  We perform high-resolution magnetohydrodynamics simulations of binary neutron
star mergers in numerical relativity on the Japanese supercomputer K. The
neutron stars and merger remnants are covered by a grid spacing of 70\,m, which
yields the highest-resolution results among those derived so far. By an
in-depth resolution study, we clarify several amplification mechanisms of
magnetic fields during the binary neutron star merger for the first time.
First, the Kelvin-Helmholtz instability developed in the shear layer at the
onset of the merger significantly amplifies the magnetic fields. A hypermassive
neutron star (HMNS) formed after the merger is then subject to the
nonaxisymmetric magnetorotational instability, which amplifies the magnetic
field in the HMNS. These two amplification mechanisms cannot be found with
insufficient-resolution runs. We also show that the HMNS eventually collapses
to a black hole surrounded by an accretion torus which is strongly magnetized
at birth.
