  The rotation of sunspots around their umbral center has long been considered
as an important process in leading to solar eruptions, but the underlying
mechanism remains unclear. A prevailing physical picture on how sunspot
rotation leads to eruption is that, by twisting the coronal magnetic field
lines from their footpoints, the rotation can build up a magnetic flux rope and
drive it into some kinds of ideal magnetohydrodynamics (MHD) instabilities
which initiate eruptions. Here with a data-inspired MHD simulation we studied
the rotation of a large sunspot in solar active region NOAA 12158 leading to a
major eruption, and found that it is distinct from prevailing theories based on
ideal instabilities of twisted flux rope. The simulation suggests that, through
successive rotation of the sunspot, the coronal magnetic field is sheared with
a central current sheet created progressively within the sheared arcade before
the eruption, but without forming a flux rope. Then the eruption is instantly
triggered once fast reconnection sets in at the current sheet, while a highly
twisted flux rope is created during the eruption. Furthermore, the simulation
reveals an intermediate evolution stage between the quasi-static energy-storage
phase and the impulsive eruption-acceleration phase. This stage may correspond
to the slow-rise phase in observation and it enhances building up of the
current sheet.

Solar and Stellar Astrophysics;Space Physics