  Active Regions (ARs) that exhibit compact Polarity Inversion Lines (PILs) are
known to be very flare-productive. However, the physical mechanisms behind this
statistical inference have not been demonstrated conclusively. We show that
such PILs can occur due to the collision between two emerging flux tubes nested
within the same AR. In such multipolar ARs, the flux tubes may emerge
simultaneously or sequentially, each initially producing a bipolar magnetic
region (BMR) at the surface. During each flux tube's emergence phase, the
magnetic polarities can migrate such that opposite polarities belonging to
different BMRs collide, resulting in shearing and cancellation of magnetic
flux. We name this process 'collisional shearing' to emphasize that the
shearing and flux cancellation develops due to the collision. Collisional
shearing is a process different from the known concept of flux cancellation
occurring between polarities of a single bipole, a process that has been
commonly used in many numerical models. High spatial and temporal resolution
observations from the Solar Dynamics Observatory for two emerging ARs, AR11158
and AR12017, show the continuous cancellation of up to 40% of the unsigned
magnetic flux of the smallest BMR, which occurs at the collisional PIL for as
long as the collision persists. The flux cancellation is accompanied by a
succession of solar flares and CMEs, products of magnetic reconnection along
the collisional PIL. Our results suggest that the quantification of magnetic
cancellation driven by collisional shearing needs to be taken into
consideration in order to improve the prediction of solar energetic events and
space weather.
