  We investigate the possible development of magnetohydrodynamical
instabilities in the EULAG-MHD "millenium simulation" of Passos & Charbonneau
(2014). This simulation sustains a large-scale magnetic cycle characterized by
solar-like polarity reversals taking place on a regular multidecadal cadence,
and in which zonally-oriented bands of strong magnetic field accumulate below
the convective layers, in response to turbulent pumping from above in
successive magnetic half-cycles. Key aspects of this simulation include low
numerical dissipation and a strongly subadiabatic fluid layer underlying the
convectively unstable layers corresponding to the modeled solar convection
zone. These properties are conducive to the growth and development of
two-dimensional instabilities otherwise suppressed by stronger dissipation. We
find evidence for the action of a non-axisymmetric magnetoshear instability
operating in the upper portions of the stably stratified fluid layers. We also
investigate the possibility that the Tayler instability may be contributing to
the destabilization of the large-scale axisymmetric magnetic component at high
latitudes. On the basis of our analyses, we propose a global dynamo scenario
whereby the magnetic cycle is driven primarily by turbulent dynamo action in
the convecting layers, but MHD instabilities accelerate the dissipation of the
magnetic field pumped down into the overshoot and stable layers, thus perhaps
significantly influencing the magnetic cycle period. Support for this scenario
is found in the distinct global dynamo behaviors observed in an otherwise
identical EULAG-MHD simulations, using a different degree of subadiabaticity in
the stable fluid layers underlying the convection zone.
