Solar Orbiter / PAS

Thematic Group: PEPS
Scientific Leader: Philippe Louarn

Description / Background

Several decades after HELIOS, Solar Orbiter is the new probe to explore the inner regions of the heliosphere with instruments capable of finely characterizing the local environment and simultaneously studying the dynamics of coronal structures.

Solar Orbiter’s orbits will be in near-corotation with the Sun. The probe will remain several days in the same solar wind stream, to observe its evolution and correlate it with the structures observed on the surface of the Sun.

Scientific objectives and instrumentation

The objective is to discover the fundamental processes linking the internal and surface phenomena of the Sun, the general mass and energy flows in the corona, as well as the formation and acceleration of the solar wind.

The instrument suite SWA (Solar Wind Analyser) is dedicated to the measurement of the solar wind and its various components.  It brings together:

(1) a ‘Proton-Alfa’ (PAS) sensor that measures the main component of the solar wind,

(2) a ‘Heavy Ions Sensor’ (HIS) that measures the fine composition of the heavy ions of the solar wind, thus their mass and charge state,

(3) an ‘electron’ sensor (EAS) that measures the electronic component.

The PAS sensor (Proton-Alfa Sensor) is the instrument of the SWA suite designed to measure the ion distribution functions of the solar wind, from 200 eV to 20 KeV. The aim is to determine the 3D distribution functions of the ions constituting the solar wind (in practice, protons and alfas). The goal is not only to compute the density, velocity and temperature of this main component of the solar wind (the ‘fluid’ parameters) but, more generally, to characterize all the deviations from the ‘classical’ Maxwellian functions: temperature anisotropy, beams, heating and acceleration traces etc…, which will result from the kinetic processes of energy and moment exchanges in this fluid without collision.

In normal mode, the instrument will measure the 3D ion distribution function every 4 seconds, in the form of matrices of 96 energies, 11 azimuthal angles and 9 elevation angles. The energy and elevation angle are selected by imposing particular values on the different high voltages supplying the deflection electrodes and the electrostatic analyzer (Figure 3). The 11 azimuth angles correspond to the 11 detectors of the instrument (‘channeltron’-type charge amplifiers).
In burst mode, the measurement rate will be up to 20 Hz, with reduced distribution functions, e.g. 24 energies, 5 deflections and 7 azimuths.
These measurements of distribution functions at frequencies significantly higher than the ionic frequencies characteristic of the medium will be the first and will provide an opening for the study of energy dissipation and turbulence.

April 15, 2020 : The first hours of observations.
We see a change in the characteristics of the solar wind at 4:34 pm.

Involvement of IRAP

Design and realization of the PAS sensor.

PAS Schematics
Exploded view of the deflector system and the analyzer showing the different electrodes whose voltages must be synchronized to allow the selection of particle angles and energy


MSSL, INAF (Frascati), University of Prague, LPP, SWRI.