Flux Transport & Emergence (FT&E)

Theme Leads: Bill Abbett & Aimee Norton

Figure 4: Select COFFIES models show A) fields in CZ from EULAG-MHD model (Stejko, 2017), B) HPS code simulations of flux emerging from the tachocline; the background field quenches the rise of bundles with certain helicities (Manek et al., 2021), C) buoyant magnetic fields generated in turbulent convection including a thin vertical shear layer from Pencil code (Guerrero et al., 2011), D) near-surface convective velocities for a 1.47Msun main sequence star from StellarBox (Kitiashvili et al., 2020)

Observed magnetic activity results directly from the emergence of magnetic flux through the photosphere and into the corona. This flux is generated in the turbulent layers of the Sun’s convection zone through dynamo processes operating at both large and small scales (Fig. 4A,C). Long-term observations of magnetic field evolution put relevant constraints on our physical understanding of the dynamo and the emergence of active region (AR)-scale flux. For example, since most ARs emerge in accordance with Joy’s law (i.e., with bipoles oriented east-west with trailing polarities systematically displaced poleward), this suggests that emerging flux structures are coherent, toroidal, and likely exist at scales subject to Coriolis forces during the process of emergence. Similarly, Hale’s polarity law (i.e., the leading polarity of most ARs switches sign across the equator and from cycle to cycle) implies a corresponding antisymmetry in the underlying toroidal fields.

The physics of the storage, generation, and subsequent emergence of magnetic flux, from deep interior to visible surface, remains a matter of great debate. The standard model posits that weak poloidal fields generate strong toroidal fields at or near the strongly sheared tachocline (Corbard et al., 1999; Guerrero et al., 2016; Kosovichev, 1996; Tobias et al., 2004; Vasil et al., 2008). These toroidal fields are then subjected to dynamic or magnetic instabilities (Dikpati et al., 2020) that produce coherent structures (e.g., magnetic flux tubes or Ω-loops; see e.g., Caligari et al. 1995; Fan et al. 1996; Wissink et al. 2000) that buoyantly rise through the convection zone (Fig. 4B; see e.g., Abbett et al. 2004; Fan et al. 2003; Knizhnik et al. 2021; Manek et al. 2021) and emerge through the near surface layers, producing observed AR properties (Fisher et al., 1995; Hale et al., 1919; Wang et al., 1989). However, recent radiative MHD models of near surface magnetoconvection show that bipolar magnetic regions can potentially form in the absence of preexisting structures. In such a scenario, the toroidal flux self-organizes into isolated magnetic structures simply by interacting with convective flows. Concentrations of magnetic flux at the visible surface then coalesce and form naturally (Chen et al., 2017; Cheung et al., 2010), not through the bodily emergence of a coherent flux tube. So how best to distinguish between these ideas? During Phase I, we initiated collaborative code-coupling efforts between the CV and SL Teams to answer this question as a part of the broader effort to address COFFIES SQ2 and SQ3.

Global models (Fig. 4A) also address these unresolved issues regarding the location, formation, and nature of emergence of strong magnetic fields. For example, anelastic global simulations of the deep interior show how preexisting, magnetically buoyant structures with magnetic fieldsabove equipartition values can rise cohesively through the turbulent convection zone (Fan, 2008), while others show how strong toroidal layers of equipartition or sub-equipartition magnetic fields can be self-consistently generated within the bulk of a (rapidly) rotating convection zone. Convective flows then act to generate magnetic loops that ascend through the interior through a combination of magnetic buoyancy and advection via convective flows (Nelson et al., 2011, 2013). However, these types of large-scale flux emergence models do not include the top 5-10 pressure scale heights near the surface. Similarly, models that include the low solar atmosphere and the near-surface shear layer do not typically extend to the deeper layers treated by the global models. This is due to the distinct physical properties and disparate spatial and temporal scales characteristic of each regime.

To make transformative progress, we must connect these regions in order to characterize the transport of magnetic flux, energy, and helicity across the entirety of the convective interior over multiple scales from the tachocline to the photosphere, where direct observations constrain our models. This is a critical component of our effort and is integral to each of the four SQs. To achieve this breakthrough, we must address fundamental problems of scale and develop new techniques to ensure that dynamic models can properly describe relevant physical processes over multiple scales. Members of COFFIES have worked independently on aspects of this problem, and in Phase I we built new collaborative teams to pursue this effort. In Phase II, COFFIES will capitalize on these essential inter-team collaborations. Specifically, the MI, DY, and CV Teams will develop a framework to couple the physics of the tachocline to the turbulent, differentially rotating plasma of the convective interior and, with the SL Team, couple the highly stratified convective surface layers and the low atmosphere to the fields and flows characteristic of the deep interior. This topic was addressed in Phase I collaborations between the SL and CV Teams, and will expand in Phase II to include the MI Team. The HS and SL Teams will provide critical observations to constrain COFFIES numerical models and, where possible, be directly integrated into the models.

In addition to our fundamental objective above, we will integrate modeling and observational projects of the DY, CV, SL, and MI Teams to (1) determine the physical mechanisms underlying the observed emergence of ARs over the solar cycle, and (2) better understand the origin and evolution of magnetic helicity (Fig. 4B). Finally, we will join observational and theoretical efforts from the HS and DY Teams to (3) understand what the presence of activity nests and active longitudes reveal about the paths of flux emergence, and better understand how these features constrain current dynamo models. The Center will approach each of these topics in a holistic and integrated way by facilitating the individual expertise and combined work effort of each Science Team.