  Solar eruptions are spectacular magnetic explosions in the Sun's corona, and
how they are initiated remains unclear. Prevailing theories often rely on
special magnetic topologies that may not generally exist in the pre-eruption
source region of corona. Here, using fully three-dimensional
magnetohydrodynamic simulations with high accuracy, we show that solar
eruptions can be initiated in a single bipolar configuration with no additional
special topology. Through photospheric shearing motion alone, an electric
current sheet forms in the highly sheared core field of the magnetic arcade
during its quasi-static evolution. Once magnetic reconnection sets in, the
whole arcade is expelled impulsively, forming a fast-expanding twisted flux
rope with a highly turbulent reconnecting region underneath. The simplicity and
efficacy of this scenario argue strongly for its fundamental importance in the
initiation of solar eruptions.
