Context :

Aims : We investigate the role played by a pre-supernova ( SN ) ambient magnetic field on the dynamics of the expanding remnant of SN 1987A and the origin and evolution of the radio emission from the remnant , in particular , during the interaction of the blast wave with the nebula surrounding the SN .

Methods : We model the evolution of SN 1987A from the breakout of the shock wave at the stellar surface to the expansion of its remnant through the surrounding nebula by three-dimensional magnetohydrodynamic simulations .
The model considers the radiative cooling , the deviations from equilibrium of ionization , the deviation from temperature-equilibration between electrons and ions , and a plausible configuration of the pre-SN ambient magnetic field .
We explore strengths of the pre-SN magnetic field ranging between 1 \mu G and 100 \mu G at the inner edge of the nebula and we assume an average field strength at the stellar surface B _ { 0 } \approx 3 kG .
From the simulations , we synthesize both thermal X-ray and non-thermal radio emission and compare the model results with observations .

Results : The presence of an ambient magnetic field with strength in the range considered does not change significantly the overall evolution of the remnant .
Nevertheless , the magnetic field reduces the erosion and fragmentation of the dense equatorial ring after the impact of the SN blast wave .
As a result , the ring survives the passage of the blast , at least , during the time covered by the simulations ( 40 years ) .
Our model is able to reproduce the morphology and lightcurves of SN 1987A in both X-ray and radio bands .
The model reproduces the observed radio emission if the flux originating from the reverse shock is heavily suppressed .
In this case , the radio emission originates mostly from the forward shock traveling through the H II region and this may explain why the radio emission seems to be insensitive to the interaction of the blast with the ring .
Possible mechanisms for the suppression of emission from the reverse shock are investigated .
We find that synchrotron self-absorption and free-free absorption have negligible effects on the emission during the interaction with the nebula .
We suggest that the emission from the reverse shock at radio frequencies might be limited by highly magnetized ejecta .

Conclusions :