  While free/non-potential magnetic energy is a necessary element of any active
phenomenon in the solar corona, its role as a marker of the trigger of eruptive
process remains elusive. Based on the unique decomposition of the magnetic
field into potential and non-potential components, magnetic energy and helicity
can also both be uniquely decomposed into two quantities. Using two 3D MHD
parametric simulations of a configuration that can produce coronal jets, we
compare the dynamics of the magnetic energies and of the relative magnetic
helicities. Both simulations share the same initial set-up and line-tied
bottom-boundary driving profile. However, they differ by the duration of the
forcing. In one simulation the system is driven sufficiently so that an helical
jet is induced. The generation of the jet is however markedly delayed: a
relatively long phase of lower-intensity reconnection takes place before the
jet is eventually induced. In the other reference simulation, the system is
driven during a shorter time, and no jet is produced. As expected, we observe
that the Jet producing simulation contains a higher value of non-potential
energy and non-potential helicity. Focusing on the phase between the end of the
driving-phase and the jet generation, we note that magnetic energies remain
relatively constant, while magnetic helicities have a noticeable evolution.
During this post-driving phase, the ratio of the non-potential to total
magnetic energy very slightly decreases while the helicity eruptivity index
significantly increases. The jet is generated when the system is at the highest
value of this helicity eruptivity index. This proxy critically decreases during
the jet generation phase. The free energy also decreases but does not present
any peak when the jet is being generated.
