Understanding Magnetic Flux Emergence

Understanding Magnetic Flux Emergence

The strong toroidal magnetic field produced by rotational shear at the base of the convection zone is thought to be the source of buoyant flux loops that rise and emerge to form magnetic active regions on the solar surface. Subsequently, the emerged field evolves in the photosphere, chromosphere, and corona where they play a critical role in structuring and forcing into the atmospheric layers. Observational, theoretical, and numerical investigations that HAO scientists have undertaken provide a vital understanding of how the precursor structures for space‐weather events such as flares and coronal mass ejections (CMEs) form in the outer atmosphere as well as of the steady mass and energy transport originating from the base of the atmosphere. The main objective is to understand the emergence, growth, decay, and distribution of the surface fields on all spatial scales, their transport by diffusion and flows, the effects of the flux eruption on the surrounding and overlying atmospheric regions, and their eventual contributions to solar irradiance variability.

HAO scientists are making progress towards three‐dimensional global magneto‐hydrodynamic (MHD) simulations in which turbulent convection, stratification, and rotation are combined to yield a dynamo that self‐consistently generates buoyant magnetic loops for the Sun and Sun‐like stars. Recent steps in this direction involve simulating convection and dynamo action in a fast rotating spherical shell with solar stratification. The simulations of fast‐rotating Sun‐like stars have demonstrated that strong “wreaths” of toroidal magnetic field are formed by dynamo action in the convection zone, and that the strongest portions of those wreaths will rise to the top of the convection zone via a combination of magnetic buoyancy instabilities and advection by giant convective cells. Conversely, for the slower solar case, the effects of the convective flow will allow the inserted flux tubes with weaker field strengths (below 50 kG at the bottom of the solar convection zone) to develop emerging loops. Those loops have properties that are consistent with those observed in solar active regions.

The solar photosphere is a transition region in which the primary energy transport mechanism switches from convection to radiative transfer. At the same time, the plasma becomes partially ionized due to the lower temperatures, requiring a more complicated equation of state. The role of the magnetic field is changing too: while the gas pressure dominates the interior of the Sun, the magnetic pressure becomes the dominant contributor in or above the photosphere. Owing to the rather short density scale height, the photosphere is a highly stratified medium in which convective motions easily steepen to supersonic flows and shock waves. The combination of all these conditions makes numerical modeling of the photosphere challenging but also extremely interesting due to the strong interaction between convection, magnetic field, and radiation, and the possibility for in‐depth comparison with high‐resolution observations.

Tilt angle as a function of emergence latitude

Figure 2. Sunspot fine structure at the τ = 1 level. Quantities shown are (a) bolometric intensity, (b) radial and (c) vertical magnetic field, (d) field inclination, (e) radial, and (f) vertical flow velocities. A field inclination of 0° corresponds to vertical field with the same polarity as the umbra, 90° to horizontal, and 180° to vertical field with opposite polarity of the umbra. Radial outflows are displayed by red colors, solid contours indicate regions with more than 10 km s–1 outflow velocity. Vertical upflows are displayed by blue colors, solid contours indicate regions with more than 5 km s–1 downflow velocity.

By studying the properties of sunspots HAO scientists are learning about the complexities of the photospheric interface. High-resolution simulations are carried out to investigate the connection between the fine structures of sunspot penumbrae and the subsurface magneto-convection processes that are responsive for filamentation of the penumbra and acceleration of the eponymous Evershed outward flow of materials. Simulations at lower resolutions but on larger computational domains (up to 16 Mm deep and 75 Mm wide) are also performed to study the subsurface magnetic and flow structure of sunspots, including large scale-flows in the surrounding moat region. The sunspot fine structures are illustrated in Figure 2. Developed in partnership with the NASA Solar Dynamo Observatory (SDO) Science Center at the Colorado Research Associate (CoRA), these simulations have permitted the critical validation of helioseismic structural inversions of sunspots and their surroundings.

The diagnosis of the prevailing magnetic conditions in the lower solar atmosphere is conducted by measuring the polarization of light emitted by magnetically sensitive spectral lines. Historically, HAO has pioneered the development and use of instruments to make such spectro-polarimetric measurements, along with techniques to infer field strength and direction from those observations. Solar physics is entering a new era of magnetic field measurements, in which emerging technologies and observations from space will revolutionize our overall understanding of the Sun’s magnetism and its impact throughout the heliosphere. At the core of these investigations are the observations and detailed analysis of magnetic features as they emerge through the solar photosphere. Through involvement with the spectro-polarimeter (SP) portion of the Hinode spacecraft’s solar optical telescope (SOT) instrumentation package, HAO scientists are able to study in detail magnetic flux emergence, decay, and distribution, at a range of spatial scales.

At the smallest scales, the atmosphere of the quiet sun is pervaded by an apparently turbulent magnetic field with various scale sizes extending well below the resolution of present-day instruments. The small-scale structure of solar magnetism might arise from disruption and shredding through the action of convective flows on the field of the large-scale solar magnetic cycle as a result of a dynamo operating at the base of the solar convection zone. Alternatively, it might arise from a small-scale turbulent dynamo (SSD) operating as a result of vigorous turbulent convective motions very close to the solar surface. It is important to distinguish between these two possibilities because the fields arising from a small-scale dynamo may affect the structure of the solar atmosphere independent of the solar magnetic cycle. The SSD mechanism has been suggested by numerical magneto-convection simulations, but so far there is little observational evidence confirming or refuting its presence. The high-resolution and excellent polarimetric sensitivity of the Hinode/SP has permitted observations that suggest the SSD indeed is operating in the solar atmosphere through the detailed examination of very quiet regions.