Top left: Hα map in the velocity range −80 <V_lsr< −40 km s^-1. Notice the vertical filament that runs at l ≈ 75° and −60°<b< −20°. Top right: Faraday depth map from Oppermann et al. (2012), which also shows a filament at the same location, with a mean value of −25 rad m^-2 along its extension. The filament has a counterpart in “absorption,” visible as a trough in polarization intensity maps. Bottom left: WMAP 23 GHz polarization intensity map. Bottom right: Planck-WMAP polarization intensity map. The Hα map on the top left corner has an angular resolution of 1deg, while the other three maps have a common resolution of 3deg. The grid spacing is 15deg.