1. What drives varying large-scale plasma motions, e.g. meridional flow and differential rotation?

Helioseismic inversions have revealed the interior profile of differential rotation (DR; Schou et al., 1998; Thompson et al., 1996) and the surface meridional circulation (MC). Mean-field dynamos show that DR acts as the main source of large-scale magnetic field amplification, while meridional flows can regulate the timing of the SC. These large-scale flows are considered the “seat” of the dynamo and include a diverse realm of possible flow profiles in stars. The origin of large-scale flows from small-scale turbulence is a pervasive issue in fluid dynamics. Flows much larger than the turbulent scale arise due to delicate balances of magnetic, mechanical, and thermal stresses, as well as angular momentum conservation.

2. How do flows interact with magnetic fields to create varying solar activity cycles?

How does small-scale turbulence generate large-scale magnetic fields? While the strong large-scale toroidal field is generated by the action of shear (DR) on weaker large-scale poloidal field, regeneration of poloidal field to complete the dynamo cycle occurs through a turbulent electromotive 5force (EMF) at small-scales. To close the equations in MFED, small-scale terms must be related to the large-scale variables via an expansion. This requires parametric tensors such as the α tensor, representing the turbulent EMF, and the β tensor representing the turbulent diffusivity. There are currently very few constraints on these parameterizations. This flexibility allows these models to reproduce large-scale SC properties, but also allows several distinct scenarios for the operation of the cycle, such as successors to the Babcock-Leighton model (Babcock, 1961; Leighton, 1969) and the interface model (Parker, 1993). No conclusive evidence allows us to rule in favor of a particular model. Part of the challenge arises from the lack of direct observations of magnetic fields within the convection zone (CZ). Another is that numerical simulations cannot capture the full dynamical range required to reproduce a true turbulent solar dynamo.

3. What causes active regions to emerge when and where they do during the solar cycle?

Even if we understood the generation of large-scale cyclic magnetic fields, how do they form the characteristics that we observe? Active regions (ARs) are the main tracers of the solar toroidal field. Emergence follows systematic patterns, such as the equatorward migration of active latitudes and the systematic orientation of AR polarity in the form of Hale’s and Joy’s laws. Observed characteristics of ARs, such as tilt angle (Kosovichev et al., 2008; Li, 2018; McClintock et al., 2016), magnetic or current helicity (Liu et al. 2014; Pevtsov et al. 1995), surface area, and changing latitude, reflect properties of the interior where the field is generated and through which it emerges. Thus, understanding of flux emergence is critical to extract information about internal processes.

4. How is our understanding of solar activity informed by fields and flows on other stars?

The Sun is the only star observable with significant temporal and spatial resolution. We study its surface activity, eruptions, and derive its interior physical structure and dynamics. Observations and data-analysis used to understand the Sun informs the analysis and understanding of stars, particularly Sun-like stars. Conversely, what we learn from myriad other Sun-like stars, with different flows or spectral types, informs our understanding of the Sun.