Interaction between adjacent sheared magnetic arcades in the solar corona

Using an axisymmetric ideal-MHD model in spherical coordinates, we investigate the energy buildup of a multipole magnetic field due to photospheric shearing motions, with an emphasis on the interaction between adjacent sheared magnetic arcades. The coronal magnetic field to be sheared is a potential field that is symmetric with respect to the equator, consisting of four topologically disconnected magnetic arcades - a central arcade astride the equator, one arcade to either side of the central one, and an overlying arcade above them. Photospheric shearing is then introduced at the base of the central arcade, the flanking arcades, or both, and the associated force-free field solutions are obtained with time-dependent MHD simulations. It is found that if only one arcade is sheared, the magnetic energy of the sheared field is lower than that of the corresponding partly open field, in which the sheared arcade is fully open while the others remain closed and unsheared. This confirms a previous extension of the Aly conjecture from fully open force-free fields to partly open ones. When both the central and flanking arcades are sheared, they interact with each other: the critical azimuthal shear displacement for one arcade, at which it becomes open, depends on the shear state of the others. As a result, an arcade may become open either by virtue of an increase in its own shear or by changing the shear state of its adjacent arcades. Such an interaction may have implications for the triggering of large-scale active phenomena such as CMEs or sympathetic events that occur successively in different active regions of the Sun.