A Sino-German lambda6 cm polarization survey of the Galactic plane I. Survey strategy and results for the first survey region

(Abridged) Polarization measurements of the Galactic plane at lambda6 cm probe the interstellar medium (ISM) to larger distances compared to measurements at longer wavelengths, hence enable us to investigate properties of the Galactic magnetic fields and electron density. We are conducting a new lambda6 cm continuum and polarization survey of the Galactic plane covering 10degr < l < 230degr and |b|<5degr. Missing large-scale structures in the U and Q maps are restored based on extrapolated polarization K-band maps from the WMAP satellite. The lambda6 cm data are analyzed together with maps at other bands. We discuss some results for the first survey region, 7degr X 10degr in size, centered at (l,b)=(125.5degr, 0degr). Two new passive Faraday screens, G125.6-1.8 and G124.9+0.1, were detected. They cause significant rotation of background polarization angles but little depolarization. G124.9+0.1 was identified as a new faint HII region at a distance of 2.8 kpc. G125.6-1.8, with a size of about 46 pc, has neither correspondence in enhanced Halpha emission nor a counterpart in total intensity. A model combining foreground and background polarization modulated by the Faraday screen was developed. Using this model, we estimated the strength of the ordered magnetic field along the line of sight to be 3.9 microGauss for G124.9+0.1, and exceeding 6.4 microGauss for G125.6-1.8. We obtained an estimate of 2.5 and 6.3 mK/kpc for the average polarized and total synchrotron emissivity towards G124.9+0.1. The synchrotron emission beyond the Perseus arm is quite weak.