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The Energetic Particle Detector (EPD) Investigation is one of five particles and fields investigations on the Magnetospheric Multiscale (MMS) mission. This mission consists of four satellites operating in close proximity in elliptical, low-inclination orbits, and is focused upon the fundamental physics of magnetic reconnection. The Energetic Particle Detector (EPD) investigation aboard the four MMS spacecraft consists of two instrument designs, the EIS (Energetic Ion Spectrometer) and the FEEPS (Fly's Eye Electron Proton Spectrometer). This present paper describes FEEPS from an instrument physics and engineering point of view, and provides some test and calibration data to facilitate effective analysis and use of the flight data for scientific purposes.A FEEPS consists of six Heads, each composed of two Eyes. Each eye is a particle telescope with a single silicon detector; there are nine electron eyes and three ion eyes per FEEPS. The energy coverage is from 25 keV to 650 keV for electrons and 45 keV to 650 keV for ions. Each eye has sixteen energy channels, the spacing of which can be modified by command. The fields of view and pointing of each eye are designed to provide a broad, instantaneous field of view for the twelve eyes per FEEPS.There are two FEEPS per MMS spacecraft mounted such that the pair along with the single EIS provide more than 3π-sr instantaneous solid-angle coverage and complete coverage in the equatorial region. A twenty-second spacecraft rotation period is divided into sixty-four sectors to provide detailed temporal and spatial sampling.

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The Relativistic Proton Spectrometer (RPS) on the Radiation Belt Storm Probes spacecraft is a particle spectrometer designed to measure the flux, angular distribution, and energy spectrum of protons from ∼60 MeV to ∼2000 MeV. RPS will investigate decadesold questions about the inner Van Allen belt proton environment: a nearby region of space that is relatively unexplored because of the hazards of spacecraft operation there and the difficulties in obtaining accurate proton measurements in an intense penetrating background. RPS is designed to provide the accuracy needed to answer questions about the sources and losses of the inner belt protons and to obtain the measurements required for the nextgeneration models of trapped protons in the magnetosphere. In addition to detailed information for individual protons, RPS features count rates at a 1-second timescale, internal radiation dosimetry, and information about electrostatic discharge events on the RBSP spacecraft that together will provide new information about space environmental hazards in the Earth's magnetosphere. Keywords Scientific GoalsWithin two Earth radii of the Earth's surface there is an unexplored region of trapped particles that are a challenge to accurately measure, a challenge to engineer for spacecraft design, and a challenge to understand given particle lifetimes that can exceed decades or even centuries. The inner Van Allen belt is a nearby reservoir of charged particles from a multitude of sources. We know the existence of trapped protons with energies beyond 1 GeV, secondary particles such as positrons, electrons, and light ions 4 He, 3 He, and 2 H, electrons that diffuse inward from the outer magnetosphere, and heavy ions that originate as interstellar neutral particles. These particles execute their gyration, bounce, and drift motions in a region with a relatively strong and stable magnetic field that shelters them from transients in the geomagnetic field, yielding the longest trapping lifetimes for particles in the Earth's magnetosphere. The tenuous upper atmosphere is both a source and sink of these populations, whose influence varies over the solar cycle and episodically during times of rapid atmospheric joule heating.The Relativistic Proton Spectrometer (RPS) 223Sorting out the various sources and losses in the Inner belt is challenging but there are particle characteristics that aid the process such as: extremely high proton energies that distinguish these particles as having a galactic cosmic ray origin; heavy ion composition that identifies these as being anomalous cosmic rays whose access to the inner belt is only afforded by their being mostly singly-ionized; and antimatter composition that identifies another branch the products of nuclear collisions between galactic cosmic rays and atmospheric atoms. For electrons the picture is less clear since there is little information about the electron energy spectrum and no identifiable characteristics of an electron that originates from a nuclear interaction versus one that diffu...

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