  Several weeks after the explosion of supernova (SN) 1987A, the UV flash of
the SN illuminated a ring-like structure in the circumstellar material at about
0.65 ly from the SN. The interaction between the stellar winds from the SN
progenitor is considered to be the candidate for the formation of the
circumstellar structure. In the case that the stellar winds are spherically
symmetric, the interaction should result in a shell-like structure. However, in
this paper we show that the magnetic field in the winds causes an anisotropy
which leads to the formation of a ring-like structure. When the fast wind of
the blue supergiant phase of the progenitor sweeps up the surrounding slow wind
of the red-supergiant phase, the magnetic field as well as the wind material
are piled up in the interaction region. Since the magnetic energy increases in
proportion to the square of the amplitude, the magnetic field exhibits its
effect prominently at the interaction region; due to the magnetic pressure
force the material at lower latitudes is compressed into a ring-like structure.
It is suggested that this magnetic process can also explain the newly observed
pair of rings of the SN 1987A nebula.
