at 1420 MHz from the Canadian Galactic Plane Survey ( CGPS ) .
The structure , at { l } = { 91 \fdg 8 } , { b } = { -2 \fdg 5 } , has an extent of { \sim } 2 ^ { \circ } , within which polarization angle varies smoothly over a range of { \sim } 100 ^ { \circ } .
Polarized intensity also varies smoothly , showing a central peak within an outer shell .
This region is in sharp contrast to its surroundings , where low-level chaotic polarization structure occurs on arcminute scales .
The Faraday-rotation structure has no counterpart in radio total intensity , and is unrelated to known objects along the line of sight , which include a Lynds Bright Nebula , LBN 416 , and the star cluster M39 ( NGC7092 ) .
It is interpreted as a smooth enhancement of electron density .
The absence of a counterpart , either in optical emission or in total intensity , establishes a lower limit to its distance .
An upper limit is determined by the strong beam depolarization in this direction .
At a probable distance of 350 { \pm } 50 pc , the size of the object is 10 pc , the enhancement of electron density is 1.7 cm ^ { -3 } , and the mass of ionized gas is 23 M _ { \odot } .
It has a very smooth internal magnetic field of strength 3 \mu G , slightly enhanced above the ambient field .
G91.8 - 2.5 is the second such object to be discovered in the CGPS , and it seems likely that such structures are common in the Magneto-Ionic Medium .