The Relativistic Proton Spectrometer (RPS) for the Radiation Belt Storm Probes Mission

Abstract

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...

AE9, AP9 and SPM: New Models for Specifying the Trapped Energetic Particle and Space Plasma Environment

“…AP9 goes out to 400 MeV using a physics-based model extrapolation. Lack of high-energy proton data, a regime especially affecting on-board electronics, is the primary reason for flying the Relativistic Proton Spectrometer (RPS) on the Van Allen Probes (Mazur et al 2012). …”

“…The project has been led by principal investigators from the Aerospace Corporation and MIT Lincoln Laboratory and has involved personnel from AFRL, Boston College, Atmospheric and Environmental Research (AER) and Los Alamos National Laboratory. Development of Version 1.0 (V1.0) of AE9/AP9/SPM was under the auspices of the NRO Proton Spectrometer Belt Research (PSBR) program which also supported the development of the Relativistic Proton Spectrometer (RPS) instruments (Mazur et al 2012) currently flying on the Van Allen Probes (formerly Radiation Belt Storm Probes, or RBSP). Future versions of the model are expected to include data from RPS as well as the many other particle and plasma detectors on the Van Allen Probes.…”

Science Goals and Overview of the Radiation Belt Storm Probes (RBSP) Energetic Particle, Composition, and Thermal Plasma (ECT) Suite on NASA’s Van Allen Probes Mission

“…The integrated RBSP-ECT suite provides maximum Van Allen Probes science return using a minimum of resources. The RBSP-ECT suite particle measurements are complemented on each Van Allen Probes spacecraft by two other particle sensors which provide additional important information about ion composition in the ring current energy range, RBSPICE (Mitchell et al 2013), and of the inner zone proton populations, RPS (Mazur et al 2012). …”

Investigation of Solar Proton Access Into the Inner Magnetosphere on 11 September 2017

“…There was no accompanying geomagnetic storm, unlike the SEP event which accompanied the 6–8 September CME shock‐driven geomagnetic storm. The penetration of >60‐MeV protons into the magnetosphere during the 10–12 September 2017 solar energetic particle event was explored with the Relativistic Proton Spectrometer (Mazur et al, ) onboard Van Allen Probes (O'Brien et al, ). Lower‐energy solar protons with higher intensity can also cause serious space weather hazards and are thus deserving further attention.…”