We observed total and polarized radio continuum emission from the spiral galaxy M 101 at \lambda \lambda 6.2 cm and 11.1 cm with the Effelsberg telescope .
The angular resolutions are 2 \hbox { $ . { } ^ { \prime } $ } 5 ( =5.4 kpc ) and 4 \hbox { $ . { } ^ { \prime } $ } 4 ( =9.5 kpc ) , respectively .
We use these data to study various emission components in M 101 and properties of the magnetic field .
Separation of thermal and non-thermal emission shows that the thermal emission is closely correlated with the spiral arms , while the non-thermal emission is more smoothly distributed indicating diffusion of cosmic ray electrons away from their places of origin .
The radial distribution of both emissions has a break near R = 16 kpc ( = 7 \hbox { $ . { } ^ { \prime } $ } 4 ) , where it steepens to an exponential scale length of L \simeq 5 kpc , which is about 2.5 times smaller than at R < 16 kpc .
The distribution of the polarized emission has a broad maximum near R = 12 kpc and beyond R = 16 kpc also decreases with L \simeq 5 kpc .
It seems that near R = 16 kpc a major change in the structure of M 101 takes place , which also affects the distributions of the strength of the random and ordered magnetic field .
Beyond R =16 kpc the radial scale length of both fields is about 20 kpc , which implies that they decrease to about 0.3 \mu G at R = 70 kpc , which is the largest optical extent .
The equipartition strength of the total field ranges from nearly 10 \mu G at R < 2 kpc to 4 \mu G at R = 22 - 24 kpc .
As the random field dominates in M 101 ( B _ { ran } / B _ { ord } \simeq 2.4 ) , wavelength-independent polarization is the main polarization mechanism .
We show that energetic events causing { H \textsc { I } } shells of mean diameter < 625 pc could partly be responsible for this .
At radii < 24 kpc , the random magnetic field depends on the star formation rate/area , \Sigma _ { SFR } , with a power-law exponent of b = 0.28 \pm 0.02 .
The ordered magnetic field is generally aligned with the spiral arms with pitch angles that are about 8 \hbox { $ { } ^ { \circ } $ } larger than those of { H \textsc { I } } filaments .