August 24, 2016

SWA Instrument

The Solar Wind Analyzer (SWA) instrument suite comprises 3 sensors and a shared data processing unit (DPU), and will completely characterize the solar wind between 0.23–1.4 AU. The overarching objective of SWA is to provide comprehensive in-situ measurements of the solar wind to establish the fundamental physical links between the Sun's highly dynamic magnetized atmosphere and the solar wind in all its quiet and disturbed states. These measurements are vital to Solar Orbiter objectives addressing the origin of the solar wind, solar eruptions, shocks and the suprathermal ions that are the seed populations of hazardous solar particle events. To meet or exceed all the measurement requirements, SWA must be able to measure the three-dimensional velocity distribution functions of the major solar wind constituents—protons, alpha particles and electrons. The basic moments of the distributions, such as density, velocity, temperature tensor, and heat flux vector need to be obtained under all solar wind conditions and should be sampled sufficiently rapidly to characterize fully the fluid and kinetic state of the wind. In addition, measurements of representative high-FIP elements (the C, N, O group) and of low-FIP elements (such as Fe, Si or Mg) are required. These comprehensive and coordinated plasma measurements provided by SWA use simple, high-heritage components flown on previous missions, including Ulysses, ACE, Helios, Stereo, Wind, Soho, Cluster and Cassini. The 3 sensors are:

The 3 SWA sensors

  • The Electron Analyzer System (SWA-EAS) consists of a pair of top-hat electrostatic analyzers with aperture deflection plates mounted in the shadow of the spacecraft at the end of the instrument boom. Orthogonal mounting of the 2 sensors and the ±45° aperture deflection provides an almost full 4 field of view subject only to minor blockage by the spacecraft and its appendages. The sensor will measure electron fluxes in the energy range from ~1 eV to ~5 keV with E/E ~10-12% and an angular resolution = 10°. Moments of the electron distribution will be returned with a cadence of 3 s, although the sensor will be capable of returning full 3D distributions at lower cadence, and 2D electron pitch angle distributions at ~0.1 s cadence during short periods of burst mode.

  • The Proton-Alpha Sensor (SWA-PAS) comprises a top-hat electrostatic analyzer (EA) designed to measure the full 3D velocity distribution functions of major solar wind species, protons and alpha particles in the energy range = 0.2–20 keV/q, with E/E ~7.5%, an angular resolution = 2° across a field of view of –17.5° to +47.5° by ±22.5° about the solar direction and a cadence of 3 s. Reduced distribution functions (1.5-D) of the solar wind protons and alpha particles over a similar energy range will be returned at higher cadence (0.1 s) during burst modes.

  • The Heavy Ion Sensor (SWA-HIS) consists of an electrostatic analyzer module with ion steering to achieve the required extent of the HIS field-of-view (–33° to +63° x ±18°), coupled with a time-of-flight (TOF) telescope with solid state detectors for total ion energy measurements. HIS will measure five key properties for all ions: mass in the range 2-56 amu/q, charge (q), energy in the range 0.5–100 keV/q (for azimuth) and 0.5–16 keV/q (for elevation), E/E ~6% and direction of incidence (, ) with 6° x 6° pixel resolution. The time resolution for 3D distribution measurements is 5 minutes for a full scan in normal mode and 30 s for heavy ions or 3 s for alphas in burst mode.

SWA EAS sensor prototype
EAS sensor prototype

bpc_solar-orbiter-swa-his-pas.png

The HIS and PAS sensors require fields of view containing the Sun-direction and therefore require apertures in the spacecraft heat shield or (preferably) mounting at the edge of the heat shield in such a way that they can protrude beyond its edge.

The SWA plasma sensors are integrated into a suite, and serviced by a common DPU, which provides a single power, telemetry, and control interface to the spacecraft as well as power, switching, commanding, data handling and data compression functions for all of the sensors.