Meridional Circulation Dynamics in a Cyclic Convective Dynamo

Friday, March 24, 2017

Surface observations indicate that the speed of the solar meridional circulation in the photosphere varies in anti-phase with the solar cycle. The current explanation for the source of this variation is that inflows into active regions alter the global surface pattern of the meridional circulation.

Meridional flow variations image
Meridional flow variations induced by magnetic cycles. The images show the axial torques (convergence of the convective and magnetic angular momentum flux) averaged over (A) cycle minima and (B) cycle maxima. Red indicates a convergence of the angular momentum flux that induces a meridional flow (arrows) away from the rotation axis. Blue indicates a flux divergence that induces a flow toward the rotation axis. Numbers indicate regions that are subject to further analysis in the paper. The strong torques at mid-latitudes near the base of the convection zone (indicated by dashed lines) during cycle max (B) are due to a buildup of strong toroidal magnetic flux, which is shown by the white contours. The presence of these toroidal bands induces a prominent mid-latitude upwelling that waxes and wanes with the phase of the cycle. If similar upwellings occur in the Sun, they may have implications for magnetic flux emergence, for the solar cycle, and for the coupling between the convective envelope and the radiative interior.

When these localized inflows are integrated over a full hemisphere, they contribute to the slow down of the axisymmetric poleward horizontal component. The behavior of this large scale flow deep inside the convection zone remains largely unknown. Present helioseismic techniques are not sensitive enough to capture the dynamics of this weak large scale flow. Moreover, the large time of integration needed to map the meridional circulation inside the convection zone, also masks some of the possible dynamics on shorter timescales. In this work we examine the dynamics of the meridional circulation that emerges from a 3D MHD global simulation of the solar convection zone. Our aim is to assess and quantify the behavior of meridional circulation deep inside the convection zone, where the cyclic large-scale magnetic field can reach considerable strength. Our analyses indicate that the meridional circulation morphology and amplitude are both highly influenced by the magnetic field, via the impact of magnetic torques on the global angular momentum distribution. A dynamic feature induced by these magnetic torques is the development of a prominent upward flow at mid latitudes in the lower convection zone that occurs near the equatorward edge of the toroidal bands and that peaks during cycle maximum. Globally, the dynamo-generated large-scale magnetic field drives variations in the meridional flow, in stark contrast to the conventional kinematic flux transport view of the magnetic field being advected passively by the flow. Submitted to Astronomy and Astrophysics.

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