We present high-quality Very Large Array ( VLA ) images of the Fanaroff & Riley Class I radio galaxy 3C 31 in the frequency range 1365 to 8440 MHz with angular resolutions from 0.25 to 40 arcsec .
Our new images reveal complex , well resolved filamentary substructure in the radio jets and tails .
We also use these images to explore the spectral structure of 3C 31 on large and small scales .

We infer the apparent magnetic field structure by correcting for Faraday rotation .
Some of the intensity substructure in the jets is clearly related to structure in their apparent magnetic field : there are arcs of emission where the degree of linear polarization increases , with the apparent magnetic field parallel to the ridges of the arcs .

The spectra of the jets between 1365 and 8440 MHz are consistent with power laws within 60 arcsec of the nucleus .
The spectral indices , \alpha ( flux density \propto \nu ^ { - \alpha } ) are significantly steeper ( \alpha = 0.62 ) within \approx 7 arcsec of the nucleus than between 7 and 50 arcsec ( \alpha = 0.52 – 0.57 ) .
The spectra of the arcs and of the jet edges are also slightly flatter than the average for their surroundings .
At larger distances , the jets are clearly delimited from surrounding larger-scale emission both by their flatter radio spectra and by sharp brightness gradients .

The spectral index of 0.62 in the first 7 arcsec of 3C 31 ’ s jets is very close to that found in other FR I galaxies where their jets first brighten in the radio and where X-ray synchrotron emission is most prominent .
Farther from the nucleus , where the spectra flatten , X-ray emission is fainter relative to the radio .
The brightest X-ray emission from FR I jets is therefore not associated with the flattest radio spectra , but with a particle-acceleration process whose characteristic energy index is 2 \alpha + 1 = 2.24 .
The spectral flattening with distance from the nucleus occurs where our relativistic jet models require deceleration , and the flatter-spectra at the jet edges may be associated with transverse velocity shear .