The Proton and Alpha Sensor (PAS) for the Solar Orbiter Mission:
motivations, design, operation principle and expected scientific return
Abstract
Solar Orbiter is an ESA/NASA mission that will provide an unprecedented
opportunity to discover the fundamental connections between the rapidly
varying solar atmosphere and the solar wind. The Solar Wind Analyzer
(SWA) plasma package shall provide comprehensive in situ measurements of
the solar wind. In particular, the Proton-Alpha Sensor (PAS) will
determine the high time resolution properties solar wind ion populations
through the measurement of the 3D distribution function, density, bulk
velocity, temperature and heat Vux. The closest approach of Solar
Orbiter to the Sun is 0.28 AU. At this distance the solar wind Vow,
solar UV, and solar infrared Vuxes increase by a factor 13 compared to
near-Earth space. The PAS instrument will provide high cadence 3D
distribution function measurements (up to 13 per second) all the way
from closest approach to 1 AU. The extreme UV and thermal conditions at
closest approach precluded the use of standard ion optics schemes. We
thus developed a new ion sensor optic design combined with conventional
robust channeltron detectors. This paper describes the PAS measurement
scheme adopted for varying solar wind conditions and our approach to the
fast sampling of 3D distribution functions. We also highlight the
expected scienti:c return of the instrument.