The Sun’s magnetic field dominates the solar atmosphere. It structures the coronal plasma, drives much of the coronal dynamics, and produces all the observed energetic phenomena. One of the most striking features of solar magnetism is its ~11-year activity cycle, which is manifest in all the associated solar and heliospheric phenomena. Similar activity cycles are also observed in a broad range of stars in the right half of the Hertzsprung-Russell diagram, and the Sun is an important test case for dynamo models of stellar activity.
The Sun’s global magnetic field is generated by a dynamo generally believed to be seated in the tachocline, the shear layer at the base of the convection zone. According to flux-transport dynamo models (e.g., Dikpati and Gilman 2008), meridional circulation, and other near-surface flows transport magnetic flux from decaying active regions to the poles. There, subduction carries it to the tachocline to be reprocessed for the next cycle. This ‘conveyor belt’ scenario provides a natural explanation for the sunspot cycle and characterizing the flows that drive it will provide a crucial test of our models and may also allow us to predict the length and amplitude of future cycles. However, current models fail miserably at predicting actual global solar behavior. For example, the current sunspot minimum has been far deeper and longer than predicted by any solar modeling group, indicating that crucial elements are missing from current understanding.
A major weakness of current global dynamo models is the poor constraint of the meridional circulation at high latitudes. The exact profile and nature of the turnover from poleward flow to subduction strongly affect behavior of the resulting global dynamo (e.g., Dikpati and Charbonneau 1999), but detecting and characterizing the solar flow is essentially impossible at shallow viewing angles in the ecliptic plane.
In addition to the global dynamo, turbulent convection may drive a local dynamo that could be responsible for generating the observed weak, small-scale internetwork field, which is ubiquitous across the surface and appears to dominate the emergent unsigned flux there.
A key objective of the Solar Orbiter mission is to measure and characterize the flows that transport the solar magnetic fields: complex near-surface flows, the meridional flow, and the differential rotation at all latitudes and radii. Of particular and perhaps paramount importance for advancing our understanding of the solar dynamo and the polarity reversal of the global magnetic field is a detailed knowledge of magnetic flux transport near the poles. Hinode, peering over the Sun’s limb from a heliographic latitude of 7°, has provided a tantalizing glimpse of the Sun’s high-latitude region above 70°; however, observations from near the ecliptic lack the detail, coverage, and unambiguous interpretation needed to understand the properties and dynamics of the polar region. Thus, Solar Orbiter’s imaging of the properties and dynamics of the polar region during the out-of-the- ecliptic phase of the mission (reaching heliographic latitudes of 25° during the nominal mission and as high as 34° during the extended mission) will provide urgently needed constraints on our models of the solar dynamo.
Most of the open magnetic flux that extends into the heliosphere originates from the Sun’s polar regions, from polar coronal holes. The current solar minimum activity period, which is deeper and more extended than previously measured minima, demonstrates the importance of this polar field to the solar wind and heliosphere.
There is evidence that the solar wind dynamic pressure, composition and turbulence levels, as well as the strength of the heliospheric magnetic field, have all changed in the last few years in ways that are unprecedented in the space age. None of these changes were predicted, and current solar conditions present a challenge to our understanding of the solar dynamo and its effects on the solar system at large and the Earth in particular.
In the following sections, we discuss in more detail three interrelated questions that flow down from this top-level question: How is magnetic flux transported to and reprocessed at high solar latitudes? What are the properties of the magnetic field at high solar latitudes? Are there separate dynamo processes in the Sun?
Presentations of the SAP #4 Meeting, 27-28 October 2016, Göttingen.
- Welcome & scope of the meeting (Yannis Zouganelis).
- Helioseismology overview and Solar Orbiter (Markus Roth).
- Polar magnetic field topology (Andreas Lagg).
- Magnetoconvection (Luis Bellot Rubio).
- PHI strategy for addressing Helioseismology goals (Bjoern Loeptien).
- Some thoughts about Helioseismology, the shape of the Sun and other things (Jesper Schou).
- How METIS can contribute to Objective 4 (Daniele Spadaro).
- How SPICE can contribute to Objective 4 (Alessandra Giunta).