Pulsed, focused electrons are presented as a possible surrogate to single-event effect (SEE) testing. Electrons have the capability to penetrate deep into a material and provide an equivalent linear energy transfer (LET) in silicon to that of a heavy ion. Through experiment, modeling, and simulation of a large area pin photodiode, we have shown that this method can act as an alternative way to test for SEEs as long as the spot size of the electron bunch is small enough to saturate the local carrier collection dynamics. There are no known fundamental physical limitations to preclude pulsed, focused electrons as a tool to be used for predictive SEE capability.

Understanding the zenith angle dependence of the Martian surface radiation environment is crucial for planning future human exploration missions to Mars. In our previous research (Wimmer et al. 2015; Guo et al. 2021; Khaksarighiri et al. 2023) we extensively studied the zenith-angle dependence of the Martian surface radiation dose rate. Leveraging the same validated radiation model, calibrated with data from the Radiation Assessment Detector (RAD) on Mars, we calculated the flux of secondary downward particles reaching to the surface of Mars from various zenith angles resulting from the interaction of primary particles with the Martian atmosphere. These flux of secondary particles, coming from different zenith angles, can be integrated into a comprehensive topographic map of Mars, providing a detailed depiction of the global radiation landscape.The construction of this radiation map requires careful consideration of various factors, including atmospheric column density, local and large-scale topography offering potential shielding effects, and the input spectrum is affected by heliospheric modulation. Additionally, accounting for seasonal pressure cycles and daily atmospheric surface pressure due to thermal tides is essential. Our model specifically focused on the influence of zenith angle on atmospheric column depth and simulations tailored to the Gale Crater region, a region explored by the Curiosity rover. Applying this methodology allows us to create lookup tables of all secondary particles reaching the Martian surface from various zenith angles and evaluate the atmospheric impact. Employing these matrices alongside the incident spectrum enables the calculation of secondary particle flux from all zenith angles on the Martian surface.This method provides valuable insights into the fluctuations in radiation flux on Mars, facilitating thorough assessments of potential radiation hazards. Mission planners can leverage these data, obtaining vital information to identify secure landing areas and sheltered regions for astronauts on the Martian surface.

The topographic influence of the radiation environment on the Martian surface radiation is crucial for future human exploration. Topographic maps help assess radiation flux variations, aiding in hazard evaluation. Creating a global radiation map requires accounting for seasonally varying atmospheric density, heliospheric modulation, and topography. Here, we use a radiation model to derive the flux of secondary downward particles generated by the interaction of primary protons with the Martian atmosphere. Our model examines two key factors: (a) the dependence of atmospheric column depth on the zenith angle, affecting radiation directionality as horizon-arriving particles traverse more atmosphere than vertical ones and (b) atmospheric conditions at surface heights in Gale Crater, crucial for developing radiation dose maps that incorporate topographic effects. Our model is validated against Radiation Assessment Detector measurements and benchmarked with existing models. We construct response matrices representing the ratio of secondary particles at the Martian surface to primary inputs across zenith angles, assessing atmospheric effects. We combine these matrices with the incident spectrum to compute secondary particle fluxes from all zenith angles for Galactic Cosmic Rays and Solar Energetic Particles. These fluxes will be integrated into a topographic map of Mars in a follow-up study, providing a detailed representation of surface radiation levels across different terrains. This approach aids mission planners in identifying safe landing sites for astronauts.

AbstractThe Earth’s radiation belts are maintained by a number of acceleration, loss and transport mechanisms, and the electron fluxes at any given time are highly variable. Microbursts, which are rapid (sub-second) bursts of energetic electrons entering the atmosphere from the magnetosphere, are one of the key loss mechanisms controlling radiation belt fluxes. Such rapid bursts are typically observed from the outer radiation belt and driven by interactions with whistler mode chorus waves, but they can also occur in the inner belt and slot region, driven by lightning-generated whistlers. This lightning-induced electron precipitation is typically observed at 10s–100s keV, but here we present direct observations of this phenomenon at MeV energies. This unveils a coupling between near-Earth processes, such as lightning, and radiation belt processes, such as relativistic electron microbursts, bridging the gap between Earth weather and space weather.

The Relativistic Proton Spectrometer (RPS) on the Van Allen Probes spacecraft was a particle spectrometer designed to measure the flux, angular distribution, and energy spectrum of protons from ∼ 60 MeV to ∼ 2000 MeV . RPS provided new information about the inner Van Allen belt: a nearby region of space that had been relatively unexplored because of the difficulties of making charged particle measurements there and the associated hazards to satellite operations. We met the primary mission objective of providing accurate data for the AP9 radiation specification model at the high energies where there were little to no data prior to the Van Allen Probes mission. Along the way, we were able to demonstrate the long-term stability of parts of the Inner Belt by comparison with short-lived space science missions that operated decades prior to Van Allen Probes. The most significant surprises were the agreement between RPS and some of those historical measurements and the discovery of a trapped population of > 30 MeV leptons at the outer edge of the inner belt. This end-of-mission paper summarizes the instrument performance, calibration, data products, and specific science and engineering results, and includes suggestions for future investigations of intense radiation fields like those found within the inner belt.

A model balancing diffusive field‐line curvature (FLC) scattering loss with other local source, loss, and transport processes is used to compute the L ‐shell location of the proton radiation belt outer boundary, where intensity falls below observable levels. The FLC scattering rate increases with increasing L and proton energy, decreasing equatorial pitch angle, and increasing geomagnetic activity, as determined with the K p ‐dependent T89 external magnetic field combined with a dipole internal field. Simulated proton intensity near the outer boundary is compared to measurements from both the low‐altitude SAMPEX satellite and the high‐altitude Van Allen Probes satellites, with correction for instrumental biases caused by field‐of‐view, energy range, and the east‐west effect. Good agreement between observations and simulations is obtained for average boundary locations, time‐dependent variations caused by magnetic storms, and average equatorial pitch‐angle distributions.

George Clark, Jim Kinnison, Dan Kelly, Peter Kollmann, Wen Li, Allison Jaynes, Lauren Blum, Robert Marshall, Drew Turner, Ian Cohen, Sasha Ukhorskiy, Barry Mauk, Elias Roussos, Quentin Nénon, Sasha Drozdov, Xinlin Li, Emma Woodfield, Will Dunn, Grant Berland, Ralph Kraft, Peter Williams, Todd Smith, Kareem Sorathia , Anthony Sciola, George Hospodarsky, Xin Wu, Paul O’Brian, Mark Looper, Angelica Sicard, Andy Santo, Meagan Leary , Amanda Haapala, Fazle Siddique, Michelle Donegan, Ben Clare, Derek Emmell, Kim Slack, John Wirzburger, Daniel Sepulveda, Lew Roufberg, Jackie Perry, John Schellhase, Darrius Pergosky, Liz Able, Mike O’Neill, Cris Fernandes, Deb Chattopadhyay, Samuel Bibelhauser, Seth Kijewski, Joe Pulkowski, and Mike Furrow

Abstract Radiation transport codes have been an increasingly important tool for studying the space radiation environment, which includes high‐energy and high‐nuclear‐charge particles. The unique advantage of transport models lies in covering a wider range of particles, energies, and angles than would be attainable in a laboratory or measurable by an instrument. However, since there are several transport codes developed by different teams that have contributed heavily to the literature, differences are expected between such codes. In this work, we use five such radiation transport codes (FLUctuating KAscade, GEometry ANd Tracking, High‐Energy Transport Code‐Human Exploration and Development in Space, Monte Carlo N‐Particle, and Particle and Heavy Ion Transport code System) to study the radiation environment near the Moon, specifically the lunar “albedo” radiation, which is the radiation emitted by the lunar surface through interactions with incident galactic cosmic rays and solar energetic particles. The primary goal of this paper is to provide a general characterization of the lunar albedo radiation and to find the areas where the modeled transport codes agree and disagree by using almost identical input parameters. The results of this work show overall good agreement between the codes. However, some areas of discrepancies exist that are reported herein. Thus, this paper equips the space weather and radiation biology communities with a comparison between popular radiation transport models applied to lunar albedo radiation. The overall agreement and, in some cases, discrepancies between these transport codes provide fundamental insight necessary for assessing code reliability and identifying where further study and improvements are needed to advance our understanding of lunar albedo radiation.

Data sets are provided in support of the publication to appear in JGR-Space Physics. They include tabulated values of computed albedo neutron flux above the atmosphere, and of resulting radiation belt electron and proton source functions. Data format is described in the README files (left out of version 1.0, added to version 1.1).

A new model of cosmic ray albedo neutron flux above the atmosphere, based on Geant4 simulation of atomic and nuclear interactions, is used to calculate radiation belt neutron‐decay source functions for electrons and protons. The model covers albedo neutron energies from 0 to 100 GeV. Inclusion of the cosmic‐ray east‐west effect provides a corresponding azimuthal dependence in the albedo neutron flux, in addition to zenith angle and energy dependence, with parameterization by cosmic‐ray vertical cutoff and solar F10.7. Inclusion of Earth’s curvature allows forward scattered neutrons to escape into space at remote locations, increasing the source of high‐energy protons. Gravitational effects on low‐energy (thermal and epithermal) neutrons, and their decay in flight, reduce their flux at higher altitude, which determines the strength of the electron source. Model neutron flux and derived neutron density are in good agreement with available balloon and space‐based measurements.

Charged particle precipitation from Earth’s magnetosphere results in stunning displays of the aurora and energy transfer into the atmosphere. Some of this precipitation is caused by wave-particle interactions. In this study, we present an example of a wave-particle interaction between Electromagnetic Ion Cyclotron waves, and magnetospheric protons and electrons. This interaction resulted in a co-located isolated proton aurora and relativistic electron microbursts. While isolated proton aurora is widely believed to be caused by Electromagnetic Ion Cyclotron waves, this unique observation suggests that these waves can also scatter relativistic electron microbursts. Theoretically, nonlinear interactions between Electromagnetic Ion Cyclotron waves and electrons are necessary to produce the intense sub-second microburst precipitation. Lastly, detailed analysis of the auroral emissions suggests that no chorus waves were present during the event. This is in contrast to the most commonly associated driver of microbursts, whistler mode chorus waves, and supports other less commonly considered driving mechanisms.

Introduction: The Moon has a harsh radiation environment that poses significant challenges to future science and exploration activities. Exposure hazards from space radiation are primarily due to galactic cosmic rays (GCRs) and solar energetic particles (SEPs) that are incident at the lunar surface from all directions. The Lunar Reconnaissance Orbiter’s (LRO) Cosmic Ray Telescope for Effects of Radiation (CRaTER) instrument has been observing space radiation around the Moon since 2009 [1]. The CRaTER observations show a steady rate of GCR flux with intermittent SEP events that have much higher fluxes. During solar minimum the GCR have a higher flux rate while the SEP events are less common. On the other hand, during solar maximum the SEP events have a higher rate but the GCR flux is lower. This is due to variations in solar activity. GCRs have characteristic energies spanning from 1 MeV to 10s of GeV [2]. SEPs, however, have much lower energy ranges of 50 keV to 10 GeV.

Measurements from NASA’s Van Allen Probes have transformed our understanding of the dynamics of Earth’s geomagnetically-trapped, charged particle radiation. The Van Allen Probes were equipped with the Magnetic Electron Ion Spectrometers (MagEIS) that measured energetic and relativistic electrons, along with energetic ions, in the radiation belts. Accurate and routine measurement of these particles was of fundamental importance towards achieving the scientific goals of the mission. We provide a comprehensive review of the MagEIS suite’s on-orbit performance, operation, and data products, along with a summary of scientific results. The purpose of this review is to serve as a complement to the MagEIS instrument paper, which was largely completed before flight and thus focused on pre-flight design and performance characteristics. As is the case with all space-borne instrumentation, the anticipated sensor performance was found to be different once on orbit. Our intention is to provide sufficient detail on the MagEIS instruments so that future generations of researchers can understand the subtleties of the sensors, profit from these unique measurements, and continue to unlock the mysteries of the near-Earth space radiation environment.

We report the probable detection of energetic neutral hydrogen atoms (ENAs) at >0.8 MeV in several large solar energetic particle events observed between 1997 and 2004. The low Earth orbiting SAMPEX satellite detected transient increases of quasi-trapped equatorial protons beginning typically ∼3 hr after the X-ray flare and lasting for up to several hours. Since the magnetic cutoff rigidity is >10 GV at the magnetic latitude where the particles were observed, we interpret the signal as due to ENAs that penetrate Earth’s magnetic field and charge exchange in the upper atmosphere, whereupon the charged particles may become trapped. One event outside our survey period (2006 December 5) had previously reported solar flare ENAs, the only example of this phenomenon of which we are aware. Although the statistics are limited, the events we report suggest that the ENAs are produced as the flare-associated coronal mass wjection moves through the corona, as concluded previously for the 2006 December 5 event. The finding of ENAs emitted in conjunction with large solar flares opens a new avenue to understanding these events.

This study investigates the characterization and calibration of the high‐energy electron experiments (HEP) instrument onboard the exploration of energization and radiation in geospace (ERG). Two detector modules, HEP‐L and HEP‐H, which employ stacks of multichannel silicon strip detectors, detect electrons in the energy ranges of 70 keV–1 MeV and 700 keV–2 MeV, respectively. The detector response to electron irradiation needs to be assessed to obtain accurate electron fluxes from these detectors. In this study, we perform Monte Carlo simulations using the Geant4 particle simulation tool to reconstruct incident electron fluxes from detected count rates. Based on the simulation results, we investigate the response characteristics of the detectors when electrons with a certain range of energy are irradiated onto them. A response function is constructed by combining the simulation results for different incident energies. A response matrix is calculated by binning the response function according to the energy channels of the detector, and an inverse matrix derived from the response matrix is used to calibrate the observational data. Compared with the data obtained from another electron instrument onboard the Arase satellite (MEP‐e), whose energy range overlaps with that of the HEP, the differential flux data for the overlapping energy range (85–95 keV) are consistent with each other. The basic characteristics of the HEP detectors are thus confirmed to provide well‐calibrated data.

When high energy galactic cosmic ray (GCR) particles collide with the lunar regolith, they eject "albedo" particles from the surface. The albedo particles could be either scattered incident ions, or secondary ions and neutrons produced by the collisions of incident ions with the lunar regolith. In an effort to understand the nature of these albedo particles, we use the MCNP6 transport code to estimate the angular and energy distribution of albedo particles at an altitude of 50 km to calculate the resulting total absorbed dose rates, which are compared with measurements from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument aboard the Lunar Reconnaissance Orbiter (LRO) spacecraft. MCNP6 simulations estimate that the albedo particles account for 19.9% of the total absorbed dose rate. The albedo photons account for 8.81% of the total absorbed dose rate, the highest among albedo species followed by protons (4.97%), electrons (2.47%), positrons (1.89%), neutrons (1.17%), deuterons (0.5%), tritium ions (0.07%), helium-3 ions (0.02%), and alphas (0.01%). In addition, recent studies indicate the presence of hydrogen on the Moon. We simulate a hypothetical lunar regolith enriched with hydrogen to study its effect on the lunar albedo. The results herein show that the proton absorbed dose rate is slightly increased if hydrogen is present in the regolith, which is primarily caused by the increased flux of energetic albedo protons leaving the lunar surface.

Abstract The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) has been orbiting the Moon since 2009 aboard the Lunar Reconnaissance Orbiter (LRO). From this vantage point, it samples the interplanetary energetic particle population outside the shielding of the Earth's magnetosphere. We report the sensor's observations of galactic cosmic rays (GCRs) over a complete solar activity cycle. CRaTER is designed primarily to measure not the spectra of GCR particles outside the sensor but rather their effects on matter, and in particular, it measures the linear energy transfer (LET) or energy‐deposit spectrum in its silicon detectors. We have used the Geant4 radiation‐transport code to devise a background‐rejection algorithm to improve these measurements of LET under 9.9 g/cm2 of shielding, and the resulting observations show the changing radiation effects of GCRs as their intensity and spectrum vary with solar modulation. As of 2020 this intensity, after declining during solar maximum activity, has recovered to a level that exceeds by a few percent the historically high values seen during the deep solar minimum at the start of the LRO mission in 2009.