The Radiation Belt Storm Probes (RBSP)-Energetic Particle, Composition, and Thermal Plasma (ECT) suite contains an innovative complement of particle instruments to ensure the highest quality measurements ever made in the inner magnetosphere and radiation belts. The coordinated RBSP-ECT particle measurements, analyzed in combination with fields and waves observations and state-of-the-art theory and modeling, are necessary for understanding the acceleration, global distribution, and variability of radiation belt elec-

The HOPE mass spectrometer of the Radiation Belt Storm Probes (RBSP) mission (renamed the Van Allen Probes) is designed to measure the in situ plasma ion and electron fluxes over 4π sr at each RBSP spacecraft within the terrestrial radiation belts. The scientific goal is to understand the underlying physical processes that govern the radiation belt structure and dynamics. Spectral measurements for both ions and electrons are acquired over 1 eV to 50 keV in 36 log-spaced steps at an energy resolution E FWHM /E ≈ 15 %. The dominant ion species (H + , He + , and O + ) of the magnetosphere are identified using foil-based time-of-flight (TOF) mass spectrometry with channel electron multiplier (CEM) detectors. Angular measurements are derived using five polar pixels coplanar with the spacecraft spin axis, and up to 16 azimuthal bins are acquired for each polar pixel over time as the spacecraft spins. Ion and electron measurements are acquired on alternate spacecraft spins. HOPE incorporates several new methods to minimize and monitor the background induced by penetrating particles in the harsh environment of the radiation belts. The absolute efficiencies of detection are continuously monitored, enabling precise, quantitative measurements of electron and ion fluxes and ion species abundances throughout the mission. We describe the engineering approaches for plasma measurements in the radiation belts and present summaries of HOPE measurement strategy and performance.

The density and temperature structure of the low-energy ion population in the outer plasmasphere is examined by using data from Los Alamos National Laboratory's three-dimensional magnetospheric plasma analyzer at synchronous orbit. We define the "outer plasmasphere" as regions of dense, cold plasma observed at synchronous orbit, with no attempt to distinguish between the main plasmasphere or detached plasma regions existing beyond the plasmapause. We find that for moderate to high levels of geomagnetic activity the outer plasmasphere typically has a fine-scale density structure. The amount of variability in the density generally increases with increasing geomagnetic activity, as indicated by Kp, and the most variable intervals are associated with substorm activity. The dense (>10 ions cm•), cold (--1 eV), plasmaspherelike plasma is adjacent to, and often interspersed with, low-density (1-10 ions cm'S), warm (2-10 eV), "troughlike" plasma regions. We find that occasionally these two temperature populations can coexist, but that most often they do not. The scale size of the fine-scale structure in the dense, cold plasma regions often is of the order of 1000 km or less. These observations suggest that fine-scale density structure in the dusk sector of the outer plasmasphere is imposed by penetrating substorm electric fields.Terr . Phys., 50, 185, 1988. Taylor, H. A., H. C. Brinton and A. R. Deshmukh, Observations of irregular structure in thermal ion distributions in the duskside magnetosphere, J. Geophys. Res., 75, 2481, 1970. Taylor, H. A. Jr., J. M. Grebowsky, and W. J. Walsh, Structured variations of the plasmapause: Evidence of a corotating plasma tail, J. Geophys. Res., 76, 6806, 1971. Whipple, E. C., J. M. Warnock, and R. H. WinIcier, Effect of satellite potential on direct ion density measurements through the plasmapause, J. Geophys. Res., 79, 179, 1974.

Very-Low-Frequency (VLF) transmitters operate worldwide mostly at frequencies of 10–30 kilohertz for submarine communications. While it has been of intense scientific interest and practical importance to understand whether VLF transmitters can affect the natural environment of charged energetic particles, for decades there remained little direct observational evidence that revealed the effects of these VLF transmitters in geospace. Here we report a radially bifurcated electron belt formation at energies of tens of kiloelectron volts (keV) at altitudes of ~0.8–1.5 Earth radii on timescales over 10 days. Using Fokker-Planck diffusion simulations, we provide quantitative evidence that VLF transmitter emissions that leak from the Earth-ionosphere waveguide are primarily responsible for bifurcating the energetic electron belt, which typically exhibits a single-peak radial structure in near-Earth space. Since energetic electrons pose a potential danger to satellite operations, our findings demonstrate the feasibility of mitigation of natural particle radiation environment.