  The Solar Orbiter mission, to be launched in July 2017, will carry a suite of
remote sensing and in-situ instruments, including the Polarimetric and
Helioseismic Imager (PHI). PHI will deliver high-cadence images of the Sun in
intensity and Doppler velocity suitable for carrying out novel helioseismic
studies. The orbit of the Solar Orbiter spacecraft will reach a solar latitude
of up to 21 deg (up to 34 deg by the end of the extended mission) and thus will
enable the first local helioseismology studies of the polar regions. Here we
consider an array of science objectives to be addressed by helioseismology
within the baseline telemetry allocation (51 Gbit per orbit, current baseline)
and within the science observing windows (baseline 3 x 10 days per orbit). A
particularly important objective is the measurement of large-scale flows at
high latitudes (rotation and meridional flow), which are largely unknown but
play an important role in flux transport dynamos. The full range of
Earth-Sun-spacecraft angles provided by the orbit will enable helioseismology
from two vantage points by combining PHI with another instrument: stereoscopic
helioseismology will allow the study of the deep solar interior and a better
understanding of the physics of solar oscillations in both quiet Sun and
sunspots. We have used a model of the PHI instrument to study its performance
for helioseismology applications. As input we used a 6 hr time-series of
realistic solar magneto-convection simulation (Stagger code) and the SPINOR
radiative transfer code to synthesize the observables. The simulated power
spectra of solar oscillations show that the instrument is suitable for
helioseismology. In particular, the specified point spread function, image
jitter, and photon noise are no obstacle to a successful mission.
