Context :

Aims : The TeV BL Lac object Markarian 501 is a complex , core dominated radio source , with a one sided , twisting jet on parsec scales .
In the present work , we attempt to extend our understanding of the source physics to regions of the radio jet which have not been accessed before .

Methods : We present new observations of Mrk 501 at 1.4 and 86 GHz .
The 1.4 GHz data were obtained using the Very Large Array ( VLA ) and High Sensitivity Array ( HSA ) in November 2004 , in full polarization , with a final r.m.s .
noise of 25 \mu Jy/beam in the HSA total intensity image ; the 86 GHz observations were performed in October 2005 with the Global Millimeter VLBI Array ( GMVA ) , providing an angular resolution as good as 110 \mu \mathrm { as } \times 40 \mu \mathrm { as } .

Results : The sensitivity and resolution provided by the HSA make it possible to detect the jet up to \sim 700 milliarcseconds ( corresponding to a projected linear size of \sim 500 pc ) from its base , while the superior resolution of the 86 GHz GMVA observations probes the innermost regions of the jet down to \sim 200 Schwarzschild radii .
The brightness temperature at the jet base is in excess of 6 \times 10 ^ { 10 } K. We find evidence of limb brightening on physical scales from \la 1 pc to \sim 40 pc .
Polarization images and fits to the trend of jet width and brightness vs. distance from the core reveal a magnetic field parallel to the jet axis .

Conclusions :