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Low-mass M dwarf stars, T Tauri stars, as well as planets such as the Earth and Jupiter are permeated by large-scale magnetic fields generated by the convection-driven dynamo operating in their convection zones. These magnetic fields are often characterized by a significant time variability, most prominently expressed by the inversions of their polarity, denoted as reversals, whose mechanism has not been completely understood. This work aims to gain some insights into the mechanism that generates these reversals. With this purpose, a simplified nonlinear model is developed to investigate the role played in polarity reversals by the convective heat transfer occurring in stellar and planetary convection zones. A model result is the enhancement of the global heat transport before polarity reversals, showing the crucial role that heat transport might play in their occurrence. This role is elucidated by considering that a reversal has a greater than 70% probability of occurring during a burst of convective heat transport. This high probability has been found in 94 out of 101 numerical simulations obtained by changing characteristic model parameters. Moreover, the causal relationship between the convective heat flux growth and the magnetic field variations is highlighted by the temporal antecedence of the former relative to the latter and by convergent cross mapping, namely a statistical test for detecting causality. It would thus be expected that higher levels of temporal variability in the planetary and stellar magnetic fields may be correlated to a higher heat transfer efficiency achieved in the interior of these celestial bodies. [ABSTRACT FROM AUTHOR]

This article addresses the interesting and important problem of large-scale magnetic field generation in turbulent flows, using a self-consistent dynamo model recently developed. The main idea of this model is to consider the induction equation for the large-scale magnetic field, integrated consistently with the turbulent dynamics at smaller scales described by a magnetohydrodynamic shell model. The questions of dynamo action threshold, magnetic field saturation, magnetic field reversals, nature of the dynamo transition and the changes of small-scale turbulence as a consequence of the dynamo onset are discussed. In particular, the stability curve obtained by the model integration is shown in a very wide range of values of the magnetic Prandtl number not yet accessible by direct numerical simulation but more realistic for natural dynamos. Moreover, from our analysis it is shown that the large-scale dynamo transition displays a hysteretic behaviour and therefore a subcritical nature. The model successfully reproduces magnetic polarity reversals, showing the capability to generate persistence times which are increasing for decreasing magnetic diffusivity. Moreover, when the system reaches a statistically stationary dynamo state, where the large-scale magnetic field can abruptly reverse its polarity (magnetic reversal state) or not, keeping the same polarity (steady state), it shows an unmistakable tendency towards the energy equipartition for the turbulence at small scale. [ABSTRACT FROM PUBLISHER]