Solar Orbiter's Energetic Particle Detector (EPD) - STEIN
SupraThermal Electrons, Ions, and Neutrals (STEIN) is being developed at CAU in collaboration with the Space Sciences
Laboratory (SSL) ot the University of California in Berkeley (UCB), and
the Kyung-Hee University in South Korea.
It is a double-ended telescope, utilizing passively cooled silicon semiconductor detectors (SSDs) to measure suprathermal particles from ~3-100 keV. STEIN utilizes an electrostatic deflection system to separate electrons and ions up to ~40 keV, and neutrals up to ~10 keV.
Solar Orbiter goes close to the Sun, where the Parker spiral field will be much closer to the radial direction and the magnetic field gradient will be much larger, so charged particles will tend to be focused along the field line. Thus, to have a high probability of detecting the field-aligned suprathermal electrons, it is highly desirable to look as close to the radial direction as possible. This means that ions will also be in the field of view.
Thus, STEIN will utilize an electrostatic deflection system, consisting of an electric field region between two highly corrugated parallel plates behind an entrance aperture (see Figure for a cutaway view of the STEIN sensor), to separate electrons and ions. After passing through this deflection region, lower energy electrons and ions are swept to opposite sides (with the direction dependent on the deflection voltage polarity), where they are measured by the edge pixels, while higher energy ions and electrons (and neutrals) are largely unperturbed, and strike the center pixels. By sweeping the deflection voltage (with both polarities), we cover a range of ion and electron energies and angles. Simulations show that STEIN’s deflection system will cleanly separate electrons and ions in the edge pixels over an angular range of ~±35° (with angular resolution of ~5-10°, better for small angles) for energies from the SSD energy threshold (~2 keV for electrons, 3-4 keV for ions and neutrals) up to ~15 keV, and over a narrower angular range of ~±5-10° from ~15 keV up to ~40 keV. Above these energies, no particles will be measured by the edge pixels.
Figure: profile of STEIN
The STEIN instrument consists of two sets of these detectors and deflection systems
mounted back to back (see Figure), thereby allowing measurements of sunward and
anti-sunward electrons, ions and, possibly, neutrals.
The STEIN sensor configuration ensures that the edge pixels have no direct light path, even during potential off-pointing situations, and collimators minimize scattered light to all the detectors. Since SSDs are sensitive even to scattered light, however, the STEIN sensor is located in shadow near the end of the boom, with no illuminated regions of the spacecraft in the field of view.