The Galactic synchrotron emission is expected to be the most relevant source of astrophysical contamination in cosmic microwave background polarization measurements , at least at frequencies \nu \lower 2.0 pt \hbox { $ { < \atop \hbox { \raise 4.0 pt \hbox { $ \sim$ } } } $ } 70 GHz and at angular scales \theta \lower 2.0 pt \hbox { $ { > \atop \hbox { \raise 4.0 pt \hbox { $ \sim$ } } } $ } 30 ^ { \prime } .
We present a multifrequency analysis of the Leiden surveys , linear polarization surveys covering the Northern Celestial Hemisphere at five frequencies between 408 MHz and 1411 MHz .
By implementing specific interpolation methods to deal with these irregularly sampled data , we produced maps of the polarized diffuse Galactic radio emission with a pixel size \simeq 0.92 ^ { \circ } .
We derived the angular power spectrum ( APS ) ( PI , E , and B modes ) of the synchrotron dominated radio emission as function of the multipole , \ell .
We considered the whole covered region and some patches at different Galactic latitudes .
By fitting the APS in terms of power laws ( C _ { \ell } \sim \kappa \cdot \ell ^ { \alpha } ) , we found spectral indices that steepen with increasing frequency : from \alpha \sim - ( 1-1.5 ) at 408 MHz to \alpha \sim - ( 2-3 ) at 1411 MHz for 10 \lower 2.0 pt \hbox { $ { < \atop \hbox { \raise 4.0 pt \hbox { $ \sim$ } } } $ } \ell % \lower 2.0 pt \hbox { $ { < \atop \hbox { \raise 4.0 pt \hbox { $ \sim$ } } } $ } 100 and from \alpha \sim - 0.7 to \alpha \sim - 1.5 for lower multipoles ( the exact values depending on the considered sky region and polarization mode ) .
The bulk of this flattening at lower frequencies can be interpreted in terms of Faraday depolarization effects .
We then considered the APS at various fixed multipoles and its frequency dependence .
Using the APSs of the Leiden surveys at 820 MHz and 1411 MHz , we determined possible ranges for the rotation measure , RM , in the simple case of an interstellar medium slab model .
Also taking into account the polarization degree at 1.4 GHz , it is possible to break the degeneracy between the identified RM intervals .
The most reasonable of them turned out to be RM \sim 9 - 17 rad/m ^ { 2 } although , given the uncertainty on the measured polarization degree , RM values in the interval \sim 53 - 59 rad/m ^ { 2 } can not be excluded .