Les instabilités magnétohydrodynamiques dans EULAG-MHD
Even today, the physical mecanisms generating the solar magnetic cycle are not yet fully understood. Recent research has suggested that magnetohydrodynamical (MHD) instabilities could be present and perhaps have a significant influence on the magnetic cycle period through their ability to accelerate the destruction of established large scale magnetic fields. In the present work, we investigate the existence and operation of an in- stability operating within an EULAG-MHD simulation of solar convection. We define a magnetic energy-based proxy designed to be sensitive to the growth of the MHD instabi- lity suspected to develop in the simulation, and compare its temporal behavior to another energy-based proxy characterizing the primary large-scale magnetic cycle. Both proxies show phase-lagged cyclic behavior with similar amplitudes. We follow up on this ana- lysis by applying another method based of the identification of "unstable zones" inside our convectively stable layer, using a known linear stability criterion for the so-called "Tayler instability". The application of this stability criterion reveals an equatorward mi- gration of a small unstable zone that starts at very high latitudes at each poles, crosses the zonal bands of strong toroidal magnetic field building up in the simulation, and acce- lerates towards the equator. This "instability front" appears to accelerate the destruction of the established magnetic field and thus facilitates polarity reversal and the buildup of the opposite polarity magnetic field for the subsequent half-period of the cycle. The combination of both types of analyses enables us to propose a plausible dynamo scenario that includes the effets of this instability on the magnetic cycle as well as on the global stability of our simulation. In the context of this scenario, it must be emphasized that the polarity inversions per se are not driven by the instability, which only supports the pola- rity reversals process by means of accelerated desctruction of the established large-scale magnetic field.
Instabilities Magnetohydrodynamics Solar magnetic cycle