Helioseismology with the Solar Orbiter Mission
Björn Löptien (Max-Planck-Institut für Sonnensystemforschung)

The Solar Orbiter mission, to be launched in October 2018, 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 28 deg (up to 33 deg during the extended mission). This will enable local helioseismic studies of the polar regions of the Sun. In addition, combined observations of Solar Orbiter and another instrument will enable stereoscopic helioseismology - helioseismology from two vantage points. This potentially will allow probing the deep interior of the Sun and can lead to a better understanding of the physics of solar oscillations in both quiet Sun and sunspots. Helioseismology with Solar Orbiter will be subject to challenges that are not relevant to current missions. Major constraints for helioseismoloy will be the probably short observing time (current baseline: 3 x 10 days per orbit) and the low telemetry (minimum: 52 Gbit/orbit), which will require the usage of lossy data compression. Recent results, however, suggest that both local helioseismology and feature tracking methods are robust regarding lossy data compression. In addition, helioseismology of the solar poles requires observations close to the solar limb, even from the inclined orbit of Solar Orbiter. This gives rise to systematic errors, such as the center-to-limb effect in local helioseismology. These systematics will show a time-dependency due to the eccentric and inclined orbit of Solar Orbiter.