Solar Orbiter will provide the first detailed view of the polar subsurface layers by carrying out seismic measurements both from a high-latitude vantage point and "stereoscopically" by combining PHI data and seismic data from the ground or from NEO (e.g. SDO/HMI). These observations will reveal the patterns of differential rotation, the geometry of the meridional flow, and the properties of convection cell below the solar surface. By monitoring the temporal variations over the course of the mission, it will be possible to deduce solar-cycle variations in the flows.
Thanks to global helioseismology (e.g. Christensen- Dalsgaard 2002), the solar differential rotation has been mapped as a function of latitude and radial distance throughout most of the convection zone. For heliographic latitudes above 70°, however, the global oscillation mode inversions are uncertain, leaving an incomplete picture of the solar interior. Local helioseismology (e.g. Gizon & Birch 2005) aims to measure the 3D velocity vectors of the material flows in the solar interior, allowing studies of convective, rotational and meridional flows, as well as sunspots and active regions. Local helioseismology together with PHI observations will enable the study of high heliographic latitudes. Another important goal of Solar Orbiter is to implement stereoscopic helioseismology by combining PHI data with Doppler measurements from Earth or NEO instruments. Local helioseismic inversions from techniques such as time-distance helioseismology or helioseismic holography will be able to probe deeper into the Sun using observations from widely separated vantage points because skip distances of more than half a circumference will, at last, become accessible. This will be important for probing the tachocline at the base of the convection zone, where the dynamo is surmised to be situated.
- Track granules and magnetic features to follow their motions and mutual interaction over time (Abramenko et al., 2011; Giannattasio et al., 2013; 2014; Gosic et al., 2014; Requerey et al., 2014).
- Track mesogranules and supergranules and determine lifetime, sizes, horizontal velocities, and other properties such as helicity of the flows (e.g. Hathaway 2000, Palacios 2012).