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Solar Orbiters EPD

Solar Orbiter's Energetic Particle Detector (EPD) EPT

The Electron and Proton Telescope (EPT) has heritage from STEREO SEPT. SEPT and Solar Orbiter EPT was designed to cleanly separate and measure electrons in the energy range from 20 - 400 keV and protons from 60 - 7000 keV. The Solar Orbiter EPT electron measurements from 20 - 400 keV will cover the gap with some overlap between suprathermal electrons measured by STEP and high energy electrons measured by HET. The proton measurements from 20 - 7000 keV will cover the gap between STEP and HET.
The Electron and Proton Telescope (EPT) relies on the magnet/foil-technique, adapted from STEREO SEPT in order to measure electrons from 20 keV to 400 keV and protons from 20 keV to 7 MeV.

The energy range and energy resolution as well as the time resolution and pointing directions are optimized to provide crucial constraints on the acceleration and propagation of energetic particles. In order to determine particle anisotropies EPT consists of two dual double-ended magnet/foil particle telescopes EPT1 and EPT2: EPT1 points in the orbital plane approximately along the Parker spiral magnetic field towards and away from the Sun. EPT2 points out of this plane towards North and South.

Figure: EPT sensor

The EPT sensor schematic is shown in Figure. Each double-ended telescope has two solid state detectors (SSDs, shown in red). These two SSDs are operated in anticoincidence. One of the outer SSD looks through an absorption foil and its partner through the air gap of a magnet system. The magnet system consists of two pairs of rare-earth permanent magnets. Each pair leaves an air gap with the necessary magnetic induction to deflect the electrons. The long-range field is attenuated by placing the two oppositely polarized magnetic dipoles at close distance. Thus, the two air gaps provided by the magnet system serve two telescope systems simultaneously with antiparallel viewing directions at minimum weight penalty while satisfying the stringent requirements on magnetic cleanliness. The far-field onboard STEREO as measured with the magnetometer of the MAG instrument amounted to 1.8 nT at a distance of 3 m.

The parylene layer leaves the electron spectrum essentially unchanged but stops protons of energy up to the energy of electrons (~ 400 keV) which penetrate the first SSD. The magnet is designed to sweep away electrons below 400 keV, but leaves ions unaffected. In the absence of > 400 keV ions, the foil SSD only detects electrons, and the magnet SSD detects only ions. Ions from 20 keV to 7 MeV/n will stop in the magnet SSD and their fluxes will be cleanly measured. The contribution of > 400 keV ions to the foil SSD can then be computed and subtracted to obtain the electron fluxes.

The geometrical factor for each of the four telescopes is 0.01 cm2 sr. EPT and HET share two common Eboxes. One Ebox hosts the Sun/anti-Sun sensors of EPT and HET, the other Ebox hosts the out-of-ecliptic (or North-South) sensor heads.

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