Page tree
Skip to end of metadata
Go to start of metadata

Present state of knowledge:

MHD simulations indicate that a local turbulent dynamo should be acting in the Sun’s turbulent convection zone (Brun et al. 2004) and even in the near-surface layers (Vögler and Schüssler 2007). Hinode/SOT has detected ubiquitous horizontal magnetic fields in quiet regions of the Sun (Lites et al. 2007), which are possibly generated by local dynamo action (Pietarila Graham et al. 2009). These small, weak features (inter-network fields; Zirin 1987) bring 100 times more magnetic flux to the solar surface than the stronger features that are known to be the product of the global dynamo, and have themselves shown to be in cross-scale turbulent equilibrium (Schrijver et al. 1997). Even the smallest observable features have been shown to be formed primarily by aggregation of yet smaller, yet more prevalent features too small to resolve with current instrumentation (Lamb et al. 2008, 2009). It is, however, still uncertain whether a separate local, turbulent dynamo really is acting on the Sun and how strongly it contributes to the Sun’s magnetic flux (and magnetic energy). In particular, all solar magnetic features, from the smallest observable intergranular flux concentrations to the largest active regions, have been shown (Parnell et al. 2009) to have a power law (scale free) probability distribution function, suggesting that a single turbulent mechanism may be responsible for all observable scales of magnetic activity. 


How Solar Orbiter will address this question:

One way to distinguish between the products of a global and a local dynamo is to study the distribution of small elements of freshly emerging magnetic flux over heliographic latitude. The global dynamo, presumably owing to the structure of the differential rotation and the meridional flow near the base of the convection zone, leads to the emergence of large bipolar magnetic regions (active regions) at the solar surface at latitudes between 5° and 30° and of smaller ephemeral active regions over a larger range of latitudes, but concentrated also at low latitudes. In contrast, a local turbulent dynamo is expected to enhance field more uniformly across the surface.

Observations carried out from the ecliptic cannot quantitatively determine the latitudinal distribution of magnetic flux and in particular the emergence of small-scale magnetic features (inter-network fields) due to foreshortening and the different sensitivity of the Zeeman effect to longitudinal and transversal fields. Solar Orbiter, by flying to latitudes of 25° and higher above the ecliptic, will be able to measure weak magnetic features equally well at low and high latitudes (Martínez Pillet 2006). If the number and size (i.e., magnetic flux) distributions of such features are significantly different at high latitudes, then even the weak features are probably due to the global dynamo. If, however, they are evenly distributed, then the evidence for a significant role of a local dynamo will be greatly strengthened. Current work is confounded by viewing angle restrictions near the poles, by the ubiquitous seething horizontal eld (e.g., Harvey et al. 2007), and by small deflections in near-vertical fields, which dominate observed feature distributions near the limb of the Sun. 

Detailed sub-objectives:

4.4.1 Compare the distribution of small-scale fields at low and high latitudes (Tsuneta et al., 2008; Ito et al., 2010). Is the magnetic network equally strong between low and high latitudes? How does this distribution change between activity maximum and minimum? What is the latitude distribution of the emergence of ephemeral regions? Is this distribution dependent on the cycle phase?

The emergence, diffusion and decay of ephemeral regions near the poles and below high-latitude coronal holes should be studied for the aspect of how they feed the magnetic network (see e.g. Simon et al. 2001, ApJ 561, 49 427; Gosic et al. 2014, ApJ 797). In particular, the latitudinal dependence of this decay process would be interesting to study. What is the latitude distribution of internetwork magnetic fields and their emergence rates? Do they depend on the phase of the solar cycle? What is the latitudinal distribution of the linear-polarisation features in the quiet Sun? Is there a solar cycle dependence?


4.4.2 Joy’s law at high latitudes Extend Joy’s law at low latitudes from large to small bipolar features, i.e. from active regions to ephemeral regions. Compare the tilt angle distributions of ephemeral regions at low and high latitudes. How do these distributions change over the cycle?


4.4.3 Understand differences in size and internal structuring of magnetic concentrations in high and low latitudes and relate them to different origins or dynamic environments (e.g. Lagg et al., 2010; Martínez González, 2012; Requerey et al., 2014; 2015).