Understanding the Solar Dynamo
Understanding the Solar Dynamo
The magnetic fields that are responsible for various manifestations of solar activity, from the photosphere through the chromosphere to the corona, originate inside the Sun where the motion of highly electrically conducting plasma acts as a dynamo. HAO scientists carry out a comprehensive program of dynamo‐related research by using advanced numerical simulations of turbulent convection, studying the large‐scale differential rotation and meridional circulation in the Sun’s interior, and investigating the interactions between these flows and entrained magnetic fields.
An ongoing challenge is to understand how ordered patterns of magnetic activity such as the sunspot cycle emerge from the highly turbulent conditions in the solar convection zone. The answer lies in complex self‐organization processes linking multiple spatial and temporal scales, and involving helical flows and fields, rotational shear, global circulations, magnetic buoyancy, flux emergence, and subtle boundary layers. Simulations of global solar convection demonstrate that magnetic cycles can occur in convective dynamos even without three elements that play a crucial role in most current dynamo models of the solar activity cycle, namely magnetic flux emergence, a layer at the tachocline of rotational shear, and significant flux transport by the mean meridional circulation. Rather, agents of self‐organization include helicity and shear, forming persistent toroidal flux structures in the midst of turbulent convection zones, defying previous theoretical expectations. Recent ground‐breaking simulations demonstrate that these toroidal flux structures can become buoyantly unstable, rising toward the surface. These are the first convection simulations to exhibit the self‐consistent generation of buoyant magnetic flux tubes by rotational shear that is maintained by the convection itself.
In flux‐transport‐dynamo models, the Sun’s meridional circulation acts as a conveyor belt, where plasma flows poleward at the solar surface and returns equatorward near the base of the convection zone (“tachocline”). Just as the Earth's great oceanic conveyor‐belt carries thermal signatures that determine El Niño events, the Sun's conveyor‐belt affects the timing, amplitude and shape of a solar cycle through its transport of magnetic fields. Numerical simulations of a flux transport dynamo have demonstrated that polar fields are determined mostly by the strength of a surface poloidal source provided by the decay of tilted, bipolar active regions. The profile of a meridional flow with latitude and its change with time have much less effect on polar fields in fluxtransport dynamo models than in models focusing on the transport of magnetic flux at the solar surface. The flux‐transport dynamo models therefore offer a reasonable explanation to the weakening of polar magnetic fields in solar cycle 23, which were observed to be 50% smaller than those in solar cycle 22.
The base of the Sun's convective envelope is a region widely believed to play a key role in the solar dynamo. The stratification in this region is governed by processes of convective overshooting and element diffusion, and it gives rise to a characteristic signal in the frequencies of solar helioseismic p modes that has been used to determine the depth of the solar convection zone and to investigate the extent of convective overshoot. Recent physically motivated models incorporate a smooth transition in stratification bridging the region from the lower convection zone to the radiative interior beneath, providing better agreement with helioseismic data than would a standard solar model. Convectively excited gravity waves at the base of the convection zone can also be influenced by rotation and the strong toroidal magnetic field that is thought to be present. A simple model, investigating how internal waves with a vertical component of propagation are reflected when traveling through a layer containing a horizontal magnetic field with a strength that varies with depth, found that the interaction can prevent a portion of the downward traveling wave energy flux from reaching the deep solar interior, limiting the ability of waves to influence the rotational state of the core. Additional simulations, in which the waves were forced selfconsistently by overshoot from an overlying convection zone, revealed complex behaviors produced by wave reflection and the wave‐field interaction. The spectrum of generated waves differed considerably from the predictions of available analytic models. The changing character of the wave‐field interaction resulting from distortion of the field by waves and overshoot led to significant variations in the amount of wave energy propagating into the deep radiative interior.
Efforts to probe the nature of the dynamo are supplemented by work aimed at detecting, measuring, and modeling magnetically induced variability in the oscillations and radiative emissions of solar‐type stars. Over the past two decades, helioseismology has revolutionized our understanding of the interior structure and dynamics of the Sun. Asteroseismology will soon put this knowledge into a broader context by providing structural data for hundreds of Sun‐like stars. As shown in Figure 1 (above), asteroseismic data from the Kepler satellite mission have been used in this way to determine the precise age and radius for the Sun‐like star KIC 11026764. Stellar analogues of the solar cycle can also be sought in Kepler data, through long‐term measurements of emission in the H and K lines of singly ionized Ca. The broad range of cycle periods observed in stellar magnetic activity variations is thought to reflect differences in the rotational properties and the depths of the surface convection zones for stars with various masses and ages. For example, the 1.6‐year magnetic activity cycle in the exoplanet host star ι Hor was established: this is the shortest activity cycle so far measured for a solar‐type star.