Thomas Paul O'BRien

As we move towards proliferated space and smaller, more cost-effective satellites, miniaturization of quality science instrumentation is becoming necessary and possible. This paper describes the Relativistic Electron Magnetic Spectrometer (REMS), a miniaturization of NASA’s Van Allen Probes’ MagEIS-Medium instrument, which was designed to measure radiation belt electrons. REMS was designed, built, and delivered for NASA’s GTOSat mission (Blum et al. in Proc. SPIE 11389, micro- and nanotechnology sensors, systems, and applications XII, 113892E, 2020), and was integrated onto the spacecraft bus in 2021. REMS’s electron energy range is one decade and can be set between ∼30–1500 keV. The GTOSat REMS main rate electron energy centroids range from 136–1088 keV. REMS has 9 electron main rate energy channels and 32 electron histogram channels per detector pixel, allowing for more precise energy measurements that enable background subtraction as was accomplished with MagEIS. GTOSat REMS measures ∼400–9000 keV protons. The angular resolution ranges from ∼3°–20° depending on the REMS orientation and the detector pixel in question. On the spinning GTOSat, REMS provides a pitch angle resolution down to 5.63° (32 sectors per spin) for the main rate electron data product, 30° for the proton data product, and 22.5° for the histogram data product. A different sectoring choice would alter the pitch angle resolution. At the time of writing, the mission is awaiting a launch. In the meantime, REMS has been included in additional mission concepts and proposals. The REMS instrument provides high quality energy and pitch angle resolved measurements of electrons and protons, enabling future exploration and monitoring of Earth’s radiation belts with small satellites.

Whitepaper #450 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: space weather applications; infrastructure/workforce/other programmatic. Additional topics: space weather research/operations/research loop; other programmatic; […]

Whitepaper #036 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: space weather applications. Additional topics: space weather research/operations/research loop; space-based missions/projects; space weather

We describe a new version of the SHELLS model, which specifies the outer electron belt environment as observed by the Van Allen Probes using the Kp Index and data from Polar‐orbiting Operational Environmental Satellites and the Meteorological Operational satellites as inputs. The updated model combines the radial, angular and energy dependence into a single neural network, allowing for specification of the electron environment for a set of user input energy, L‐shell and B_mirror values. We demonstrate that the model can accurately reproduce the outer electron radiation belt dynamics using both out‐of‐sample data from the Van Allen Probes and measurements from the Combined X‐ray Dosimeter instrument on GPS. This updated model will provide valuable characterization of the internal charging hazard for non‐GEO orbits, particularly in the post‐Van Allen Probes era.

We explore the hypothesis that electron precipitation curtains such as those observed by the AeroCube‐6 satellite pair can be produced by electron microbursts. Precipitation curtains are latitudinal structures of stable precipitation that persist for timescales of 10s of seconds or longer. The electrons involved have energies of 10–100s of keV. The microburst formation hypothesis states that a source region in the equatorial region produces a series of very low frequency chorus wave emissions. Each of these emissions in turn produces a microburst of electron precipitation, filling the drift and bounce loss cone on the local field line. Electrons in the drift loss cone remain on the field line and bounce‐phase mix over subsequent bounces while also drifting in azimuth. When observed at downstream azimuths by a satellite equipped with an integral energy sensor, no bounce phase structure remains, or, equivalently, the same time profile is present when two such satellites pass by many seconds apart. The spatial structure that remains reflects the pattern of microburst sources. Statistical studies of where and when curtains occur have indicated that some, but not all, curtains could be caused by microbursts. We use test particle tracing in a dipole magnetic field to show that spatially stationary source regions generating periodic microbursts can produce curtain signatures azimuthally downstream. We conclude that one viable explanation for many of the curtains observed by the AeroCube‐6 pair is the accumulation of drift‐dispersed microburst electron byproducts in the drift loss cone.

We examine drift phase structure in the electron radiation belt observations to differentiate radial transport mechanisms. Impulsive electrostatic or electromagnetic fields can cause radial transport and produce drift echoes (periodic drift phase structures with energy‐dependent period). Narrow‐band standing electromagnetic wave fields can also cause radial transport, while producing energy‐independent periodic drift phase structures. Broad‐band, random‐phase electromagnetic wave fields can cause radial transport, but do not necessarily produce drift phase structure. We present results of three case studies showing little association between drift phase structure and approximately MeV electron flux enhancements in the outer belt. We estimate the amplitude of drift phase structures expected for impulsive or narrow‐band interactions to compete with broad‐band, random‐phase waves. We show that the observed drift phase structure is typically much smaller than would be present if either impulses or narrow‐band waves were the dominant cause of radial transport. We conclude that radial transport is primarily consistent with the broad‐band, random‐phase, small perturbations assumed in quasilinear diffusion theory, although we cannot rule out the unlikely possibility that radial transport plays little role in radiation belt dynamics.

Key Point: The editors thank the 2021 peer reviewers

We explore the hypothesis that electron precipitation curtains such as those observed by the AeroCube-6 satellite pair can be produced by electron microbursts. Precipitation curtains are latitudinal structures of stable precipitation that persist for timescales of 10s of seconds or longer. The electrons involved have energies of 10s-100s of keV. The microburst formation hypothesis states that a source region in the equatorial region produces a series of very low frequency chorus wave emissions. Each of these emissions in turn produces a microburst of electron precipitation, filling the drift and bounce loss cone on the local field line. Electrons in the drift loss cone remain on the field line and bounce-phase mix over subsequent bounces while also drifting in azimuth. When observed at downstream azimuths by a satellite equipped with an integral energy sensor, no bounce phase structure remains, or, equivalently, the same time profile is present when two such satellites pass by many seconds apart. The spatial structure that remains reflects the pattern of microburst sources. Statistical studies of where and when curtains occur have indicated that some, but not all, curtains could be caused by microbursts. We use test particle tracing in a dipole magnetic field to show that spatially stationary source regions generating periodic microbursts can produce curtain signatures azimuthally downstream. We conclude that one viable explanation for many of the curtains observed by the AeroCube-6 pair is the accumulation of drift-dispersed microburst electron byproducts in the drift loss cone.

We examine drift phase structure in the electron radiation belt observations to differentiate radial transport mechanisms. Impulsive electrostatic or electromagnetic fields can cause radial transport and produce drift echoes (periodic drift phase structures with energy-dependent period). Narrow-band standing electromagnetic wave fields can also cause radial transport, while producing energy-independent periodic drift phase structures. Broad-band, random-phase electromagnetic wave fields can cause radial transport, but do not necessarily produce drift phase structure. We present results of three case studies showing little association between drift phase structure and ~MeV electron flux enhancements in the outer belt. We estimate the amplitude of drift phase structures expected for impulsive or narrow-band interactions to compete with broad-band, random-phase waves. We show that the observed drift phase structure is typically much smaller than would be present if either impulses or narrow-band waves were the dominant cause of radial transport. We conclude that radial transport is primarily consistent with the broad-band, random-phase, small perturbations assumed in quasilinear diffusion theory, although we cannot rule out the unlikely possibility that radial transport plays little role in radiation belt dynamics.