2.2. Magnetic field strengths
The average and maximum nonthermal and polarized intensities
at 
6 cm and at 3'
resolution were used to compute the average and maximum equipartition
strengths of the total field and its regular component, assuming a ratio
of cosmic-ray protons to electrons of 100 and pathlengths through the
emitting regions of 3 kpc and 2 kpc (taking into account the different
inclinations). Equipartition is probably valid on average over the ring
of M31 but invalid within the ring
(Sect. 3.2), so the maximum field
strengths are lower limits.
The results are summarized in Table 3. The two
galaxies are similar in most of their radio continuum properties.
The main differences between M31 and M33 are the morphology (the "ring" in M31, a diffuse disk in M33) and the degree of regularity of the
magnetic field which is much higher in M31.
Corrected for thermal contribution, the nonthermal degree
of polarization at 6
cm at 3' resolution becomes 33% in M31, only 10% in M33. No other spiral galaxy reveals a degree of
polarization as high as M31, even when observed with higher spatial
resolution.
| M31 | M33 | |
| Spectral index of the integrated emission | 0.76 ± 0.04 | 0.75 ± 0.10 |
| Nonthermal spectral index | 1.0 ± 0.1 | 0.9 ± 0.1 |
| Thermal fraction at
6 cm |
 35% |
30% |
| Average total field strength | 6 ±2 µG | 6 ± 2 µG |
| Maximum total field strength |  8 ±
2 µG |
9 ± 2 µG |
| Average regular field strength | 4 ± 1 µG | 2 ± 1 µG |
| Maximum regular field strength | 6 ±
1 µG |
4 ± 1 µG |
| Scale length of the disk at
6 cm |
3.3 ± 0.1 kpc* | 2.3 ± 0.1 kpc |
| *beyond 10 kpc radius | ||