  The recent study of Blackman and Subramanian (Paper I) indicates that large
scale helical magnetic fields are resilient to turbulent diffusion in the sense
that helical fields stronger than a critical value, decay on slow (~resistively
mediated), rather than fast ($\sim$ turbulent) time scales. This gives more
credence to potential fossil field origin models of the magnetic fields in
stars, galaxies and compact objects. We analyze a suite of direct numerical
simulations (DNS) of decaying large scale helical magnetic fields in the
presence of non-helical turbulence to further study the physics of helical
field decay. We study two separate cases: (1) the initial field is large enough
to decay resistively, is tracked until it transitions to decay fast, and the
critical large scale helical field at that transition is sought; (2) the case
of Paper I, wherein there is a critical initial helical field strength below
which the field undergoes fast decay right from the beginning. In case (1),
both DNS and solutions of the two scale model (from Paper 1), reveal that the
transition energy, $E_{c1}$, is independent of the scale of the turbulent
forcing, within a small range of $\Rm$. We also find that the kinetic alpha,
$\alpha_K$, is subdominant to magnetic alpha, $\alpha_M$, in the DNS,
justifying an assumption in the two scale model. For case (2), we show exact
solutions of two scale model in the limit of $\eta \rightarrow 0$ in fully
helical case, leading to the transition energy, $E_{c2} = (k_1/\kf)^2 M_{eq}$,
where $k_1$ and $\kf$ are the large scale and small turbulent forcing scale
respectively and $M_{eq}$ is the equipartition magnetic energy. The DNS in this
case agree qualitatively with the two scale model but the $R_M$ currently
achievable, is too small to satisfy a condition $3/R_M << (k_1/k_f)^2$,
necessary to robustly reveal the transition, $E_{c2}$ (Abridged).
