  Solar eruptions are explosive release of coronal magnetic field energy as
manifested in solar flares and coronal mass ejection. Observations have shown
that the core of eruption-productive regions are often a sheared magnetic
arcade, i.e., a single bipolar configuration, and, particularly, the
corresponding magnetic polarities at the photosphere are elongated along a
strong-gradient polarity inversion line (PIL). It remains unclear what
mechanism triggers the eruption in a single bipolar field and why the one with
a strong PIL is eruption-productive. Recently, using high accuracy simulations,
we have established a fundamental mechanism of solar eruption initiation that a
bipolar field as driven by quasi-static shearing motion at the photosphere can
form an internal current sheet, and then fast magnetic reconnection triggers
and drives the eruption. Here we investigate the behavior of the fundamental
mechanism with different photospheric magnetic flux distributions, i.e.,
magnetograms, by combining theoretical analysis and numerical simulation. Our
study shows that the bipolar fields of different magnetograms, as sheared
continually, all exhibit similar evolutions from the slow storage to fast
release of magnetic energy in accordance with the fundamental mechanism, which
demonstrates the robustness of the mechanism. We further found that the
magnetograms with stronger PIL produce larger eruptions, and the key reason is
that the sheared bipolar fields with stronger PIL can achieve more
non-potentiality, and their internal current sheet can form at a lower height
and with a larger current density, by which the reconnection can be more
efficient. This also provides a viable trigger mechanism for the observed
eruptions in active region with strong PIL.
