Space Physics Seminar fall-2017

Solar active regions, torsional oscillations, and meridional flows:
Understanding and modeling the variability with time of the solar differential rotation

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Observations reveal coherent large-scale flows in the solar convection zone that are intimately related to the solar magnetic activity cycle. Solar active regions, the birthplaces of sunspots and sunspot groups, are generally flanked by bands of faster-than-average and slower-than-average zonal motions that extend to great depths within the convective zone. These ‘torsional oscillations’ may be viewed as perturbations of the mean differential rotation of the Sun. The torsional oscillations coexist with two types of meridional motions. The first of these flows generally poleward from the equator in each hemisphere, with velocities of 10-20 m s-1. Smaller scale flows (with velocities of 5-10 m s-1) are in addition seen to converge toward the active regions; the speeds of these flows vary with the phase of the sunspot cycle. The meridional flows and torsional oscillations exhibit spatiotemporal correlations with each other and with the progressive evolution of the magnetic activity cycle. Such correlations suggest the existence of physical relationships linking the phenomena of the active regions, the torsional oscillations, and the meridional flows. These linked phenomena are likely to be closely connected with the mechanism(s) responsible for the excitation of the solar dynamo.

The chicken-and-egg problem of determining causality has proven to be a difficult one. Does the progressive motion with time of the active regions (as seen in the butterfly diagram) arise due to the large scale differential motions of solar materials, or is it the other way around? Or might these intertwined and mutually-correlated phenomena arise due to some other cause? A recently developed dynamical hypothesis may potentially shed light on this question. We have traditionally considered the rotational motions and the orbital motions of the Sun (and other extended bodies) to be entirely independent and uncoupled, aside from certain weak effects attributed to tidal friction. This has allowed us to treat the angular momenta of the solar rotation and the solar orbital motion separately as closed systems. This long-standing assumption has recently been called into question. An equation describing a weak non-tidal coupling of the orbital and rotational motions was derived in Shirley (Plan. Space Sci. 141, 1-16, 2017). In the current presentation we will describe the nature and likely consequences of the proposed orbit-spin coupling mechanism for the solar differential rotation. In addition we make note of some earlier solar-physical observations and studies that may be cited in support of the physical hypothesis. If time permits we may additionally review pertinent results obtained from general circulation modeling of the Mars atmosphere with orbit-spin coupling (Mischna, M.A., and J. H. Shirley, Plan. Space Sci 141, 45-72, 2017).