The Near-Surface Shear Layer (NSSL)

Theme Leads: Rudi Komm & Nick Featherstone

The solar convection zone also possesses a boundary layer of radial rotational shear near the surface. This NSSL extends to a depth of ∼35 Mm and is characterized by a 5% reduction of the differential rotation rate (Howe, 2009, review). Unlike the tachocline, the NSSL can be observed in detail by helioseismic techniques. Those observations raise a number of questions regarding the maintenance of the NSSL and its linkage to the broader dynamo process that are well suited to COFFIES . We begin with (1) Characterizing NSSL dynamics and those processes responsible for its maintenance. Exactly how this region is maintained remains a major puzzle. Rapid near-surface convection, such as supergranulation, certainly plays an important role in diminishing the differential rotation (Foukal et al., 1975), and other effects, such as gyroscopic pumping (Miesch et al., 2011) and thermal wind balance (Choudhuri, 2021) may also contribute to its maintenance. While some numerical simulations confirm the importance of these effects, solar-like shear has been found only in the equatorial regions of models (e.g., Guerrero et al., 2013; Matilsky et al., 2019).

Helioseismic studies of the NSSL have mainly focused on its solar cycle variations (e.g., Basu, 2021; Komm et al., 2020) and its changes in the proximity of ARs (e.g., Braun, 2019). The HS Team will derive measurements of convective flows (meridional circulation and differential rotation) of the NSSL from the surface to about 0.95 Rsun on time scales of days to years in longitude and latitude, including the solar cycle variation using global (Basu, 2016, review) and local techniques (Gizon et al., 2005, review). The helioseismic inferences will be validated by applying the same techniques to model data from the DY and MI Teams and will also be reconciled with flows derived by the SL Team. These observations will be used to measure several derived fluid properties, such as Reynolds stress and kinetic helicity, that will allow us to explore the dynamical processes that maintain the NSSL. We will determine whether the NSSL is in thermal wind balance (Choudhuri, 2021), and assess the relative importance of gyroscopic pumping in its maintenance. These analyses will be carried out in tandem with the CV Team, linking unobservable deep convection to near-surface observations.

This combined modeling/observational effort will help us address another outstanding question: (2) what does the presence of the NSSL imply about deep-convection dynamics? Deep-seated convective flows are thought to sustain solar differential rotation and are therefore key to the dynamo process (Miesch, 2005, review). Yet their near-surface signatures are much weaker than expected, if not absent all together. Instead, photospheric convection is dominated by supergranulation (e.g., Hart, 1956; Rincon et al., 2018) with a horizontal spatial scale nearly 10x smaller than the 200 Mm-sized motions expected to sustain the solar differential rotation (Miesch, 2005). Observations of large-scale convection have been substantially weaker than expected, by roughly a factor of 10 (Hathaway et al., 2021), and helioseismic attempts to probe their depth dependence have led to contradictory results (Greer et al., 2015; Hanasoge et al., 2012; Proxauf, 2021). Collectively, these inconsistencies are often referred to as the convective conundrum, best summarized as, “How does the Sun’s convection transport energy and drive differential rotation with such apparently weak flows?” Addressing this topic is a major component of the NSSL Theme that requires intensive collaboration among all five COFFIES Science Teams.

The final component is to (3) understand the relationship of the NSSL to broader dynamo processes. The role played by the NSSL in mediating the generation and timing of solar magnetism remains unclear, but emergent magnetic flux that serves as the source of ARs must rise through the NSSL. Following emergence, the AR evolution is governed by near-surface flows that influence its dispersal and transport. It is believed that this newly generated poloidal field is then pumped back into the interior where differential rotation produces the toroidal fields of the next cycle. The relative contributions of meridional circulation and turbulent pumping to this submersion process and its ultimate relation to the dynamo cycle remain unclear. Similar questions surround the role of turbulent transport by near-surface convection (effectively acting as a magnetic diffusivity) and the importance of kinetic helicity, which contributes to the active generation of magnetic field.