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The Polarimetric and Helioseismic Imager (SO/PHI), formerly known as VIM (the Visible-light Imager and Magnetograph), is a proposal to ESA in response to the Announcement of Opportunity for the Solar Orbiter mission. The contributing institutions are listed below:
PHI will provide maps of the magnetic vector and of the line-of-sight (LoS) velocity in the solar photosphere. It will thus probe the deepest layers of the Sun (including the solar interior using helioseismology) of all the instruments on Solar Orbiter or the Solar Probe Plus (NASA). Since the magnetic field anchored at the solar surface produces most of the structures and energetic events in the upper solar atmosphere and significantly influences the heliosphere, PHI plays a key role in reaching the science goals of Solar Orbiter. Extrapolations of the magnetic field observed by PHI into the Sun’s upper atmosphere and heliosphere will provide the information needed for other optical and in situ instruments to analyze and understand the data recorded by them in a proper physical context. PHI will address and resolve basic questions in solar physics by studying the Sun at high resolution from close up and from high latitudes up to 35°. In particular, it will be central for answering all three top-level science questions of the Heliophysical Explorers (HELEX) program as posed in the Joint Science and Technology Definition Team (JSTDT) report:
PHI will also be a key instrument for reaching three of the four top-level science goals listed in the de Solar Orbiter Science Requirements Document (Sci-RD):
The proposed instrument is designed to completely fulfill the science goals of the HELEX program. In addition, PHI will allow important questions related to magneto-convection and the variability of solar luminosity to be addressed by flying outside the Sun-Earth line and going to high latitudes. PHI will provide high-resolution and full-disk measurements of the photospheric vector magnetic field and line-of-sight (LOS) velocity as well as the continuum intensity in the visible wavelength range at a cadence of one set of observables per minute. The LOS velocity maps will have the accuracy and stability to allow detailed helioseismic investigations of the solar interior, in particular of the solar convection zone. PHI will be composed of two telescopes. The off-axis Ritchey-Chrétien High Resolution Telescope (HRT) will image a fraction of the solar disk at a resolution reaching 150 km at perihelion (the same resolution as the Extreme Ultraviolet Imager’s high resolution channels will have). The refractor Full Disk Telescope (FDT) will be able to image the full solar disk at all phases of the orbit. It incorporates an off-pointing capability. Each telescope will have its own Polarization Modulation Package (PMP) located early in the optical path in order to minimize polarization cross-talk effects. Polarimetry at a signal to noise level of 103 is baselined for PHI. The HRT and the FDT will sequentially send light to a Fabry-Pérot filtergraph system (~ 100 mÅ spectral resolution) and onto a 2048 — 2048 pixel CMOS sensor. PHI will have its own Image Stabilization System (ISS) that will compensate spacecraft jitter or other disturbances. This system will be composed of a limb sensor and separate rapid tip-tilt mirrors for the FDT and the HRT. PHI is of novel design. However, its core elements either have space heritage or are based on technologies developed by the PHI team for the balloon-borne Sunrise mission. Space qualification for these technologies has been started successfully. Additionally, backup solutions have been identified for all critical items. Participating institutes:
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