Present state of knowledge:
In the last decades, the mapping of surface and subsurface flow fields at low and middle latitudes has seen major advances, largely due to the availability of high-quality data from the SOHO's Michelson Doppler Imager (MDI) instrument. These data have provided accurate knowledge of differential rotation, the low latitude, near-surface part of the meridional flows, and the near-surface torsional oscillations, which are rhythmic changes in the rotation speed that travel from mid-latitudes both equatorward and poleward (Howe et al. 2006). Local helioseismic techniques have also reached a level of maturity that allows the three-dimensional structure of the shallow velocity field beneath the solar surface to be determined.
Despite these advances, progress in understanding the operation of the solar dynamo depends on how well we understand differential rotation and the meridional flows near the poles of the Sun. However, because of the lack of out-of-the-ecliptic observations, the near-polar flow fields remain poorly mapped, as does the differential rotation at high latitudes (see Beck 2000; Thompson et al. 2003). The meridional flow, in particular, the very foundation of the flux transport dynamo, is not well characterized above ~50° latitude; it is not even certain that it consists of only one cell in each hemisphere. The return flow, believed to occur at the base of the convection zone, is entirely undetermined save for the requirement of mass conservation. All these flows must be better constrained observationally in order to help solve the puzzle of the solar cycle and to advance our understanding of the operation of the solar dynamo (and, more broadly, of stellar dynamos generally).
How Solar Orbiter will address this question:
Solar Orbiter will measure or infer local and convective flows, rotation, and meridional circulation in the photosphere and in the subsurface convection zone at all heliographic latitudes including, during the later stages of the nominal mission, at the critical near-polar latitudes. Solar Orbiter will reveal the patterns of differential rotation, the geometry of the meridional flow, the structure of subduction areas around the poles where the solar plasma dives back into the Sun, and the properties of convection cells below the solar surface. This will be achieved through correlation tracking of small features, direct imaging of Doppler shifts, and helioseismic observations (including the first from a high-latitude vantage point). By monitoring the temporal variations over the course of the mission, it will be possible to deduce solar cycle variations in the flows.
Solar Orbiter will resolve small-scale magnetic features near the poles, even within the nominal mission phase, and right up to the poles during the extended mission. It will determine the detailed surface flow field through tracking algorithms. Such algorithms provide only inconclusive results when applied to polar data obtained from near-Earth orbit due to the foreshortening. Doppler maps of the line-of-sight velocity component will complement the correlation tracking measurements and will also reveal convection, rotation, and meridional circulation flows.
Time series of Doppler and intensity maps will be used to probe the three-dimensional mass flows in the upper layers of the convection zone, at high heliographic latitudes. The flows will be inferred using the methods of local helioseismology (e.g., Gizon and Birch 2005): time-distance helioseismology, ring diagram analysis, helioseismic holography, and direct modeling. Using SOHO/MDI Dopplergrams, it was demonstrated that even complex velocity fields can be derived with a single day of data (e.g., Jackiewicz et al. 2008).
The deeper layers of the convection zone will be studied using both local and the global methods of helioseismology. Moreover, Solar Orbiter will provide the first opportunity to implement the novel technique of stereoscopic helioseismology to probe flows and structural heterogeneities deep in the convection zone, even reaching down to the tachocline. Combining Solar Orbiter observations with ground- or space-based helioseismic observations from 1 AU (e.g., GONG or SDO) will open new windows into the Sun. Looking at the Sun from two distinct viewing angles will increase the observed fraction of the Sun’s surface and will benefit global helioseismology because the modes of oscillation will be easier to disentangle (reduction of spatial leaks). With stereoscopic helioseismology, new acoustic ray paths can be taken into account to probe deeper layers in the interior, including the bottom of the convection zone.